[v2 1/7] x86/mpx: Do not use SIB index if index points to R/ESP

Ricardo Neri
2016-12-24T01:37:39Z

Section 2.2.1.2 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.

An example of such instruction could be

insn -0x80(%rsp)

This is represented as:

[opcode] 4c 24 80

ModR/M: mod: 1, reg: 1: r/m: 4 (R/ESP)
SIB 24: sc: 0, index: 100 (R/ESP), base(R/ESP): 100
Displacement -0x80

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

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

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Qiaowei Ren <qiaowei.ren@intel.com>
Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
Cc: x86@kernel.org
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
arch/x86/mm/mpx.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 324e571..6a75a75 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, SP is not used as index. Check is done after
+ * looking at REX.X This is because R12 can be used as an
+ * index.
+ */
+ if (regno == 4 && X86_MODRM_RM(insn->modrm.value) != 3)
+ return 0;
break;

case REG_TYPE_BASE:
@@ -161,7 +168,10 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
goto out_err;

base = regs_get_register(regs, base_offset);
- indx = regs_get_register(regs, indx_offset);
+ if (indx_offset)
+ indx = regs_get_register(regs, indx_offset);
+ else
+ indx = 0;
addr = base + indx * (1 << X86_SIB_SCALE(sib));
} else {
addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
--
2.9.3