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

[Sean Christopherson <seanjc@google.com>]

On Fri, Sep 19, 2025, Borislav Petkov wrote:
> On Fri, Aug 08, 2025 at 10:23:56AM -0700, Sean Christopherson wrote:
> > 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.
> 
> So I did some staring... I guess this fix is trying to address our insn
> decoder shortcoming and calls it "weirdness", right?
> 
> $ objdump -d a.out | awk -f ./arch/x86/tools/objdump_reformat.awk | ./arch/x86/tools/insn_decoder_test 
> ./arch/x86/tools/insn_decoder_test: warning: Found an x86 instruction decoder bug, please report this.
> ./arch/x86/tools/insn_decoder_test: warning:    0:      62 83 c5 05 0f 08 ff    vpalignr $0xff,(%r8),%xmm23,%xmm17{%k5}
> ./arch/x86/tools/insn_decoder_test: warning: objdump says 7 bytes, but insn_get_length() says 6
> ./arch/x86/tools/insn_decoder_test: warning: Decoded and checked 1 instructions with 1 failures
> 
> Looks like it.
> 
> a.out has:
> 
> 0000000000000000 <.text>:
>    0:   62 83 c5 05 0f 08 ff    vpalignr $0xff,(%r8),%xmm23,%xmm17{%k5}
> 
> I guess just adding the insn to the table doesn't fix it.

vpalignr is just one example of an unhandled instruction, that's not what I find
weird.

The "weirdness" I am referring to is purely speculative; what I was trying to say
is that I deliberate went with a "bad" check on nbytes, i.e. it really should be
"insn->opcode.nbytes == 2".  But I didn't want to risk breaking some bizarre
userspace that happened to be relying on a quirk of the kernel's decoder (I
haven't dug into the decoder, so I genuinely have/had no idea what all could
happen).

> Masami?
> 
> ---
> diff --git a/arch/x86/lib/x86-opcode-map.txt b/arch/x86/lib/x86-opcode-map.txt
> index 262f7ca1fb95..a23ff3c16908 100644
> --- a/arch/x86/lib/x86-opcode-map.txt
> +++ b/arch/x86/lib/x86-opcode-map.txt
> @@ -23,7 +23,7 @@
>  #
>  # AVX Superscripts
>  #  (ev): this opcode requires EVEX prefix.
> -#  (es): this opcode requires EVEX prefix and is SCALABALE.
> +#  (es): this opcode requires EVEX prefix and is SCALABLE.
>  #  (evo): this opcode is changed by EVEX prefix (EVEX opcode)
>  #  (v): this opcode requires VEX prefix.
>  #  (v1): this opcode only supports 128bit VEX.
> @@ -867,7 +867,7 @@ AVXcode: 3
>  0c: vblendps Vx,Hx,Wx,Ib (66)
>  0d: vblendpd Vx,Hx,Wx,Ib (66)
>  0e: vpblendw Vx,Hx,Wx,Ib (66),(v1)
> -0f: palignr Pq,Qq,Ib | vpalignr Vx,Hx,Wx,Ib (66),(v1)
> +0f: palignr Pq,Qq,Ib | vpalignr Vx,Hx,Wx,Ib (66),(v1) | vpalignr Vx,kz,Hx,Wx,Ib (ev)
>  14: vpextrb Rd/Mb,Vdq,Ib (66),(v1)
>  15: vpextrw Rd/Mw,Vdq,Ib (66),(v1)
>  16: vpextrd/q Ey,Vdq,Ib (66),(v1)
> 
> 
> > 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
> > 
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette