[PATCH v7 05/26] x86/mpx: Do not use SIB.base if its value is 101b and ModRM.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 is base = 101b and the mod part
of the ModRM byte is zero, the base port on the effective address
computation is null. In this case, a 32-bit displacement follows the SIB
byte. This is obtained when the instruction decoder parses the operands.

To signal this scenario, a -EDOM error is returned to indicate callers that
they should ignore the base.

Cc: Borislav Petkov 
Cc: Andy Lutomirski 
Cc: Dave Hansen 
Cc: Adam Buchbinder 
Cc: Colin Ian King 
Cc: Lorenzo Stoakes 
Cc: Qiaowei Ren 
Cc: Peter Zijlstra 
Cc: Nathan Howard 
Cc: Adan Hawthorn 
Cc: Joe Perches 
Cc: Ravi V. Shankar 
Cc: [e-mail]
Signed-off-by: Ricardo Neri 
---
 arch/x86/mm/mpx.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 7397b81..30aef92 100644
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -122,6 +122,15 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,

case REG_TYPE_BASE:
    	regno = X86_SIB_BASE(insn->sib.value);
+		/*
+		 * If ModRM.mod is 0 and SIB.base == 5, the base of the
+		 * register-indirect addressing is 0. In this case, a
+		 * 32-bit displacement is expected in this case; the
+		 * instruction decoder finds such displacement for us.
+		 */
+		if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5)
+			return -EDOM;
+
    	if (X86_REX_B(insn->rex_prefix.value))
    		regno += 8;
    	break;
@@ -162,16 +171,21 @@ 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 (base_offset == -EDOM)
+				base = 0;
+			else if (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 (indx_offset == -EDOM)
    			indx = 0;
    		else if (indx_offset < 0)
@@ -179,7 +193,6 @@ 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);