<s>
Meltdown	B-Architecture
is	O
one	O
of	O
the	O
two	O
original	O
transient	O
execution	O
CPU	B-Device
vulnerabilities	O
(	O
the	O
other	O
being	O
Spectre	B-Error_Name
)	O
.	O
</s>
<s>
Meltdown	B-Architecture
affects	O
Intel	B-Device
x86	I-Device
microprocessors	I-Device
,	O
IBM	B-Device
POWER	I-Device
processors	I-Device
,	O
and	O
some	O
ARM-based	B-Architecture
microprocessors	I-Architecture
.	O
</s>
<s>
It	O
allows	O
a	O
rogue	O
process	O
to	O
read	O
all	O
memory	B-Architecture
,	O
even	O
when	O
it	O
is	O
not	O
authorized	O
to	O
do	O
so	O
.	O
</s>
<s>
Meltdown	B-Architecture
affects	O
a	O
wide	O
range	O
of	O
systems	O
.	O
</s>
<s>
At	O
the	O
time	O
of	O
disclosure	O
(	O
2018	O
)	O
,	O
this	O
included	O
all	O
devices	O
running	O
any	O
but	O
the	O
most	O
recent	O
and	O
patched	O
versions	O
of	O
iOS	B-Application
,	O
Linux	B-Operating_System
,	O
macOS	B-Application
,	O
or	O
Windows	O
.	O
</s>
<s>
Accordingly	O
,	O
many	O
servers	O
and	O
cloud	B-Architecture
services	I-Architecture
were	O
impacted	O
,	O
as	O
well	O
as	O
a	O
potential	O
majority	O
of	O
smart	B-Device
devices	I-Device
and	O
embedded	B-Architecture
devices	I-Architecture
using	O
ARM-based	O
processors	O
(	O
mobile	O
devices	O
,	O
smart	O
TVs	O
,	O
printers	O
and	O
others	O
)	O
,	O
including	O
a	O
wide	O
range	O
of	O
networking	O
equipment	O
.	O
</s>
<s>
A	O
purely	O
software	O
workaround	O
to	O
Meltdown	B-Architecture
has	O
been	O
assessed	O
as	O
slowing	O
computers	O
between	O
5	O
and	O
30	O
percent	O
in	O
certain	O
specialized	O
workloads	O
,	O
although	O
companies	O
responsible	O
for	O
software	O
correction	O
of	O
the	O
exploit	O
reported	O
minimal	O
impact	O
from	O
general	O
benchmark	O
testing	O
.	O
</s>
<s>
Meltdown	B-Architecture
was	O
issued	O
a	O
Common	O
Vulnerabilities	O
and	O
Exposures	O
ID	O
of	O
,	O
also	O
known	O
as	O
Rogue	B-Architecture
Data	I-Architecture
Cache	I-Architecture
Load	I-Architecture
(	O
RDCL	O
)	O
,	O
in	O
January	O
2018	O
.	O
</s>
<s>
It	O
was	O
disclosed	O
in	O
conjunction	O
with	O
another	O
exploit	O
,	O
Spectre	B-Error_Name
,	O
with	O
which	O
it	O
shares	O
some	O
characteristics	O
.	O
</s>
<s>
The	O
Meltdown	B-Architecture
and	O
Spectre	B-Error_Name
vulnerabilities	I-Error_Name
are	O
considered	O
"	O
catastrophic	O
"	O
by	O
security	O
analysts	O
.	O
</s>
<s>
Several	O
procedures	O
to	O
help	O
protect	O
home	O
computers	O
and	O
related	O
devices	O
from	O
the	O
Meltdown	B-Architecture
and	O
Spectre	B-Error_Name
security	O
vulnerabilities	O
have	O
been	O
published	O
.	O
</s>
<s>
Meltdown	B-Architecture
patches	O
may	O
produce	O
performance	O
loss	O
.	O
</s>
<s>
Spectre	B-Error_Name
patches	O
have	O
been	O
reported	O
to	O
significantly	O
reduce	O
performance	O
,	O
especially	O
on	O
older	O
computers	O
;	O
on	O
the	O
then-newest	O
(	O
2017	O
)	O
eighth-generation	B-Device
Core	I-Device
platforms	O
,	O
benchmark	O
performance	O
drops	O
of	O
2	O
–	O
14	O
percent	O
have	O
been	O
measured	O
.	O
</s>
<s>
On	O
18	O
January	O
2018	O
,	O
unwanted	O
reboots	O
,	O
even	O
for	O
newer	O
Intel	O
chips	O
,	O
due	O
to	O
Meltdown	B-Architecture
and	O
Spectre	B-Error_Name
patches	O
,	O
were	O
reported	O
.	O
</s>
<s>
Nonetheless	O
,	O
according	O
to	O
Dell	O
:	O
"	O
No	O
'	O
real-world	O
'	O
exploits	O
of	O
these	O
vulnerabilities	O
[	O
i.e.	O
,	O
Meltdown	B-Architecture
and	O
Spectre ]	O
have	O
been	O
reported	O
to	O
date	O
[	O
26	O
January	O
2018 ]	O
,	O
though	O
researchers	O
have	O
produced	O
proof-of-concepts.	O
"	O
</s>
<s>
On	O
15	O
March	O
2018	O
,	O
Intel	O
reported	O
that	O
it	O
will	O
redesign	O
its	O
CPUs	B-Device
to	O
help	O
protect	O
against	O
the	O
Meltdown	B-Architecture
and	O
related	O
Spectre	B-Error_Name
vulnerabilities	I-Error_Name
(	O
especially	O
,	O
Meltdown	B-Architecture
and	O
Spectre-V2	O
,	O
but	O
not	O
Spectre-V1	O
)	O
,	O
and	O
expects	O
to	O
release	O
the	O
newly	O
redesigned	O
processors	O
later	O
in	O
2018	O
.	O
</s>
<s>
On	O
8	O
October	O
2018	O
,	O
Intel	O
is	O
reported	O
to	O
have	O
added	O
hardware	O
and	O
firmware	O
mitigations	O
regarding	O
Spectre	B-Error_Name
and	O
Meltdown	B-Architecture
vulnerabilities	I-Architecture
to	O
its	O
latest	O
processors	O
.	O
</s>
<s>
Meltdown	B-Architecture
exploits	I-Architecture
a	O
race	B-Operating_System
condition	I-Operating_System
,	O
inherent	O
in	O
the	O
design	O
of	O
many	O
modern	O
CPUs	B-Device
.	O
</s>
<s>
This	O
occurs	O
between	O
memory	B-Architecture
access	O
and	O
privilege	O
checking	O
during	O
instruction	B-General_Concept
processing	O
.	O
</s>
<s>
Additionally	O
,	O
combined	O
with	O
a	O
cache	B-General_Concept
side-channel	O
attack	O
,	O
this	O
vulnerability	O
allows	O
a	O
process	O
to	O
bypass	O
the	O
normal	O
privilege	O
checks	O
that	O
isolate	O
the	O
exploit	O
process	O
from	O
accessing	O
data	O
belonging	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
other	O
running	O
processes	B-Operating_System
.	O
</s>
<s>
The	O
vulnerability	O
allows	O
an	O
unauthorized	O
process	O
to	O
read	O
data	O
from	O
any	O
address	O
that	O
is	O
mapped	O
to	O
the	O
current	O
process	O
's	O
memory	B-Architecture
space	O
.	O
</s>
<s>
Since	O
instruction	B-General_Concept
pipelining	I-General_Concept
is	O
in	O
the	O
affected	O
processors	O
,	O
the	O
data	O
from	O
an	O
unauthorized	O
address	O
will	O
almost	O
always	O
be	O
temporarily	O
loaded	O
into	O
the	O
CPU	B-General_Concept
's	I-General_Concept
cache	I-General_Concept
during	O
out-of-order	B-General_Concept
execution	I-General_Concept
—	O
from	O
which	O
the	O
data	O
can	O
be	O
recovered	O
.	O
</s>
<s>
This	O
can	O
occur	O
even	O
if	O
the	O
original	O
read	O
instruction	B-General_Concept
fails	O
due	O
to	O
privilege	O
checking	O
,	O
or	O
if	O
it	O
never	O
produces	O
a	O
readable	O
result	O
.	O
</s>
<s>
Since	O
many	O
operating	B-General_Concept
systems	I-General_Concept
map	B-Architecture
physical	B-Architecture
memory	I-Architecture
,	O
kernel	B-Operating_System
processes	B-Operating_System
,	O
and	O
other	O
running	O
user	B-Operating_System
space	I-Operating_System
processes	B-Operating_System
into	O
the	O
address	O
space	O
of	O
every	O
process	O
,	O
Meltdown	B-Architecture
effectively	O
makes	O
it	O
possible	O
for	O
a	O
rogue	O
process	O
to	O
read	O
