Re: [PATCH 3.19 v4 2/2] x86: Enforce maximum instruction size in the instruction decoder

[Andy Lutomirski <luto@amacapital.net>]

On Jan 15, 2015 4:37 AM, "Masami Hiramatsu"
<masami.hiramatsu.pt@hitachi.com> wrote:
>
> (2015/01/14 6:49), Andy Lutomirski wrote:
> > x86 instructions cannot exceed 15 bytes, and the instruction decoder
> > should enforce that.  Prior to 6ba48ff46f76, the instruction length
> > limit was implicitly set to 16, which was an approximation of 15,
> > but there is currently no limit at all.
> >
> > Fix the decoder to reject instructions that exceed 15 bytes.
> > A subsequent patch (targetted for 3.20) will fix MAX_INSN_SIZE.
>
> Hmm, is there any problem to just change MAX_INSN_SIZE to 15?

I don't want to do that for 3.19.  It's kind of late.

>
> > Other than potentially confusing some of the decoder sanity checks,
> > I'm not aware of any actual problems that omitting this check would
> > cause.
> >
> > Fixes: 6ba48ff46f76 x86: Remove arbitrary instruction size limit in instruction decoder
> > Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> > ---
> >  arch/x86/lib/insn.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
> > index 2480978b31cc..7b80745d2c5a 100644
> > --- a/arch/x86/lib/insn.c
> > +++ b/arch/x86/lib/insn.c
> > @@ -52,6 +52,13 @@
> >   */
> >  void insn_init(struct insn *insn, const void *kaddr, int buf_len, int x86_64)
> >  {
> > +     /*
> > +      * Instructions longer than 15 bytes are invalid even if the
> > +      * input buffer is long enough to hold them.
> > +      */
> > +     if (buf_len > 15)
> > +             buf_len = 15;
> > +
>
> Without changing the MAX_INSN_SIZE, this looks very odd, since all other
> code suppose that the max length of an instruction is 16 (MAX_INSN_SIZE)
> except here.

I thought this was your suggestion.  Did I misunderstand?

If you think the current code is okay for 3.19, I can fold the two
patches together and send for 3.20.

--Andy

>
> Thank you,
>
> >       memset(insn, 0, sizeof(*insn));
> >       insn->kaddr = kaddr;
> >       insn->end_kaddr = kaddr + buf_len;
> >
>
>
> --
> Masami HIRAMATSU
> Software Platform Research Dept. Linux Technology Research Center
> Hitachi, Ltd., Yokohama Research Laboratory
> E-mail: masami.hiramatsu.pt@hitachi.com
>
>