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SubjectRe: [PATCH 3.19 2/3] x86, mpx: Short-circuit the instruction decoder for unexpected opcodes
On Mon, Dec 29, 2014 at 4:52 PM, Andy Lutomirski <luto@amacapital.net> 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@amacapital.net>
> ---
> 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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