any	O
physical	O
,	O
kernel	B-Operating_System
or	O
other	O
processes	B-Operating_System
 '	O
mapped	O
memory	B-Architecture
—	O
regardless	O
of	O
whether	O
it	O
should	O
be	O
able	O
to	O
do	O
so	O
.	O
</s>
<s>
Defenses	O
against	O
Meltdown	B-Architecture
would	O
require	O
avoiding	O
the	O
use	O
of	O
memory	B-Architecture
mapping	O
in	O
a	O
manner	O
vulnerable	O
to	O
such	O
exploits	O
(	O
i.e.	O
</s>
<s>
a	O
software-based	O
solution	O
)	O
or	O
avoidance	O
of	O
the	O
underlying	O
race	B-Operating_System
condition	I-Operating_System
(	O
i.e.	O
</s>
<s>
a	O
modification	O
to	O
the	O
CPUs	B-Device
 '	O
microcode	B-Device
or	O
execution	O
path	O
)	O
.	O
</s>
<s>
The	O
vulnerability	O
is	O
viable	O
on	O
any	O
operating	B-General_Concept
system	I-General_Concept
in	O
which	O
privileged	O
data	O
is	O
mapped	O
into	O
virtual	B-Architecture
memory	I-Architecture
for	O
unprivileged	O
processes	B-Operating_System
—	O
which	O
includes	O
many	O
present-day	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Meltdown	B-Architecture
could	O
potentially	O
impact	O
a	O
wider	O
range	O
of	O
computers	O
than	O
presently	O
identified	O
,	O
as	O
there	O
is	O
little	O
to	O
no	O
variation	O
in	O
the	O
microprocessor	B-Architecture
families	O
used	O
by	O
these	O
computers	O
.	O
</s>
<s>
A	O
Meltdown	B-Architecture
attack	I-Architecture
cannot	O
be	O
detected	O
if	O
it	O
is	O
carried	O
out	O
.	O
</s>
<s>
On	O
8	O
May	O
1995	O
,	O
a	O
paper	O
called	O
"	O
The	O
Intel	B-Operating_System
80x86	I-Operating_System
Processor	O
Architecture	O
:	O
Pitfalls	O
for	O
Secure	O
Systems	O
"	O
published	O
at	O
the	O
1995	O
IEEE	O
Symposium	O
on	O
Security	O
and	O
Privacy	O
warned	O
against	O
a	O
covert	O
timing	O
channel	O
in	O
the	O
CPU	B-General_Concept
cache	I-General_Concept
and	O
translation	B-Architecture
lookaside	I-Architecture
buffer	I-Architecture
(	O
TLB	O
)	O
.	O
</s>
<s>
In	O
July	O
2012	O
,	O
Apple	O
's	O
XNU	B-Operating_System
kernel	I-Operating_System
(	O
used	O
in	O
macOS	B-Application
,	O
iOS	B-Application
and	O
tvOS	B-Operating_System
,	O
among	O
others	O
)	O
adopted	O
kernel	B-Operating_System
address	O
space	O
layout	O
randomization	O
(	O
KASLR	O
)	O
with	O
the	O
release	O
of	O
OS	B-Operating_System
X	I-Operating_System
Mountain	I-Operating_System
Lion	I-Operating_System
10.8	B-Operating_System
.	O
</s>
<s>
In	O
essence	O
,	O
the	O
base	O
of	O
the	O
system	O
,	O
including	O
its	O
kernel	B-Application
extensions	I-Application
(	O
kexts	B-Application
)	O
and	O
memory	B-Architecture
zones	O
,	O
is	O
randomly	O
relocated	O
during	O
the	O
boot	O
process	O
in	O
an	O
effort	O
to	O
reduce	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
vulnerability	O
to	O
attacks	O
.	O
</s>
<s>
In	O
March	O
2014	O
,	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
adopted	O
KASLR	O
to	O
mitigate	O
address	O
leaks	O
.	O
</s>
<s>
On	O
8	O
August	O
2016	O
,	O
Anders	O
Fogh	O
and	O
Daniel	O
Gruss	O
presented	O
"	O
Using	O
Undocumented	O
CPU	B-Device
Behavior	O
to	O
See	O
Into	O
Kernel	B-Operating_System
Mode	O
and	O
Break	O
KASLR	O
in	O
the	O
Process	O
"	O
at	O
the	O
Black	O
Hat	O
2016	O
conference	O
.	O
</s>
<s>
of	O
TU	O
Graz	O
published	O
"	O
ARMageddon	O
:	O
Cache	B-General_Concept
Attacks	O
on	O
Mobile	O
Devices	O
"	O
in	O
the	O
proceedings	O
of	O
the	O
25th	O
USENIX	B-Operating_System
security	I-Operating_System
symposium	I-Operating_System
.	O
</s>
<s>
Even	O
though	O
focused	O
on	O
ARM	B-Architecture
,	O
it	O
laid	O
the	O
groundwork	O
for	O
the	O
attack	O
vector	O
.	O
</s>
<s>
On	O
27	O
December	O
2016	O
,	O
at	O
33C3	O
,	O
Clémentine	O
Maurice	O
and	O
Moritz	O
Lipp	O
of	O
TU	O
Graz	O
presented	O
their	O
talk	O
"	O
What	O
could	O
possibly	O
go	O
wrong	O
with	O
insert	O
x86	B-Operating_System
instruction	B-General_Concept
here	O
?	O
</s>
<s>
Side	O
effects	O
include	O
side-channel	O
attacks	O
and	O
bypassing	O
kernel	B-Operating_System
ASLR	O
"	O
which	O
outlined	O
already	O
what	O
was	O
coming	O
.	O
</s>
<s>
of	O
Vrije	O
Universiteit	O
Amsterdam	O
published	O
their	O
findings	O
of	O
how	O
address	O
space	O
layout	O
randomization	O
(	O
ASLR	O
)	O
could	O
be	O
abused	O
on	O
cache-based	O
architectures	O
at	O
the	O
NDSS	O
Symposium	O
.	O
</s>
<s>
On	O
27	O
March	O
2017	O
,	O
researchers	O
at	O
Graz	O
University	O
of	O
Technology	O
in	O
Austria	O
developed	O
a	O
proof-of-concept	O
that	O
could	O
grab	O
RSA	B-Architecture
keys	O
from	O
Intel	O
SGX	O
enclaves	O
running	O
on	O
the	O
same	O
system	O
within	O
five	O
minutes	O
by	O
using	O
certain	O
CPU	B-Device
instructions	O
in	O
lieu	O
of	O
a	O
fine-grained	O
timer	O
to	O
exploit	O
cache	B-General_Concept
DRAM	O
side-channels	O
.	O
</s>
<s>
A	O
presentation	O
on	O
the	O
resulting	O
KAISER	B-Application
technique	O
was	O
submitted	O
for	O
the	O
Black	O
Hat	O
congress	O
in	O
July	O
2017	O
,	O
but	O
was	O
rejected	O
by	O
the	O
organizers	O
.	O
</s>
<s>
Nevertheless	O
,	O
this	O
work	O
led	O
to	O
kernel	B-Application
page-table	I-Application
isolation	I-Application
(	O
KPTI	B-Application
,	O
originally	O
known	O
as	O
KAISER	B-Application
)	O
in	O
2017	O
,	O
which	O
was	O
confirmed	O
to	O
eliminate	O
a	O
large	O
class	O
of	O
security	O
bugs	O
,	O
including	O
some	O
limited	O
protection	O
against	O
the	O
not-yet-discovered	O
Meltdown	B-Architecture
–	O
a	O
fact	O
confirmed	O
by	O
the	O
Meltdown	B-Architecture
authors	O
.	O
</s>
<s>
In	O
July	O
2017	O
,	O
research	O
made	O
public	O
on	O
the	O
CyberWTF	O
website	O
by	O
security	O
researcher	O
Anders	O
Fogh	O
outlined	O
the	O
use	O
of	O
a	O
cache	B-General_Concept
timing	O
attack	O
to	O
read	O
kernel	B-Operating_System
space	I-Operating_System
data	O
by	O
observing	O
the	O
results	O
of	O
speculative	O
operations	O
conditioned	O
on	O
data	O
fetched	O
with	O
invalid	O
privileges	O
.	O
</s>
<s>
Meltdown	B-Architecture
was	O
discovered	O
independently	O
by	O
Jann	O
Horn	O
from	O
Google	B-Application
's	I-Application
Project	O
Zero	O
,	O
Werner	O
Haas	O
and	O
Thomas	O
Prescher	O
from	O
Cyberus	O
Technology	O
,	O
and	O
Daniel	O
Gruss	O
,	O
Moritz	O
Lipp	O
,	O
Stefan	O
Mangard	O
and	O
Michael	O
Schwarz	O
from	O
Graz	O
University	O
of	O
Technology	O
.	O
</s>
<s>
The	O
same	O
research	O
teams	O
that	O
