[v6 PATCH 03/21] x86/mpx: Do not use R/EBP as base in the SIB byte with Mod = 0

[Ricardo Neri]

Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
Developer's Manual volume 2A states that when a SIB byte is used and the
base of the SIB byte points to R/EBP (i.e., base = 5) and the mod part
of the ModRM byte is zero, the value of such register will not be used
as part of the address computation. To signal this, a -EDOM error is
returned to indicate callers that they should ignore the value.

Also, for this particular case, a displacement of 32-bits should follow
the SIB byte if the mod part of ModRM is equal to zero. The instruction
decoder ensures that this is the 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: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Nathan Howard <liverlint@xxxxxxxxx>
Cc: Adan Hawthorn <adanhawthorn@xxxxxxxxx>
Cc: Joe Perches <joe@xxxxxxxxxxx>
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 | 29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index d9e92d6..ef7eb67 100644
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -121,6 +121,17 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
 
 	case REG_TYPE_BASE:
 		regno = X86_SIB_BASE(insn->sib.value);
+		/*
+		 * If mod is 0 and register R/EBP (regno=5) is indicated in the
+		 * base part of the SIB byte, the value of such register should
+		 * not be used in the address computation. Also, a 32-bit
+		 * displacement is expected in this case; the instruction
+		 * decoder takes care of it. This is true for both R13 and
+		 * R/EBP as REX.B will not be decoded.
+		 */
+		if (regno == 5 && X86_MODRM_MOD(insn->modrm.value) == 0)
+			return -EDOM;
+
 		if (X86_REX_B(insn->rex_prefix.value))
 			regno += 8;
 		break;
@@ -161,16 +172,21 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 		eff_addr = regs_get_register(regs, addr_offset);
 	} else {
 		if (insn->sib.nbytes) {
+			/*
+			 * Negative values in the base and index offset means
+			 * an error when decoding the SIB byte. Except -EDOM,
+			 * which means that the registers should not be used
+			 * in the address computation.
+			 */
 			base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE);
-			if (base_offset < 0)
+			if (unlikely(base_offset == -EDOM))
+				base = 0;
+			else if (unlikely(base_offset < 0))
 				goto out_err;
+			else
+				base = regs_get_register(regs, base_offset);
 
 			indx_offset = get_reg_offset(insn, regs, REG_TYPE_INDEX);
-			/*
-			 * A negative offset generally means a error, except
-			 * -EDOM, which means that the contents of the register
-			 * should not be used as index.
-			 */
 			if (unlikely(indx_offset == -EDOM))
 				indx = 0;
 			else if (unlikely(indx_offset < 0))
@@ -178,7 +194,6 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
 			else
 				indx = regs_get_register(regs, indx_offset);
 
-			base = regs_get_register(regs, base_offset);
 			eff_addr = base + indx * (1 << X86_SIB_SCALE(sib));
 		} else {
 			addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
-- 
2.9.3