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| author | Sean Christopherson <seanjc@google.com> | 2025-08-08 10:23:56 -0700 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2025-10-19 16:23:18 +0200 |
| commit | efc947f836e215c4e9a1f52d922819652d848ef7 (patch) | |
| tree | 4e8e370a7826dae6ef4059cbf2808dc7969e6b0a | |
| parent | 8fc01985b735b1c2d6e263564e27839497c87b33 (diff) | |
| download | linux-efc947f836e215c4e9a1f52d922819652d848ef7.tar.gz linux-efc947f836e215c4e9a1f52d922819652d848ef7.tar.bz2 linux-efc947f836e215c4e9a1f52d922819652d848ef7.zip | |
x86/umip: Check that the instruction opcode is at least two bytes
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>
| -rw-r--r-- | arch/x86/kernel/umip.c | 4 |
1 files 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) { |