5.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Sean Christopherson seanjc@google.com
commit 32278c677947ae2f042c9535674a7fff9a245dd3 upstream.
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.
Analyzed by Nick Bray ncbray@google.com.
Fixes: 1e5db223696a ("x86/umip: Add emulation code for UMIP instructions") Reported-by: Dan Snyder dansnyder@google.com Signed-off-by: Sean Christopherson seanjc@google.com Signed-off-by: Borislav Petkov (AMD) bp@alien8.de Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: stable@vger.kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/kernel/umip.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/arch/x86/kernel/umip.c +++ b/arch/x86/kernel/umip.c @@ -155,8 +155,8 @@ static int identify_insn(struct insn *in 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) {