[PATCH 3.19 v4 1/2] x86, mpx: Short-circuit the instruction decoder for unexpected opcodes

[Andy Lutomirski <luto@amacapital.net>]

This reduces the degree to which we're exposing the instruction decoder
to malicious user code at very little complexity cost.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 arch/x86/mm/mpx.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
index 67ebf5751222..a73004330732 100644
--- a/arch/x86/mm/mpx.c
+++ b/arch/x86/mm/mpx.c
@@ -230,6 +230,22 @@ static int mpx_insn_decode(struct insn *insn,
 	 */
 	if (!nr_copied)
 		return -EFAULT;
+
+	/*
+	 * We only _really_ need to decode bndcl/bndcn/bndcu
+	 * Error out on anything else.  Check this before decoding the
+	 * instruction to reduce our exposure to intentionally bad code
+	 * to some extent.  Note that this shortcut can incorrectly return
+	 * -EINVAL instead of -EFAULT under some circumstances.  This
+	 * discrepancy has no effect.
+	 */
+	if (nr_copied < 2)
+		goto bad_opcode;
+	if (buf[0] != 0x0f)
+		goto bad_opcode;
+	if (buf[1] != 0x1a && buf[1] != 0x1b)
+		goto bad_opcode;
+
 	insn_init(insn, buf, nr_copied, x86_64);
 	insn_get_length(insn);
 	/*
@@ -244,15 +260,6 @@ static int mpx_insn_decode(struct insn *insn,
 		return -EFAULT;
 
 	insn_get_opcode(insn);
-	/*
-	 * We only _really_ need to decode bndcl/bndcn/bndcu
-	 * Error out on anything else.
-	 */
-	if (insn->opcode.bytes[0] != 0x0f)
-		goto bad_opcode;
-	if ((insn->opcode.bytes[1] != 0x1a) &&
-	    (insn->opcode.bytes[1] != 0x1b))
-		goto bad_opcode;
 
 	return 0;
 bad_opcode:
-- 
2.1.0