Re: [PATCH 3.19 v2 2/3] x86, mpx: Short-circuit the instruction decoder for unexpected opcodes

[Andy Lutomirski]

On Wed, Jan 14, 2015 at 11:43 AM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
> On 01/12/2015 03:57 PM, Andy Lutomirski wrote:
>>>> >> -     /*
>>>> >> -      * 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;
>>> >
>>> > Otherwise, this looks OK to me.  Have you tested this at all?  I know
>>> > you don't have any MPX hardware, but you can still hack something in to
>>> > point the instruction decoder at an MPX binary.
>> I haven't tested this at all.  ISTM it's more likely that any test
>> hack I write for this will mask any problem than that it will be a
>> real test.
>
> This is completely and totally broken when there is an instruction
> prefix.  Instruction prefixes which occur before the opcodes in the
> buffer, so buf[0] is not necessarily insn->opcode.bytes[0].
>
> This was immediately obvious when I actually ran this code for the first
> time, even on hardware without MPX.

My apologies -- I misunderstood what insn->opcode.bytes did, and I
assumed that the original code intentionally did the same thing.

Please drop this patch.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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