discovered	O
Meltdown	B-Architecture
also	O
discovered	O
Spectre	B-Error_Name
.	O
</s>
<s>
In	O
October	O
2017	O
,	O
Kernel	B-Operating_System
ASLR	O
support	O
on	O
amd64	O
was	O
added	O
to	O
NetBSD-current	O
,	O
making	O
NetBSD	B-Device
the	O
first	O
totally	O
open-source	O
BSD	O
system	O
to	O
support	O
kernel	B-Operating_System
address	O
space	O
layout	O
randomization	O
(	O
KASLR	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
partially	O
open-source	O
Apple	B-Operating_System
Darwin	I-Operating_System
,	O
which	O
forms	O
the	O
foundation	O
of	O
macOS	B-Application
and	O
iOS	B-Application
(	O
among	O
others	O
)	O
,	O
is	O
based	O
on	O
FreeBSD	B-Operating_System
;	O
KASLR	O
was	O
added	O
to	O
its	O
XNU	B-Operating_System
kernel	I-Operating_System
in	O
2012	O
as	O
noted	O
above	O
.	O
</s>
<s>
On	O
14	O
November	O
2017	O
,	O
security	O
researcher	O
Alex	O
Ionescu	O
publicly	O
mentioned	O
changes	O
in	O
the	O
new	O
version	O
of	O
Windows	B-Operating_System
10	I-Operating_System
that	O
would	O
cause	O
some	O
speed	O
degradation	O
without	O
explaining	O
the	O
necessity	O
for	O
the	O
changes	O
,	O
just	O
referring	O
to	O
similar	O
changes	O
in	O
Linux	B-Operating_System
.	O
</s>
<s>
After	O
affected	O
hardware	O
and	O
software	O
vendors	O
had	O
been	O
made	O
aware	O
of	O
the	O
issue	O
on	O
28	O
July	O
2017	O
,	O
the	O
two	O
vulnerabilities	O
were	O
made	O
public	O
jointly	O
,	O
on	O
3	O
January	O
2018	O
,	O
several	O
days	O
ahead	O
of	O
the	O
coordinated	O
release	O
date	O
of	O
9	O
January	O
2018	O
as	O
news	O
sites	O
started	O
reporting	O
about	O
commits	O
to	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
and	O
mails	O
to	O
its	O
mailing	O
list	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
patches	O
were	O
not	O
available	O
for	O
some	O
platforms	O
,	O
such	O
as	O
Ubuntu	B-Operating_System
,	O
when	O
the	O
vulnerabilities	O
were	O
disclosed	O
.	O
</s>
<s>
On	O
28	O
January	O
2018	O
,	O
Intel	O
was	O
reported	O
to	O
have	O
shared	O
news	O
of	O
the	O
Meltdown	B-Architecture
and	O
Spectre	B-Error_Name
security	O
vulnerabilities	O
with	O
Chinese	O
technology	O
companies	O
before	O
notifying	O
the	O
U.S.	O
government	O
of	O
the	O
flaws	O
.	O
</s>
<s>
The	O
security	O
vulnerability	O
was	O
called	O
Meltdown	B-Architecture
because	O
"	O
the	O
vulnerability	O
basically	O
melts	O
security	O
boundaries	O
which	O
are	O
normally	O
enforced	O
by	O
the	O
hardware	O
"	O
.	O
</s>
<s>
On	O
8	O
October	O
2018	O
,	O
Intel	O
was	O
reported	O
to	O
have	O
added	O
hardware	O
and	O
firmware	O
mitigations	O
regarding	O
Spectre	B-Error_Name
and	O
Meltdown	B-Architecture
vulnerabilities	I-Architecture
to	O
its	O
latest	O
processors	O
.	O
</s>
<s>
Researchers	O
attempted	O
to	O
compromise	O
CPU	B-Device
protection	O
mechanisms	O
using	O
code	O
to	O
exploit	O
weaknesses	O
in	O
memory	B-Architecture
protection	O
and	O
the	O
instruction	B-General_Concept
.	O
</s>
<s>
They	O
also	O
attempted	O
but	O
failed	O
to	O
exploit	O
CPU	B-Device
operations	O
for	O
memory	B-Architecture
alignment	O
,	O
division	O
by	O
zero	O
,	O
supervisor	O
modes	O
,	O
segment	O
limits	O
,	O
invalid	O
opcodes	O
,	O
and	O
non-executable	O
code	O
.	O
</s>
<s>
Meltdown	B-Architecture
relies	O
on	O
a	O
CPU	B-Device
race	B-Operating_System
condition	I-Operating_System
that	O
can	O
arise	O
between	O
instruction	B-General_Concept
execution	O
and	O
privilege	O
checking	O
.	O
</s>
<s>
Put	O
briefly	O
,	O
the	O
instruction	B-General_Concept
execution	O
leaves	O
side	O
effects	O
that	O
constitute	O
information	O
not	O
hidden	O
to	O
the	O
process	O
by	O
the	O
privilege	O
check	O
.	O
</s>
<s>
The	O
process	O
carrying	O
out	O
Meltdown	B-Architecture
then	O
uses	O
these	O
side	O
effects	O
to	O
infer	O
the	O
values	O
of	O
memory	B-Architecture
mapped	I-Architecture
data	I-Architecture
,	O
bypassing	O
the	O
privilege	O
check	O
.	O
</s>
<s>
The	O
following	O
provides	O
an	O
overview	O
of	O
the	O
exploit	O
,	O
and	O
the	O
memory	B-Architecture
mapping	O
that	O
is	O
its	O
target	O
.	O
</s>
<s>
The	O
attack	O
is	O
described	O
in	O
terms	O
of	O
an	O
Intel	B-Device
processor	I-Device
running	O
Microsoft	B-Application
Windows	I-Application
or	O
Linux	B-Operating_System
,	O
the	O
main	O
test	O
targets	O
used	O
in	O
the	O
original	O
paper	O
,	O
but	O
it	O
also	O
affects	O
other	O
processors	O
and	O
operating	B-General_Concept
systems	I-General_Concept
,	O
including	O
macOS	B-Application
(	O
aka	O
OS	O
X	O
)	O
,	O
iOS	B-Application
,	O
and	O
Android	B-Application
.	O
</s>
<s>
Modern	O
computer	B-Device
processors	I-Device
use	O
a	O
variety	O
of	O
techniques	O
to	O
gain	O
high	O
levels	O
of	O
efficiency	O
.	O
</s>
<s>
Four	O
widely	O
used	O
features	O
are	O
particularly	O
relevant	O
to	O
Meltdown	B-Architecture
:	O
</s>
<s>
Virtual	O
(	O
paged	O
)	O
memory	B-Architecture
,	O
also	O
known	O
as	O
memory	B-Architecture
mapping	O
–	O
used	O
to	O
make	O
memory	B-Architecture
access	O
more	O
efficient	O
and	O
to	O
control	O
which	O
processes	B-Operating_System
can	O
access	O
which	O
areas	O
of	O
memory.A	O
modern	O
computer	O
usually	O
runs	O
many	O
processes	B-Operating_System
in	O
parallel	O
.	O
</s>
<s>
In	O
an	O
operating	B-General_Concept
system	I-General_Concept
such	O
as	O
Windows	O
or	O
Linux	B-Operating_System
,	O
each	O
process	O
is	O
given	O
the	O
impression	O
that	O
it	O
alone	O
has	O
complete	O
use	O
of	O
the	O
computer	O
's	O
physical	B-Architecture
memory	I-Architecture
,	O
and	O
may	O
do	O
with	O
it	O
as	O
it	O
likes	O
.	O
</s>
<s>
In	O
reality	O
it	O
will	O
be	O
allocated	O
memory	B-Architecture
to	O
use	O
from	O
the	O
physical	B-Architecture
memory	I-Architecture
,	O
which	O
acts	O
as	O
a	O
"	O
pool	O
"	O
of	O
available	O
memory	B-Architecture
,	O
when	O
it	O
first	O
tries	O
to	O
use	O
any	O
given	O
memory	B-Architecture
address	O
(	O
by	O
trying	O
to	O
read	O
or	O
write	O
to	O
it	O
)	O
.	O
</s>
<s>
This	O
allows	O
multiple	O
processes	B-Operating_System
,	O
including	O
the	O
kernel	B-Operating_System
or	O
operating	B-General_Concept
system	I-General_Concept
itself	O
,	O
to	O
co-habit	O
on	O
the	O
same	O
system	O
,	O
but	O
retain	O
their	O
individual	O
activity	O
and	O
integrity	O
without	O
being	O
affected	O
by	O
