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

[Dave Hansen <dave.hansen@linux.intel.com>]

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.