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: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
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);
--
2.9.3
--
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