other	O
running	O
processes	B-Operating_System
,	O
and	O
without	O
being	O
vulnerable	O
to	O
interference	O
or	O
unauthorized	O
data	O
leaks	O
caused	O
by	O
a	O
rogue	O
process	O
.	O
</s>
<s>
Privilege	B-Operating_System
levels	I-Operating_System
,	O
or	O
protection	O
domains	O
–	O
provide	O
a	O
means	O
by	O
which	O
the	O
operating	B-General_Concept
system	I-General_Concept
can	O
control	O
which	O
processes	B-Operating_System
are	O
authorized	O
to	O
read	O
which	O
areas	O
of	O
virtual	O
memory.As	O
virtual	B-Architecture
memory	I-Architecture
permits	O
a	O
computer	O
to	O
refer	O
to	O
vastly	O
more	O
memory	B-Architecture
than	O
it	O
will	O
ever	O
physically	O
contain	O
,	O
the	O
system	O
can	O
be	O
greatly	O
sped	O
up	O
by	O
"	O
mapping	O
"	O
every	O
process	O
and	O
their	O
in-use	O
memory	B-Architecture
–	O
in	O
effect	O
all	O
memory	B-Architecture
of	O
all	O
active	O
processes	B-Operating_System
–	O
into	O
every	O
process	O
's	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
In	O
some	O
systems	O
all	O
physical	B-Architecture
memory	I-Architecture
is	O
mapped	O
as	O
well	O
,	O
for	O
further	O
speed	O
and	O
efficiency	O
.	O
</s>
<s>
This	O
is	O
usually	O
considered	O
safe	O
,	O
because	O
the	O
operating	B-General_Concept
system	I-General_Concept
can	O
rely	O
on	O
privilege	B-Operating_System
controls	I-Operating_System
built	O
into	O
the	O
processor	O
itself	O
,	O
to	O
limit	O
which	O
areas	O
of	O
memory	B-Architecture
any	O
given	O
process	O
is	O
permitted	O
to	O
access	O
.	O
</s>
<s>
An	O
attempt	O
to	O
access	O
authorized	O
memory	B-Architecture
will	O
immediately	O
succeed	O
,	O
and	O
an	O
attempt	O
to	O
access	O
unauthorized	O
memory	B-Architecture
will	O
cause	O
an	O
exception	B-General_Concept
and	O
void	O
the	O
read	O
instruction	B-General_Concept
,	O
which	O
will	O
fail	O
.	O
</s>
<s>
Either	O
the	O
calling	O
process	O
or	O
the	O
operating	B-General_Concept
system	I-General_Concept
directs	O
what	O
will	O
happen	O
if	O
an	O
attempt	O
is	O
made	O
to	O
read	O
from	O
unauthorized	O
memory	B-Architecture
–	O
typically	O
it	O
causes	O
an	O
error	O
condition	O
and	O
the	O
process	O
that	O
attempted	O
to	O
execute	O
the	O
read	O
will	O
be	O
terminated	O
.	O
</s>
<s>
As	O
unauthorized	O
reads	O
are	O
usually	O
not	O
part	O
of	O
normal	O
program	O
execution	O
,	O
it	O
is	O
much	O
faster	O
to	O
use	O
this	O
approach	O
than	O
to	O
pause	O
the	O
process	O
every	O
time	O
it	O
executes	O
some	O
function	O
that	O
requires	O
privileged	O
memory	B-Architecture
to	O
be	O
accessed	O
,	O
to	O
allow	O
that	O
memory	B-Architecture
to	O
be	O
mapped	O
into	O
a	O
readable	O
address	O
space	O
.	O
</s>
<s>
Instruction	B-General_Concept
pipelining	I-General_Concept
and	O
speculative	B-General_Concept
execution	I-General_Concept
–	O
used	O
to	O
allow	O
instructions	O
to	O
execute	O
in	O
the	O
most	O
efficient	O
manner	O
possible	O
–	O
if	O
necessary	O
allowing	O
them	O
to	O
run	O
out	O
of	O
order	O
or	O
in	O
parallel	O
across	O
various	O
processing	O
units	O
within	O
the	O
CPU	B-Device
–	O
so	O
long	O
as	O
the	O
final	O
outcome	O
is	O
correct.Modern	O
processors	O
commonly	O
contain	O
numerous	O
separate	O
execution	B-General_Concept
units	I-General_Concept
,	O
and	O
a	O
scheduler	O
that	O
decodes	O
instructions	O
and	O
decides	O
,	O
at	O
the	O
time	O
they	O
are	O
executed	O
,	O
the	O
most	O
efficient	O
way	O
to	O
execute	O
them	O
.	O
</s>
<s>
This	O
might	O
involve	O
the	O
decision	O
that	O
two	O
instructions	O
can	O
execute	O
at	O
the	O
same	O
time	O
,	O
or	O
even	O
out	O
of	O
order	O
,	O
on	O
different	O
execution	B-General_Concept
units	I-General_Concept
(	O
known	O
as	O
"	O
instruction	B-General_Concept
pipelining	I-General_Concept
"	O
)	O
.	O
</s>
<s>
So	O
long	O
as	O
the	O
correct	O
outcome	O
is	O
still	O
achieved	O
,	O
this	O
maximizes	O
efficiency	O
by	O
keeping	O
all	O
of	O
the	O
processor	O
's	O
execution	B-General_Concept
units	I-General_Concept
in	O
use	O
as	O
much	O
as	O
possible	O
.	O
</s>
<s>
Some	O
instructions	O
,	O
such	O
as	O
conditional	B-General_Concept
branches	I-General_Concept
,	O
will	O
lead	O
to	O
one	O
of	O
two	O
different	O
outcomes	O
,	O
depending	O
on	O
a	O
condition	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
the	O
CPU	B-Device
may	O
not	O
yet	O
know	O
which	O
branch	O
to	O
take	O
.	O
</s>
<s>
Rather	O
than	O
wait	O
to	O
learn	O
the	O
correct	O
option	O
,	O
the	O
CPU	B-Device
may	O
proceed	O
immediately	O
(	O
speculative	B-General_Concept
execution	I-General_Concept
)	O
.	O
</s>
<s>
If	O
it	O
executes	O
the	O
incorrect	O
option	O
,	O
the	O
CPU	B-Device
will	O
attempt	O
to	O
discard	O
all	O
effects	O
of	O
its	O
incorrect	O
guess	O
.	O
</s>
<s>
CPU	B-General_Concept
cache	I-General_Concept
–	O
a	O
modest	O
amount	O
of	O
memory	B-Architecture
within	O
the	O
CPU	B-Device
used	O
to	O
ensure	O
it	O
can	O
work	O
at	O
high	O
speed	O
,	O
to	O
speed	O
up	O
memory	B-Architecture
access	O
,	O
and	O
to	O
facilitate	O
"	O
intelligent	O
"	O
execution	O
of	O
instructions	O
in	O
an	O
efficient	O
manner.From	O
the	O
perspective	O
of	O
a	O
CPU	B-Device
,	O
the	O
computer	O
's	O
physical	B-Architecture
memory	I-Architecture
is	O
slow	O
to	O
access	O
.	O
</s>
<s>
Also	O
the	O
instructions	O
a	O
CPU	B-Device
runs	O
are	O
very	O
often	O
repetitive	O
,	O
or	O
access	O
the	O
same	O
or	O
similar	O
memory	B-Architecture
numerous	O
times	O
.	O
</s>
<s>
To	O
maximize	O
efficient	O
use	O
of	O
the	O
CPU	B-Device
's	O
resources	O
,	O
modern	O
CPUs	B-Device
often	O
have	O
a	O
modest	O
amount	O
of	O
very	O
fast	O
on-chip	O
memory	B-Architecture
,	O
known	O
as	O
CPU	B-General_Concept
cache	I-General_Concept
.	O
</s>
<s>
When	O
data	O
is	O
accessed	O
or	O
an	O
instruction	B-General_Concept
is	O
read	O
from	O
physical	B-Architecture
memory	I-Architecture
,	O
a	O
copy	O
of	O
that	O
information	O
is	O
routinely	O
saved	O
in	O
the	O
CPU	B-General_Concept
cache	I-General_Concept
at	O
the	O
same	O
time	O
.	O
</s>
<s>
If	O
the	O
CPU	B-Device
later	O
needs	O
the	O
same	O
instruction	B-General_Concept
or	O
memory	B-Architecture
contents	O
again	O
,	O
it	O
can	O
obtain	O
it	O
with	O
minimal	O
delay	O
from	O
its	O
own	O
cache	B-General_Concept
rather	O
than	O
waiting	O
