[v3 PATCH 01/10] x86/mpx: Do not use SIB index if index points to R/ESP

From: Ricardo Neri
Date: Wed Jan 25 2017 - 15:25:26 EST

Section of the Intel 64 and IA-32 Architectures Software
Developer's Manual volume 2A states that when memory addressing is used
(i.e., mod part of ModR/M is not 3), a SIB byte is used and the index of
the SIB byte points to the R/ESP (i.e., index = 4), the index should not be
used in the computation of the memory address.

In these cases the address is simply the value present in the register
pointed by the base part of the SIB byte plus the displacement byte.

An example of such instruction could be

insn -0x80(%rsp)

This is represented as:

[opcode] 4c 23 80

ModR/M=0x4c: mod: 0x1, reg: 0x1: r/m: 0x4(R/ESP)
SIB=0x23: sc: 0, index: 0x100(R/ESP), base: 0x11(R/EBX):
Displacement -0x80

The correct address is (base) + displacement; no index is used.

We can achieve the desired effect of not using the index by making
get_reg_offset return a negative offset in this particular case. A negative
offset indicates callers that they should not use the index to calculate
the address. This is equivalent to using a index of zero when multiplying
it by the base.

Care is taken to allow R12 to be used as index, which is a valid scenario.

Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
arch/x86/mm/mpx.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index af59f80..9d15f6b 100644
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -109,6 +109,13 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
regno = X86_SIB_INDEX(insn->sib.value);
if (X86_REX_X(insn->rex_prefix.value))
regno += 8;
+ /*
+ * If mod !=3, register R/ESP (regno=4) is not used as index in
+ * the address computation. Check is done after looking at REX.X
+ * This is because R12 (regno=12) can be used as an index.
+ */
+ if (regno == 4 && X86_MODRM_MOD(insn->modrm.value) != 3)
+ return -EINVAL;

@@ -157,11 +164,16 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
goto out_err;

indx_offset = get_reg_offset(insn, regs, REG_TYPE_INDEX);
+ /*
+ * A negative offset means that the register cannot be
+ * be used as an index.
+ */
if (indx_offset < 0)
- goto out_err;
+ indx = 0;
+ else
+ indx = regs_get_register(regs, indx_offset);

base = regs_get_register(regs, base_offset);
- indx = regs_get_register(regs, indx_offset);
addr = base + indx * (1 << X86_SIB_SCALE(sib));
} else {
addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);