[PATCH 1/3] x86/umip: Check that the instruction opcode is at least two bytes

[Sean Christopherson <seanjc@google.com>]

When checking for a potential UMIP violation on #GP, verify the decoder
found at least two opcode bytes to avoid false positives when the kernel
encounters an unknown instruction that starts with 0f.  Because the array
of opcode.bytes is zero-initialized by insn_init(), peeking at bytes[1]
will misinterpret garbage as a potential SLDT or STR instruction, and can
incorrectly trigger emulation.

E.g. if a vpalignr instruction

   62 83 c5 05 0f 08 ff     vpalignr xmm17{k5},xmm23,XMMWORD PTR [r8],0xff

hits a #GP, the kernel emulates it as STR and squashes the #GP (and
corrupts the userspace code stream).

Arguably the check should look for exactly two bytes, but no three byte
opcodes use '0f 00 xx' or '0f 01 xx' as an escape, i.e. it should be
impossible to get a false positive if the first two opcode bytes match
'0f 00' or '0f 01'.  Go with a more conservative check with respect to the
existing code to minimize the chances of breaking userspace, e.g. due to
decoder weirdness.

Fixes: 1e5db223696a ("x86/umip: Add emulation code for UMIP instructions")
Reported-by: Dan Snyder <dansnyder@google.com>
Analyzed-by; Nick Bray <ncbray@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kernel/umip.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
index 5a4b21389b1d..406ac01ce16d 100644
--- a/arch/x86/kernel/umip.c
+++ b/arch/x86/kernel/umip.c
@@ -156,8 +156,8 @@ static int identify_insn(struct insn *insn)
 	if (!insn->modrm.nbytes)
 		return -EINVAL;
 
-	/* All the instructions of interest start with 0x0f. */
-	if (insn->opcode.bytes[0] != 0xf)
+	/* The instructions of interest have 2-byte opcodes: 0F 00 or 0F 01. */
+	if (insn->opcode.nbytes < 2 || insn->opcode.bytes[0] != 0xf)
 		return -EINVAL;
 
 	if (insn->opcode.bytes[1] == 0x1) {
-- 
2.50.1.703.g449372360f-goog