for	O
a	O
request	O
related	O
to	O
physical	B-Architecture
memory	I-Architecture
to	O
take	O
place	O
.	O
</s>
<s>
They	O
provide	O
the	O
basis	O
for	O
most	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
and	O
processors	O
.	O
</s>
<s>
Meltdown	B-Architecture
exploits	I-Architecture
the	O
way	O
these	O
features	O
interact	O
to	O
bypass	O
the	O
CPU	B-Device
's	O
fundamental	O
privilege	B-Operating_System
controls	I-Operating_System
and	O
access	O
privileged	O
and	O
sensitive	O
data	O
from	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
other	O
processes	B-Operating_System
.	O
</s>
<s>
To	O
understand	O
Meltdown	B-Architecture
,	O
consider	O
the	O
data	O
that	O
is	O
mapped	O
in	O
virtual	B-Architecture
memory	I-Architecture
(	O
much	O
of	O
which	O
the	O
process	O
is	O
not	O
supposed	O
to	O
be	O
able	O
to	O
access	O
)	O
and	O
how	O
the	O
CPU	B-Device
responds	O
when	O
a	O
process	O
attempts	O
to	O
access	O
unauthorized	O
memory	B-Architecture
.	O
</s>
<s>
The	O
process	O
is	O
running	O
on	O
a	O
vulnerable	O
version	O
of	O
Windows	O
,	O
Linux	B-Operating_System
,	O
or	O
macOS	B-Application
,	O
on	O
a	O
64-bit	B-Device
processor	I-Device
of	O
a	O
vulnerable	O
type	O
.	O
</s>
<s>
The	O
CPU	B-Device
encounters	O
an	O
instruction	B-General_Concept
accessing	O
the	O
value	O
,	O
A	O
,	O
at	O
an	O
address	O
forbidden	O
to	O
the	O
process	O
by	O
the	O
virtual	B-Architecture
memory	I-Architecture
system	O
and	O
the	O
privilege	O
check	O
.	O
</s>
<s>
Because	O
of	O
speculative	B-General_Concept
execution	I-General_Concept
,	O
the	O
instruction	B-General_Concept
is	O
scheduled	O
and	O
dispatched	O
to	O
an	O
execution	B-General_Concept
unit	I-General_Concept
.	O
</s>
<s>
This	O
execution	B-General_Concept
unit	I-General_Concept
then	O
schedules	O
both	O
the	O
privilege	O
check	O
and	O
the	O
memory	B-Architecture
access	O
.	O
</s>
<s>
The	O
CPU	B-Device
encounters	O
an	O
instruction	B-General_Concept
accessing	O
address	O
Base+A	O
,	O
with	O
Base	O
chosen	O
by	O
the	O
attacker	O
.	O
</s>
<s>
This	O
instruction	B-General_Concept
is	O
also	O
scheduled	O
and	O
dispatched	O
to	O
an	O
execution	B-General_Concept
unit	I-General_Concept
.	O
</s>
<s>
The	O
privilege	O
check	O
informs	O
the	O
execution	B-General_Concept
unit	I-General_Concept
that	O
the	O
address	O
of	O
the	O
value	O
,	O
A	O
,	O
involved	O
in	O
the	O
access	O
is	O
forbidden	O
to	O
the	O
process	O
(	O
per	O
the	O
information	O
stored	O
by	O
the	O
virtual	B-Architecture
memory	I-Architecture
system	O
)	O
,	O
and	O
thus	O
the	O
instruction	B-General_Concept
should	O
fail	O
and	O
subsequent	O
instructions	O
should	O
have	O
no	O
effect	O
.	O
</s>
<s>
Because	O
these	O
instructions	O
were	O
speculatively	O
executed	O
,	O
however	O
,	O
the	O
data	O
at	O
Base+A	O
may	O
have	O
been	O
cached	O
before	O
the	O
privilege	O
check	O
–	O
and	O
may	O
not	O
have	O
been	O
undone	O
by	O
the	O
execution	B-General_Concept
unit	I-General_Concept
(	O
or	O
any	O
other	O
part	O
of	O
the	O
CPU	B-Device
)	O
.	O
</s>
<s>
If	O
this	O
is	O
indeed	O
the	O
case	O
,	O
the	O
mere	O
act	O
of	O
caching	B-General_Concept
constitutes	O
a	O
leak	O
of	O
information	O
in	O
and	O
of	O
itself	O
.	O
</s>
<s>
At	O
this	O
point	O
,	O
Meltdown	B-Architecture
intervenes	O
.	O
</s>
<s>
The	O
process	O
executes	O
a	O
timing	O
attack	O
by	O
executing	O
instructions	O
referencing	O
memory	B-Architecture
operands	O
directly	O
.	O
</s>
<s>
To	O
be	O
effective	O
,	O
the	O
operands	O
of	O
these	O
instructions	O
must	O
be	O
at	O
addresses	O
which	O
cover	O
the	O
possible	O
address	O
,	O
Base+A	O
,	O
of	O
the	O
rejected	O
instruction	B-General_Concept
's	O
operand	O
.	O
</s>
<s>
Because	O
the	O
data	O
at	O
the	O
address	O
referred	O
to	O
by	O
the	O
rejected	O
instruction	B-General_Concept
,	O
Base+A	O
,	O
was	O
cached	O
nevertheless	O
,	O
an	O
instruction	B-General_Concept
referencing	O
the	O
same	O
address	O
directly	O
will	O
execute	O
faster	O
.	O
</s>
<s>
The	O
process	O
can	O
detect	O
this	O
timing	O
difference	O
and	O
determine	O
the	O
address	O
,	O
Base+A	O
,	O
that	O
was	O
calculated	O
for	O
the	O
rejected	O
instruction	B-General_Concept
–	O
and	O
thus	O
determine	O
the	O
value	O
A	O
at	O
the	O
forbidden	O
memory	B-Architecture
address	O
.	O
</s>
<s>
Meltdown	B-Architecture
uses	O
this	O
technique	O
in	O
sequence	O
to	O
read	O
every	O
address	O
of	O
interest	O
at	O
high	O
speed	O
,	O
and	O
depending	O
on	O
other	O
running	O
processes	B-Operating_System
,	O
the	O
result	O
may	O
contain	O
passwords	O
,	O
encryption	O
data	O
,	O
and	O
any	O
other	O
sensitive	O
information	O
,	O
from	O
any	O
address	O
of	O
any	O
process	O
that	O
exists	O
in	O
its	O
memory	B-Architecture
map	B-Architecture
.	O
</s>
<s>
In	O
practice	O
,	O
because	O
cache	B-General_Concept
side-channel	O
attacks	O
are	O
slow	O
,	O
it	O
is	O
faster	O
to	O
extract	O
data	O
one	O
bit	O
at	O
a	O
time	O
(	O
only	O
attacks	O
needed	O
to	O
read	O
a	O
byte	O
,	O
rather	O
than	O
if	O
it	O
tried	O
to	O
read	O
all	O
8	O
bits	O
at	O
once	O
)	O
.	O
</s>
<s>
The	O
impact	O
of	O
Meltdown	B-Architecture
depends	O
on	O
the	O
design	O
of	O
the	O
CPU	B-Device
,	O
the	O
design	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
(	O
specifically	O
how	O
it	O
uses	O
memory	B-Architecture
paging	O
)	O
,	O
and	O
the	O
ability	O
of	O
a	O
malicious	O
party	O
to	O
get	O
any	O
code	O
run	O
on	O
that	O
system	O
,	O
as	O
well	O
as	O
the	O
value	O
of	O
any	O
data	O
it	O
could	O
read	O
if	O
able	O
to	O
execute	O
.	O
</s>
<s>
CPU	B-Device
–	O
Many	O
of	O
the	O
most	O
widely	O
used	O
modern	O
CPUs	B-Device
from	O
the	O
late	O
1990s	O
until	O
early	O
2018	O
have	O
the	O
required	O
exploitable	O
design	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
possible	O
to	O
mitigate	O
it	O
within	O
CPU	B-Device
design	O
.	O
</s>
<s>
A	O
CPU	B-Device
that	O
could	O
detect	O
and	O
avoid	O
memory	B-Architecture
access	O
for	O
unprivileged	O
instructions	O
,	O
or	O
was	O
not	O
susceptible	O
to	O
cache	B-General_Concept
timing	O
attacks	O
or	O
similar	O
probes	O
,	O
or	O
removed	O
cache	B-General_Concept
entries	O
upon	O
non-privilege	O
detection	O
(	O
and	O
did	O
not	O
allow	O
other	O
processes	B-Operating_System
to	O
access	O
them	O
