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

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 <[email protected]>
Cc: Adam Buchbinder <[email protected]>
Cc: Colin Ian King <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Qiaowei Ren <[email protected]>
Cc: Ravi V. Shankar <[email protected]>
Cc: [email protected]
Signed-off-by: Ricardo Neri <[email protected]>
---
 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