[PATCH] x86, mpx: fix instruction decoder condition

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

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.

Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: x86@xxxxxxxxxx
---

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

diff -puN arch/x86/mm/mpx.c~mpx-rex arch/x86/mm/mpx.c
--- a/arch/x86/mm/mpx.c~mpx-rex	2015-11-30 15:33:32.393469377 -0800
+++ b/arch/x86/mm/mpx.c	2015-11-30 16:12:14.805379908 -0800
@@ -101,19 +101,19 @@ static int get_reg_offset(struct insn *i
 	switch (type) {
 	case REG_TYPE_RM:
 		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;
 		break;
 
 	case REG_TYPE_INDEX:
 		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;
 		break;
 
 	case REG_TYPE_BASE:
 		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;
 		break;
 
_
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