x86/mpx: Fix instruction decoder condition

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@oracle.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Cc: Dave Hansen <dave@sr71.net>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20151201003113.D800C1E0@viggo.jf.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 1202d5c..b2fd67d 100644 (file)
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -101,19 +101,19 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
        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;