[v2 2/7] x86/mpx: Fail when implicit zero-displacement is used along with R/EBP

[Ricardo Neri]

Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
Developer's Manual volume 2A states that when memory addressing with no
explicit displacement (i.e, mod part of ModR/M is 0), a SIB byte is used
and the base of the SIB byte points to (R/EBP) (i.e., base = 5), an
explicit displacement of 0 must be used.

Make the address decoder to return -EINVAL in such a case.

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 | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 6a75a75..71681d0 100644
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -120,6 +120,13 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
 
 	case REG_TYPE_BASE:
 		regno = X86_SIB_BASE(insn->sib.value);
+		if (regno == 5 && X86_MODRM_RM(insn->modrm.value) == 0) {
+			WARN_ONCE(1, "An explicit displacement is required when %sBP used as SIB base.",
+				  (IS_ENABLED(CONFIG_X86_64) && insn->x86_64) ?
+				  "R13 or R" : "E");
+			return -EINVAL;
+		}
+
 		if (X86_REX_B(insn->rex_prefix.value))
 			regno += 8;
 		break;
-- 
2.9.3