[PATCH v7 05/26] x86/mpx: Do not use SIB.base if its value is 101b and ModRM.mod = 0

Ricardo Neri
2017-05-05T18:17:03Z

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 <bp@suse.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Qiaowei Ren <qiaowei.ren@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Nathan Howard <liverlint@gmail.com>
Cc: Adan Hawthorn <adanhawthorn@gmail.com>
Cc: Joe Perches <joe@perches.com>
Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
Cc: x86@kernel.org
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
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);
--
2.9.3