until	O
authorized	O
)	O
as	O
part	O
of	O
abandoning	O
the	O
instruction	B-General_Concept
,	O
would	O
not	O
be	O
able	O
to	O
be	O
exploited	O
in	O
this	O
manner	O
.	O
</s>
<s>
Some	O
observers	O
consider	O
that	O
all	O
software	O
solutions	O
will	O
be	O
"	O
workarounds	O
"	O
and	O
the	O
only	O
true	O
solution	O
is	O
to	O
update	O
affected	O
CPU	B-Device
designs	O
and	O
remove	O
the	O
underlying	O
weakness	O
.	O
</s>
<s>
Operating	B-General_Concept
system	I-General_Concept
–	O
Most	O
of	O
the	O
widely	O
used	O
and	O
general-purpose	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
use	O
privilege	B-Operating_System
levels	I-Operating_System
and	O
virtual	B-Architecture
memory	I-Architecture
mapping	O
as	O
part	O
of	O
their	O
design	O
.	O
</s>
<s>
Meltdown	B-Architecture
can	O
access	O
only	O
those	O
pages	O
that	O
are	O
memory	B-Architecture
mapped	O
so	O
the	O
impact	O
will	O
be	O
greatest	O
if	O
all	O
active	O
memory	B-Architecture
and	O
processes	B-Operating_System
are	O
memory	B-Architecture
mapped	O
in	O
every	O
process	O
and	O
have	O
the	O
least	O
impact	O
if	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
designed	O
so	O
that	O
almost	O
nothing	O
can	O
be	O
reached	O
in	O
this	O
manner	O
.	O
</s>
<s>
An	O
operating	B-General_Concept
system	I-General_Concept
might	O
also	O
be	O
able	O
to	O
mitigate	O
in	O
software	O
to	O
an	O
extent	O
by	O
ensuring	O
that	O
probe	O
attempts	O
of	O
this	O
kind	O
will	O
not	O
reveal	O
anything	O
useful	O
.	O
</s>
<s>
Modern	O
operating	B-General_Concept
systems	I-General_Concept
use	O
memory	B-Architecture
mapping	O
to	O
increase	O
speed	O
so	O
this	O
could	O
lead	O
to	O
performance	O
loss	O
.	O
</s>
<s>
Virtual	B-Architecture
machine	I-Architecture
–	O
A	O
Meltdown	B-Architecture
attack	I-Architecture
cannot	O
be	O
used	O
to	O
break	O
out	O
of	O
a	O
virtual	B-Architecture
machine	I-Architecture
,	O
i.e.	O
,	O
in	O
fully	O
virtualized	O
machines	O
guest	O
user	B-Operating_System
space	I-Operating_System
can	O
still	O
read	O
from	O
guest	O
kernel	B-Operating_System
space	I-Operating_System
,	O
but	O
not	O
from	O
host	O
kernel	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
The	O
bug	O
enables	O
reading	O
memory	B-Architecture
from	O
address	O
space	O
represented	O
by	O
the	O
same	O
page	B-General_Concept
table	I-General_Concept
,	O
meaning	O
the	O
bug	O
does	O
not	O
work	O
between	O
virtual	O
tables	O
.	O
</s>
<s>
That	O
is	O
,	O
guest-to-host	O
page	B-General_Concept
tables	I-General_Concept
are	O
unaffected	O
,	O
only	O
guest-to-same-guest	O
or	O
host-to-host	O
,	O
and	O
of	O
course	O
host-to-guest	O
since	O
the	O
host	O
can	O
already	O
access	O
the	O
guest	O
pages	O
.	O
</s>
<s>
This	O
means	O
different	O
VMs	O
on	O
the	O
same	O
fully	O
virtualized	O
hypervisor	B-Operating_System
cannot	O
access	O
each	O
other	O
's	O
data	O
,	O
but	O
different	O
users	O
on	O
the	O
same	O
guest	O
instance	O
can	O
access	O
each	O
other	O
's	O
data	O
.	O
</s>
<s>
Embedded	B-Architecture
device	I-Architecture
–	O
Among	O
the	O
vulnerable	O
chips	O
are	O
those	O
designed	O
by	O
ARM	B-Architecture
and	O
Intel	O
designed	O
for	O
standalone	O
and	O
embedded	B-Architecture
devices	I-Architecture
,	O
such	O
as	O
mobile	O
phones	O
,	O
smart	O
TVs	O
,	O
networking	O
equipment	O
,	O
vehicles	O
,	O
hard	O
drives	O
,	O
industrial	O
control	O
,	O
and	O
the	O
like	O
.	O
</s>
<s>
For	O
example	O
,	O
an	O
ARM	B-Architecture
processor	I-Architecture
in	O
a	O
cellphone	O
or	O
Internet	B-Operating_System
of	I-Operating_System
Things	I-Operating_System
"	O
smart	O
"	O
device	O
may	O
be	O
vulnerable	O
,	O
but	O
the	O
same	O
processor	O
used	O
in	O
a	O
device	O
that	O
cannot	O
download	O
and	O
run	O
new	O
code	O
,	O
such	O
as	O
a	O
kitchen	O
appliance	O
or	O
hard	O
drive	O
controller	O
,	O
is	O
believed	O
to	O
not	O
be	O
exploitable	O
.	O
</s>
<s>
The	O
attack	O
can	O
reveal	O
the	O
content	O
of	O
any	O
memory	B-Architecture
that	O
is	O
mapped	O
into	O
a	O
user	O
address	O
space	O
,	O
even	O
if	O
otherwise	O
protected	O
.	O
</s>
<s>
For	O
example	O
,	O
before	O
kernel	B-Application
page-table	I-Application
isolation	I-Application
was	O
introduced	O
,	O
most	O
versions	O
of	O
Linux	B-Operating_System
mapped	O
all	O
physical	B-Architecture
memory	I-Architecture
into	O
the	O
address	O
space	O
of	O
every	O
user-space	B-Operating_System
process	O
;	O
the	O
mapped	O
addresses	O
are	O
(	O
mostly	O
)	O
protected	O
,	O
making	O
them	O
unreadable	O
from	O
user-space	B-Operating_System
and	O
accessible	O
only	O
when	O
transitioned	O
into	O
the	O
kernel	B-Operating_System
.	O
</s>
<s>
The	O
existence	O
of	O
these	O
mappings	O
makes	O
transitioning	O
to	O
and	O
from	O
the	O
kernel	B-Operating_System
faster	O
,	O
but	O
is	O
unsafe	O
in	O
the	O
presence	O
of	O
the	O
Meltdown	B-Architecture
vulnerability	I-Architecture
,	O
as	O
the	O
contents	O
of	O
all	O
physical	B-Architecture
memory	I-Architecture
(	O
which	O
may	O
contain	O
sensitive	O
information	O
such	O
as	O
passwords	O
belonging	O
to	O
other	O
processes	B-Operating_System
or	O
the	O
kernel	B-Operating_System
)	O
can	O
then	O
be	O
obtained	O
via	O
the	O
above	O
method	O
by	O
any	O
unprivileged	O
process	O
from	O
user-space	B-Operating_System
.	O
</s>
<s>
According	O
to	O
researchers	O
,	O
"	O
every	O
Intel	B-Device
processor	I-Device
that	O
implements	O
out-of-order	B-General_Concept
execution	I-General_Concept
is	O
potentially	O
affected	O
,	O
which	O
is	O
effectively	O
every	O
processor	O
since	O
1995	O
(	O
except	O
Intel	B-General_Concept
Itanium	I-General_Concept
and	O
Intel	B-Device
Atom	I-Device
before	O
2013	O
)	O
.	O
"	O
</s>
<s>
The	O
vulnerability	O
is	O
expected	O
to	O
impact	O
major	O
cloud	O
providers	O
,	O
such	O
as	O
Amazon	B-Application
Web	I-Application
Services	I-Application
(	O
AWS	B-Application
)	O
and	O
Google	B-General_Concept
Cloud	I-General_Concept
Platform	I-General_Concept
.	O
</s>
<s>
Cloud	O
providers	O
allow	O
users	O
to	O
execute	O
programs	O
on	O
the	O
same	O
physical	O
servers	O
where	O
sensitive	O
data	O
might	O
be	O
stored	O
,	O
and	O
rely	O
on	O
safeguards	O
provided	O
by	O
the	O
CPU	B-Device
to	O
prevent	O
unauthorized	O
access	O
to	O
the	O
privileged	O
memory	B-Architecture
