linux-kernel - Re: [PATCH 3.19 v4 1/2] x86, mpx: Short-circuit the instruction decoder for unexpected opcodes

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Date:	Tue, 20 Jan 2015 14:28:07 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>
Subject: Re: [PATCH 3.19 v4 1/2] x86, mpx: Short-circuit the instruction
 decoder for unexpected opcodes

On Jan 20, 2015 5:59 AM, "Thomas Gleixner" <tglx@...utronix.de> wrote:
>
> On Tue, 13 Jan 2015, Andy Lutomirski wrote:
>
> > This reduces the degree to which we're exposing the instruction decoder
> > to malicious user code at very little complexity cost.
> >
> > Signed-off-by: Andy Lutomirski <luto@...capital.net>
> > ---
> >  arch/x86/mm/mpx.c | 25 ++++++++++++++++---------
> >  1 file changed, 16 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
> > index 67ebf5751222..a73004330732 100644
> > --- a/arch/x86/mm/mpx.c
> > +++ b/arch/x86/mm/mpx.c
> > @@ -230,6 +230,22 @@ 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 can incorrectly return
> > +      * -EINVAL instead of -EFAULT under some circumstances.  This
> > +      * discrepancy has no effect.
> > +      */
> > +     if (nr_copied < 2)
> > +             goto bad_opcode;
> > +     if (buf[0] != 0x0f)
> > +             goto bad_opcode;
> > +     if (buf[1] != 0x1a && buf[1] != 0x1b)
> > +             goto bad_opcode;
>
> These opcodes can never have a prefix? If so, then we want to add this
> to the comment.

No, I think I just misunderstood the original code.  Please disregard
this patch.

--Andy

>
> Thanks,
>
>         tglx
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