[v6 PATCH 10/21] x86/insn-eval: Do not use R/EBP as base if mod in ModRM is zero

[Ricardo Neri]

Section 2.2.1.3 of the Intel 64 and IA-32 Architectures Software
Developer's Manual volume 2A states that when the mod part of the ModRM
byte is zero and R/EBP is specified in the R/M part of such bit, the value
of the aforementioned register should not be used in the address
computation. Instead, a 32-bit displacement is expected. The instruction
decoder takes care of setting the displacement to the expected value.
Returning -EDOM signals callers that they should ignore the value of such
register when computing the address encoded in the instruction operands.

Also, callers should exercise care to correctly interpret this particular
case. In IA-32e 64-bit mode, the address is given by the displacement plus
the value of the RIP. In IA-32e compatibility mode, the value of EIP is
ignored. This correction is done for our insn_get_addr_ref.

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: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
---
 arch/x86/lib/insn-eval.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index cda6c71..ea10b03 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -250,6 +250,14 @@ 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);
+		/* if mod=0, register R/EBP is not used in the address
+		 * computation. Instead, a 32-bit displacement is expected;
+		 * the instruction decoder takes care of reading such
+		 * displacement. This is true for both R/EBP and R13, as the
+		 * REX.B bit is not decoded.
+		 */
+		if (regno == 5 && X86_MODRM_MOD(insn->modrm.value) == 0)
+			return -EDOM;
 		if (X86_REX_B(insn->rex_prefix.value))
 			regno += 8;
 		break;
@@ -599,9 +607,22 @@ 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.
+			 * The only case in which we see this value is when
+			 * R/M points to R/EBP. In such a case, in 64-bit mode
+			 * the effective address is relative to tho RIP.
+			 */
+			if (addr_offset == -EDOM) {
+				eff_addr = 0;
+#ifdef CONFIG_X86_64
+				if (user_64bit_mode(regs))
+					eff_addr = (long)regs->ip;
+#endif
+			} else if (addr_offset < 0) {
 				goto out_err;
-			eff_addr = regs_get_register(regs, addr_offset);
+			} else {
+				eff_addr = regs_get_register(regs, addr_offset);
+			}
 		}
 		eff_addr += insn->displacement.value;
 	}
-- 
2.9.3