[PATCH v9 17/29] x86/insn-eval: Indicate a 32-bit displacement if ModRM.mod is 0 and ModRM.rm is 101b

Ricardo Neri
2017-10-04T03:59:02Z

Section 2.2.1.3 of the Intel 64 and IA-32 Architectures Software
Developer's Manual volume 2A states that when ModRM.mod is zero and
ModRM.rm is 101b, a 32-bit displacement follows the ModRM byte. This means
that none of the registers are used in the computation of the effective
address. A return value of -EDOM indicates callers that they should not
use the value of registers when computing the effective address for the
instruction.

In long mode, the effective address is given by the 32-bit displacement
plus the location of the next instruction. In protected mode, only the
displacement is used.

The instruction decoder takes care of obtaining the displacement.

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: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Dmitry Vyukov <dvyukov@google.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/lib/insn-eval.c | 25 ++++++++++++++++++++++---
1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index cb2734a..dd84819 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -408,6 +408,14 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
switch (type) {
case REG_TYPE_RM:
regno = X86_MODRM_RM(insn->modrm.value);
+
+ /*
+ * ModRM.mod == 0 and ModRM.rm == 5 means a 32-bit displacement
+ * follows the ModRM byte.
+ */
+ if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5)
+ return -EDOM;
+
if (X86_REX_B(insn->rex_prefix.value))
regno += 8;
break;
@@ -754,10 +762,21 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
eff_addr = base + indx * (1 << X86_SIB_SCALE(sib));
} else {
addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
- if (addr_offset < 0)
+ /*
+ * -EDOM means that we must ignore the address_offset.
+ * In such a case, in 64-bit mode the effective address
+ * relative to the RIP of the following instruction.
+ */
+ if (addr_offset == -EDOM) {
+ if (user_64bit_mode(regs))
+ eff_addr = (long)regs->ip + insn->length;
+ else
+ eff_addr = 0;
+ } else if (addr_offset < 0) {
goto out;
-
- eff_addr = regs_get_register(regs, addr_offset);
+ } else {
+ eff_addr = regs_get_register(regs, addr_offset);
+ }
}

eff_addr += insn->displacement.value;
--
2.7.4