[PATCH 4.1 001/127] x86/mpx: Fix instruction decoder condition

From: Greg Kroah-Hartman
Date: Wed Jan 27 2016 - 15:18:55 EST

4.1-stable review patch. If anyone has any objections, please let me know.


From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

commit 8e8efe0379bd93e8219ca0fc6fa80b5dd85b09cb upstream.

MPX decodes instructions in order to tell which bounds register
was violated. Part of this decoding involves looking at the "REX
prefix" which is a special instrucion prefix used to retrofit
support for new registers in to old instructions.

The X86_REX_*() macros are defined to return actual bit values:

#define X86_REX_R(rex) ((rex) & 4)

*not* boolean values. However, the MPX code was checking for
them like they were booleans. This might have led to us
mis-decoding the "REX prefix" and giving false information out to
userspace about bounds violations. X86_REX_B() actually is bit 1,
so this is really only broken for the X86_REX_X() case.

Fix the conditionals up to tolerate the non-boolean values.

Fixes: fcc7ffd67991 "x86, mpx: Decode MPX instruction to get bound violation information"
Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Dave Hansen <dave@xxxxxxxx>
Link: http://lkml.kernel.org/r/20151201003113.D800C1E0@xxxxxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

arch/x86/mm/mpx.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -120,19 +120,19 @@ static int get_reg_offset(struct insn *i
switch (type) {
regno = X86_MODRM_RM(insn->modrm.value);
- if (X86_REX_B(insn->rex_prefix.value) == 1)
+ if (X86_REX_B(insn->rex_prefix.value))
regno += 8;

regno = X86_SIB_INDEX(insn->sib.value);
- if (X86_REX_X(insn->rex_prefix.value) == 1)
+ if (X86_REX_X(insn->rex_prefix.value))
regno += 8;

regno = X86_SIB_BASE(insn->sib.value);
- if (X86_REX_B(insn->rex_prefix.value) == 1)
+ if (X86_REX_B(insn->rex_prefix.value))
regno += 8;