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

[Andy Lutomirski]

On Mon, Dec 29, 2014 at 4:52 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> This reduces the degree to which we're exposing the instruction decoder
> to malicious user code and very little complexity cost.

Don't apply this as is -- it's obviously incorrect.  I'll send a v2 in
a couple of days.

However: can an MPX instruction use a segment override?  FS and GS
seem plausible for TLS.  If so, is the current decoding logic handling
it correctly?  Presumably the byte 0 == 0x0f check will completely
prevent it from working, even if the decode logic would later forget
to correct for the segment base.

--Andy

>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
>  arch/x86/mm/mpx.c | 26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
>
> diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
> index 082ab9c4ac1c..cefa615becb3 100644
> --- a/arch/x86/mm/mpx.c
> +++ b/arch/x86/mm/mpx.c
> @@ -230,6 +230,23 @@ static int mpx_insn_decode(struct insn *insn,
>          */
>         if (!nr_copied)
>                 return -EFAULT;
> +
> +       /*
> +        * We only _really_ need to decode bndcl/bndcn/bndcu
> +        * Error out on anything else.  Check this before decoding the
> +        * instruction to reduce our exposure to intentionally bad code
> +        * to some extent.  Note that this shortcut cat incorrectly return
> +        * -EINVAL instead of -EFAULT under some circumstances.  This
> +        * discrepency has no effect.
> +        */
> +       if (nr_copied < 2)
> +               goto bad_opcode;
> +       if (insn->opcode.bytes[0] != 0x0f)
> +               goto bad_opcode;
> +       if ((insn->opcode.bytes[1] != 0x1a) &&
> +           (insn->opcode.bytes[1] != 0x1b))
> +               goto bad_opcode;
> +
>         insn_init(insn, buf, nr_copied, x86_64);
>         insn_get_length(insn);
>         /*
> @@ -244,15 +261,6 @@ static int mpx_insn_decode(struct insn *insn,
>                 return -EFAULT;
>
>         insn_get_opcode(insn);
> -       /*
> -        * 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;
>
>         return 0;
>  bad_opcode:
> --
> 2.1.0
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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