<s>
Intel	B-Device
MPX	I-Device
(	O
Memory	B-Device
Protection	I-Device
Extensions	I-Device
)	O
was	O
a	O
set	O
of	O
extensions	O
to	O
the	O
x86	B-Operating_System
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
.	O
</s>
<s>
With	O
compiler	B-Language
,	O
runtime	B-Library
library	I-Library
and	O
operating	B-General_Concept
system	I-General_Concept
support	O
,	O
Intel	B-Device
MPX	I-Device
claimed	O
to	O
enhance	O
security	O
to	O
software	O
by	O
checking	O
pointer	O
references	O
whose	O
normal	O
compile-time	O
intentions	O
are	O
maliciously	O
exploited	O
at	O
runtime	O
due	O
to	O
buffer	B-General_Concept
overflows	I-General_Concept
.	O
</s>
<s>
In	O
practice	O
,	O
there	O
have	O
been	O
too	O
many	O
flaws	O
discovered	O
in	O
the	O
design	O
for	O
it	O
to	O
be	O
useful	O
,	O
and	O
support	O
has	O
been	O
deprecated	O
or	O
removed	O
from	O
most	O
compilers	B-Language
and	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Intel	B-Device
MPX	I-Device
introduces	O
new	O
bounds	O
registers	B-General_Concept
,	O
and	O
new	O
instruction	B-General_Concept
set	I-General_Concept
extensions	O
that	O
operate	O
on	O
these	O
registers	B-General_Concept
.	O
</s>
<s>
Additionally	O
,	O
there	O
is	O
a	O
new	O
set	O
of	O
"	O
bound	O
tables	O
"	O
that	O
store	O
bounds	O
beyond	O
what	O
can	O
fit	O
in	O
the	O
bounds	O
registers	B-General_Concept
.	O
</s>
<s>
MPX	O
uses	O
four	O
new	O
128-bit	O
bounds	O
registers	B-General_Concept
,	O
BND0	O
to	O
BND3	O
,	O
each	O
storing	O
a	O
pair	O
of	O
64-bit	O
lower	O
bound	O
(	O
LB	O
)	O
and	O
upper	O
bound	O
(	O
UB	O
)	O
values	O
of	O
a	O
buffer	O
.	O
</s>
<s>
The	O
architecture	O
includes	O
two	O
configuration	O
registers	B-General_Concept
BNDCFGx	O
(	O
BNDCFGU	O
in	O
user	O
space	O
and	O
BNDCFGS	O
in	O
kernel	B-Operating_System
mode	O
)	O
,	O
and	O
a	O
status	O
register	O
BNDSTATUS	O
,	O
which	O
provides	O
a	O
memory	O
address	O
and	O
error	O
code	O
in	O
case	O
of	O
an	O
exception	O
.	O
</s>
<s>
Intel	B-Device
MPX	I-Device
was	O
introduced	O
as	O
part	O
of	O
the	O
Skylake	B-Architecture
microarchitecture	I-Architecture
.	O
</s>
<s>
Intel	O
Goldmont	B-Device
microarchitecture	O
also	O
supports	O
Intel	B-Device
MPX	I-Device
.	O
</s>
<s>
glibc	B-Language
removed	O
support	O
in	O
version	O
2.35	O
.	O
</s>
<s>
GNU	B-Application
Compiler	I-Application
Collection	I-Application
(	O
GCC	B-Application
)	O
5.0	O
added	O
support	O
for	O
MPX	O
.	O
</s>
<s>
In	O
2018	O
,	O
support	O
for	O
these	O
extensions	O
waned	O
due	O
to	O
maintenance	O
burdens	O
and	O
Intel	O
developers	O
intermittently	O
contributing	O
patches	O
,	O
resulting	O
in	O
a	O
proposal	O
to	O
drop	O
support	O
in	O
GCC	B-Application
9.0	O
.	O
</s>
<s>
Support	O
was	O
removed	O
in	O
GCC	B-Application
9.1	O
.	O
</s>
<s>
Intel	B-Language
C++	I-Language
Compiler	I-Language
(	O
icc	O
)	O
15.0	O
added	O
support	O
for	O
Intel	B-Device
MPX	I-Device
.	O
</s>
<s>
Kernel-level	O
software	O
support	O
for	O