locations	O
where	O
that	O
data	O
is	O
stored	O
,	O
a	O
feature	O
that	O
the	O
Meltdown	B-Architecture
exploit	I-Architecture
circumvents	O
.	O
</s>
<s>
The	O
original	O
paper	O
reports	O
that	O
paravirtualization	O
(	O
Xen	B-Operating_System
)	O
and	O
containers	B-Application
such	O
as	O
Docker	B-Language
,	O
LXC	B-Application
,	O
and	O
OpenVZ	B-Language
,	O
are	O
affected	O
.	O
</s>
<s>
They	O
report	O
that	O
the	O
attack	O
on	O
a	O
fully	O
virtualized	O
machine	O
allows	O
the	O
guest	O
user	B-Operating_System
space	I-Operating_System
to	O
read	O
from	O
the	O
guest	O
kernel	B-Operating_System
memory	B-Architecture
,	O
but	O
not	O
read	O
from	O
the	O
host	O
kernel	B-Operating_System
space	I-Operating_System
.	O
</s>
<s>
The	O
Meltdown	B-Architecture
vulnerability	I-Architecture
primarily	O
affects	O
Intel	B-Device
microprocessors	I-Device
,	O
but	O
the	O
ARM	B-Architecture
Cortex-A75	O
and	O
IBM	O
's	O
Power	O
microprocessors	B-Architecture
are	O
also	O
affected	O
.	O
</s>
<s>
The	O
vulnerability	O
does	O
not	O
affect	O
AMD	B-Device
microprocessors	I-Device
.	O
</s>
<s>
When	O
the	O
effect	O
of	O
Meltdown	B-Architecture
was	O
first	O
made	O
public	O
,	O
Intel	O
countered	O
that	O
the	O
flaws	O
affect	O
all	O
processors	O
,	O
but	O
AMD	O
denied	O
this	O
,	O
saying	O
"	O
we	O
believe	O
AMD	B-Device
processors	I-Device
are	O
not	O
susceptible	O
due	O
to	O
our	O
use	O
of	O
privilege	O
level	O
protections	O
within	O
paging	O
architecture	O
"	O
.	O
</s>
<s>
Researchers	O
have	O
indicated	O
that	O
the	O
Meltdown	B-Architecture
vulnerability	I-Architecture
is	O
exclusive	O
to	O
Intel	B-Device
processors	I-Device
,	O
while	O
the	O
Spectre	B-Error_Name
vulnerability	I-Error_Name
can	O
possibly	O
affect	O
some	O
Intel	O
,	O
AMD	O
,	O
and	O
ARM	B-Architecture
processors	I-Architecture
.	O
</s>
<s>
However	O
,	O
ARM	B-Architecture
announced	O
that	O
some	O
of	O
their	O
processors	O
were	O
vulnerable	O
to	O
Meltdown	B-Architecture
.	O
</s>
<s>
Google	B-Application
has	O
reported	O
that	O
any	O
Intel	B-Device
processor	I-Device
since	O
1995	O
with	O
out-of-order	B-General_Concept
execution	I-General_Concept
is	O
potentially	O
vulnerable	O
to	O
the	O
Meltdown	B-Architecture
vulnerability	I-Architecture
(	O
this	O
excludes	O
Itanium	B-General_Concept
and	O
pre-2013	O
Intel	B-Device
Atom	I-Device
CPUs	I-Device
)	O
.	O
</s>
<s>
Intel	O
introduced	O
speculative	B-General_Concept
execution	I-General_Concept
to	O
their	O
processors	O
with	O
Intel	O
's	O
P6	B-Device
family	O
microarchitecture	O
with	O
the	O
Pentium	B-Device
Pro	I-Device
IA-32	B-Device
microprocessor	B-Architecture
in	O
1995	O
.	O
</s>
<s>
ARM	B-Architecture
has	O
reported	O
that	O
the	O
majority	O
of	O
their	O
processors	O
are	O
not	O
vulnerable	O
,	O
and	O
published	O
a	O
list	O
of	O
the	O
specific	O
processors	O
that	O
are	O
affected	O
.	O
</s>
<s>
The	O
ARM	B-Architecture
Cortex-A75	O
core	O
is	O
affected	O
directly	O
by	O
both	O
Meltdown	B-Architecture
and	O
Spectre	B-Error_Name
vulnerabilities	I-Error_Name
,	O
and	O
Cortex-R7	O
,	O
Cortex-R8	O
,	O
Cortex-A8	B-Application
,	O
Cortex-A9	B-Application
,	O
Cortex-A15	O
,	O
Cortex-A17	B-Application
,	O
Cortex-A57	O
,	O
Cortex-A72	O
and	O
Cortex-A73	O
cores	O
are	O
affected	O
only	O
by	O
the	O
Spectre	B-Error_Name
vulnerability	I-Error_Name
.	O
</s>
<s>
This	O
contradicts	O
some	O
early	O
statements	O
made	O
about	O
the	O
Meltdown	B-Architecture
vulnerability	I-Architecture
as	O
being	O
Intel-only	O
.	O
</s>
<s>
A	O
large	O
portion	O
of	O
the	O
then-current	O
mid-range	O
Android	B-Application
handsets	O
use	O
the	O
Cortex-A53	O
or	O
Cortex-A55	O
in	O
an	O
octa-core	B-Architecture
arrangement	O
and	O
are	O
not	O
affected	O
by	O
either	O
the	O
Meltdown	B-Architecture
or	O
Spectre	B-Error_Name
vulnerability	I-Error_Name
as	O
they	O
do	O
not	O
perform	O
out-of-order	B-General_Concept
execution	I-General_Concept
.	O
</s>
<s>
This	O
includes	O
devices	O
with	O
the	O
Qualcomm	B-Architecture
Snapdragon	I-Architecture
630	O
,	O
Snapdragon	B-Architecture
626	O
,	O
Snapdragon	B-Architecture
625	O
,	O
and	O
all	O
Snapdragon	B-Architecture
4xx	O
processors	O
based	O
on	O
A53	O
or	O
A55	O
cores	O
.	O
</s>
<s>
Also	O
,	O
no	O
Raspberry	B-Operating_System
Pi	I-Operating_System
computers	O
are	O
vulnerable	O
to	O
either	O
Meltdown	B-Architecture
or	O
Spectre	B-Error_Name
,	O
except	O
the	O
newly-released	O
Raspberry	B-Operating_System
Pi	I-Operating_System
4	O
,	O
which	O
uses	O
the	O
ARM	B-Architecture
Cortex-A72	O
CPU	B-Device
.	O
</s>
<s>
IBM	O
has	O
also	O
confirmed	O
that	O
its	O
Power	O
CPUs	B-Device
are	O
affected	O
by	O
both	O
CPU	B-Device
attacks	O
.	O
</s>
<s>
Red	O
Hat	O
has	O
publicly	O
announced	O
that	O
the	O
exploits	O
are	O
also	O
for	O
IBM	B-Device
System	I-Device
Z	I-Device
,	O
POWER8	B-Device
,	O
and	O
POWER9	B-Device
systems	O
.	O
</s>
<s>
Oracle	O
has	O
stated	O
that	O
V9-based	O
SPARC	O
systems	O
(	O
T5	O
,	O
M5	O
,	O
M6	O
,	O
S7	O
,	O
M7	O
,	O
M8	O
,	O
M10	O
,	O
M12	O
processors	O
)	O
are	O
not	O
affected	O
by	O
Meltdown	B-Architecture
,	O
though	O
older	O
SPARC	O
processors	O
that	O
are	O
no	O
longer	O
supported	O
may	O
be	O
impacted	O
.	O
</s>
<s>
Mitigation	O
of	O
the	O
vulnerability	O
requires	O
changes	O
to	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
code	O
,	O
including	O
increased	O
isolation	O
of	O
kernel	B-Operating_System
memory	B-Architecture
from	O
user-mode	B-Operating_System
processes	B-Operating_System
.	O
</s>
<s>
Linux	B-Operating_System
kernel	I-Operating_System
developers	O
have	O
referred	O
to	O
this	O
measure	O
as	O
kernel	B-Application
page-table	I-Application
isolation	I-Application
(	O
KPTI	B-Application
)	O
.	O
</s>
<s>
KPTI	B-Application
patches	O
have	O
been	O
developed	O
for	O
Linux	B-Operating_System
kernel	I-Operating_System
4.15	O
,	O
and	O
have	O
been	O
released	O
as	O
a	O
backport	O
in	O
kernels	B-Operating_System
4.14.11	O
,	O
4.9.75	O
.	O
</s>
<s>
Red	O
Hat	O
released	O
kernel	B-Operating_System
updates	O
to	O
their	O
Red	O
Hat	O
Enterprise	O
Linux	B-Operating_System
distributions	O
version	O
6	O
and	O
version	O
7	O
.	O
</s>
<s>
CentOS	O
also	O
already	O
released	O
their	O
kernel	B-Operating_System
