[PATCH v4 02/17] 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 
Cc: Adam Buchbinder 
Cc: Colin Ian King 
Cc: Lorenzo Stoakes 
Cc: Qiaowei Ren 
Cc: Ravi V. Shankar 
Cc: [e-mail]
Signed-off-by: Ricardo Neri 
---
 arch/x86/mm/mpx.c | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 6a034bc..f660ddf 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;
@@ -160,16 +171,22 @@ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs)
    	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)
-				goto out_err;
+				if (base_offset == -EDOM)
+					base = 0;
+				else
+					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 (indx_offset < 0)
    			if (indx_offset == -EDOM)
    				indx = 0;
@@ -178,7 +195,6 @@ 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);
    		addr = base + indx * (1 << X86_SIB_SCALE(sib));
    	} else {
    		addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);