Intel	B-Device
MPX	I-Device
was	O
merged	O
into	O
the	O
Linux	O
kernel	B-Operating_System
mainline	O
in	O
kernel	B-Operating_System
version	O
3.19	O
,	O
which	O
was	O
released	O
on	O
February	O
8	O
,	O
2015	O
.	O
</s>
<s>
In	O
2018	O
,	O
Thomas	O
Gleixner	O
proposed	O
removing	O
MPX	O
support	O
from	O
Linux	O
kernel	B-Operating_System
4.18	O
.	O
</s>
<s>
A	O
study	O
examined	O
a	O
detailed	O
cross-layer	O
dissection	O
of	O
the	O
MPX	O
system	O
stack	O
and	O
comparison	O
with	O
three	O
prominent	O
software-based	O
memory	O
protection	O
mechanisms	O
(	O
AddressSanitizer	B-Application
,	O
SAFECode	O
,	O
and	O
SoftBound	O
)	O
and	O
presents	O
the	O
following	O
conclusions	O
.	O
</s>
<s>
Even	O
though	O
Intel	B-Device
MPX	I-Device
is	O
a	O
specially	O
designed	O
hardware-assisted	O
approach	O
with	O
its	O
own	O
added	O
set	O
of	O
hardware	O
registers	B-General_Concept
,	O
it	O
is	O
not	O
faster	O
than	O
any	O
of	O
the	O
software-based	O
approaches	O
.	O
</s>
<s>
New	O
Intel	B-Device
MPX	I-Device
instructions	O
can	O
cause	O
up	O
to	O
4×	O
slowdown	O
in	O
the	O
worst	O
case	O
,	O
although	O
compiler	B-Language
optimizations	O
amortize	O
it	O
and	O
lead	O
to	O
runtime	O
overheads	O
of	O
~	O
50%	O
on	O
average	O
.	O
</s>
<s>
In	O
contrast	O
to	O
the	O
other	O
software-based	O
solutions	O
,	O
Intel	B-Device
MPX	I-Device
provides	O
no	O
protection	O
against	O
temporal	O
memory	O
safety	O
errors	O
.	O
</s>
<s>
Intel	B-Device
MPX	I-Device
does	O
not	O
support	O
multithreading	O
inherently	O
,	O
which	O
can	O
lead	O
to	O
unsafe	O
data	O
races	O
in	O
legacy	O
threaded	O
programs	O
and	O
if	O
compilers	B-Language
do	O
not	O
synchronize	O
bounds	O
explicitly	O
.	O
</s>
<s>
Intel	B-Device
MPX	I-Device
does	O
not	O
support	O
several	O
common	O
C/C	O
++	O
programming	O
idioms	O
due	O
to	O
restrictions	O
on	O
the	O
allowed	O
memory	O
layout	O
.	O
</s>
<s>
Intel	B-Device
MPX	I-Device
is	O
conflicting	O
with	O
some	O
other	O
ISA	O
extensions	O
resulting	O
in	O
performance	O
and	O
security	O
issues	O
.	O
</s>
<s>
More	O
specifically	O
,	O
these	O
issues	O
arise	O
when	O
Intel	B-Device
MPX	I-Device
is	O
used	O
in	O
combination	O
with	O
other	O
hardware	O
based	O
protection	O
mechanisms	O
such	O
as	O
Intel	B-Operating_System
TSX	I-Operating_System
and	O
Intel	O
SGX	O
.	O
</s>
<s>
In	O
addition	O
,	O
a	O
review	O
concluded	O
MPX	O
was	O
not	O
production	O
ready	O
,	O
and	O
AddressSanitizer	B-Application
was	O
a	O
better	O
option	O
.	O
</s>
<s>
A	O
review	O
by	O
Kostya	O
Serebryany	O
at	O
Google	O
,	O
AddressSanitizer	B-Application
's	O
developer	O
,	O
had	O
similar	O
findings	O
.	O
</s>
<s>
Another	O
study	O
exploring	O
the	O
scope	O
of	O
Spectre	B-Error_Name
and	O
Meltdown	B-Architecture
security	O
vulnerabilities	O
discovered	O
that	O
Meltdown	B-Architecture
can	O
be	O
used	O
to	O
bypass	O
Intel	B-Device
MPX	I-Device
,	O
using	O
the	O
Bound	O
Range	O
Exceeded	O
(	O
#BR	O
)	O
hardware	O
exception	O
.	O
</s>