updates	O
to	O
CentOS6	O
and	O
CentOS7	O
.	O
</s>
<s>
Apple	O
included	O
mitigations	O
in	O
macOS	B-Application
10.13.2	O
,	O
iOS	B-Application
11.2	O
,	O
and	O
tvOS	O
11.2	O
.	O
</s>
<s>
Apple	O
has	O
stated	O
that	O
watchOS	B-Device
and	O
the	O
Apple	B-Device
Watch	I-Device
are	O
not	O
affected	O
.	O
</s>
<s>
Additional	O
mitigations	O
were	O
included	O
in	O
a	O
Safari	O
update	O
as	O
well	O
a	O
supplemental	O
update	O
to	O
macOS	B-Application
10.13	O
,	O
and	O
iOS	B-Application
11.2.2	O
.	O
</s>
<s>
Microsoft	O
released	O
an	O
emergency	O
update	O
to	O
Windows	B-Operating_System
10	I-Operating_System
,	O
8.1	B-Device
,	O
and	O
7	B-Device
SP1	I-Device
to	O
address	O
the	O
vulnerability	O
on	O
3	O
January	O
2018	O
,	O
as	O
well	O
as	O
Windows	B-Device
Server	I-Device
(	O
including	O
Server	B-Device
2008	I-Device
R2	I-Device
,	O
Server	O
2012	O
R2	O
,	O
and	O
Server	B-Device
2016	I-Device
)	O
and	O
Windows	B-Operating_System
Embedded	I-Operating_System
Industry	I-Operating_System
.	O
</s>
<s>
These	O
patches	O
are	O
incompatible	O
with	O
third-party	O
antivirus	O
software	O
that	O
use	O
unsupported	O
kernel	B-Operating_System
calls	O
;	O
systems	O
running	O
incompatible	O
antivirus	O
software	O
will	O
not	O
receive	O
this	O
or	O
any	O
future	O
Windows	O
security	O
updates	O
until	O
it	O
is	O
patched	O
,	O
and	O
the	O
software	O
adds	O
a	O
special	O
registry	B-Protocol
key	I-Protocol
affirming	O
its	O
compatibility	O
.	O
</s>
<s>
The	O
update	O
was	O
found	O
to	O
have	O
caused	O
issues	O
on	O
systems	O
running	O
certain	O
AMD	B-Device
CPUs	I-Device
,	O
with	O
some	O
users	O
reporting	O
that	O
their	O
Windows	O
installations	O
did	O
not	O
boot	O
at	O
all	O
after	O
installation	O
.	O
</s>
<s>
On	O
9	O
January	O
2018	O
,	O
Microsoft	O
paused	O
the	O
distribution	O
of	O
the	O
update	O
to	O
systems	O
with	O
affected	O
CPUs	B-Device
while	O
it	O
investigated	O
and	O
addressed	O
this	O
bug	O
.	O
</s>
<s>
It	O
was	O
reported	O
that	O
implementation	O
of	O
KPTI	B-Application
may	O
lead	O
to	O
a	O
reduction	O
in	O
CPU	B-Device
performance	O
,	O
with	O
some	O
researchers	O
claiming	O
up	O
to	O
30%	O
loss	O
in	O
performance	O
,	O
depending	O
on	O
usage	O
,	O
though	O
Intel	O
considered	O
this	O
to	O
be	O
an	O
exaggeration	O
.	O
</s>
<s>
It	O
was	O
reported	O
that	O
Intel	B-Device
processor	I-Device
generations	O
that	O
support	O
process-context	O
identifiers	O
(	O
PCID	O
)	O
,	O
a	O
feature	O
introduced	O
with	O
Westmere	B-Device
and	O
available	O
on	O
all	O
chips	O
from	O
the	O
Haswell	B-Device
architecture	O
onward	O
,	O
were	O
not	O
as	O
susceptible	O
to	O
performance	O
losses	O
under	O
KPTI	B-Application
as	O
older	O
generations	O
that	O
lack	O
it	O
.	O
</s>
<s>
This	O
is	O
because	O
the	O
selective	O
translation	B-Architecture
lookaside	I-Architecture
buffer	I-Architecture
(	O
TLB	O
)	O
flushing	O
enabled	O
by	O
PCID	O
(	O
also	O
called	O
address	O
space	O
number	O
or	O
ASN	O
under	O
the	O
Alpha	O
architecture	O
)	O
enables	O
the	O
shared	O
TLB	O
behavior	O
crucial	O
to	O
the	O
exploit	O
to	O
be	O
isolated	O
across	O
processes	B-Operating_System
,	O
without	O
constantly	O
flushing	O
the	O
entire	O
cache	B-General_Concept
–	O
the	O
primary	O
reason	O
for	O
the	O
cost	O
of	O
mitigation	O
.	O
</s>
<s>
Phoronix	O
benchmarked	O
several	O
popular	O
PC	O
games	O
on	O
a	O
Linux	B-Operating_System
system	O
with	O
Intel	O
's	O
Coffee	B-Device
Lake	I-Device
Core	O
i7-8700K	O
CPU	B-Device
and	O
KPTI	B-Application
patches	O
installed	O
,	O
and	O
found	O
that	O
any	O
performance	O
impact	O
was	O
small	O
to	O
non-existent	O
.	O
</s>
<s>
In	O
other	O
tests	O
,	O
including	O
synthetic	O
I/O	O
benchmarks	O
and	O
databases	O
such	O
as	O
PostgreSQL	B-Application
and	O
Redis	B-Operating_System
,	O
an	O
impact	O
in	O
performance	O
was	O
found	O
,	O
accounting	O
even	O
to	O
tens	O
of	O
percent	O
for	O
some	O
workloads	O
.	O
</s>
<s>
More	O
recently	O
,	O
related	O
tests	O
,	O
involving	O
AMD	O
's	O
FX	O
and	O
Intel	O
's	O
Sandybridge	B-Device
and	O
Ivybridge	B-Device
CPUs	B-Device
,	O
have	O
been	O
reported	O
.	O
</s>
<s>
Several	O
procedures	O
to	O
help	O
protect	O
home	O
computers	O
and	O
related	O
devices	O
from	O
the	O
Meltdown	B-Architecture
and	O
Spectre	B-Error_Name
security	O
vulnerabilities	O
have	O
been	O
published	O
.	O
</s>
<s>
Meltdown	B-Architecture
patches	O
may	O
produce	O
performance	O
loss	O
.	O
</s>
<s>
On	O
18	O
January	O
2018	O
,	O
unwanted	O
reboots	O
,	O
even	O
for	O
newer	O
Intel	O
chips	O
,	O
due	O
to	O
Meltdown	B-Architecture
and	O
Spectre	B-Error_Name
patches	O
,	O
were	O
reported	O
.	O
</s>
<s>
According	O
to	O
Dell	O
:	O
"	O
No	O
'	O
real-world	O
'	O
exploits	O
of	O
these	O
vulnerabilities	O
[	O
ie	O
,	O
Meltdown	B-Architecture
and	O
Spectre ]	O
have	O
been	O
reported	O
to	O
date	O
[	O
26	O
January	O
2018 ]	O
,	O
though	O
researchers	O
have	O
produced	O
proof-of-concepts.	O
"	O
</s>
<s>
On	O
25	O
January	O
2018	O
,	O
the	O
current	O
status	O
and	O
possible	O
future	O
considerations	O
in	O
solving	O
the	O
Meltdown	B-Architecture
and	O
Spectre	B-Error_Name
vulnerabilities	I-Error_Name
were	O
presented	O
.	O
</s>
<s>
In	O
March	O
2018	O
,	O
Intel	O
announced	O
that	O
it	O
had	O
designed	O
hardware	O
fixes	O
for	O
future	O
processors	O
for	O
Meltdown	B-Architecture
and	O
Spectre-V2	O
only	O
,	O
but	O
not	O
Spectre-V1	O
.	O
</s>
<s>
The	O
company	O
also	O
announced	O
it	O
had	O
developed	O
Intel	B-Device
Microcode	I-Device
workarounds	O
for	O
processors	O
dating	O
back	O
to	O
2013	O
,	O
and	O
that	O
it	O
had	O
plans	O
to	O
develop	O
them	O
for	O
most	O
processors	O
dating	O
back	O
to	O
2007	O
including	O
the	O
Core	B-Device
2	I-Device
Duo	I-Device
;	O
however	O
,	O
a	O
month	O
later	O
in	O
April	O
2018	O
,	O
it	O
announced	O
it	O
was	O
backing	O
off	O
that	O
plan	O
for	O
a	O
number	O
of	O
processor	O
families	O
and	O
that	O
no	O
processor	O
earlier	O
than	O
2008	O
would	O
have	O
a	O
patch	O
available	O
.	O
</s>
<s>
On	O
8	O
October	O
2018	O
,	O
Intel	O
was	O
reported	O
to	O
have	O
added	O
hardware	O
and	O
firmware	O
mitigations	O
regarding	O
Spectre	B-Error_Name
and	O
Meltdown	B-Architecture
vulnerabilities	I-Architecture
to	O
its	O
latest	O
processors	O
.	O
</s>
