Re: [v2 2/7] x86/mpx: Fail when implicit zero-displacement is used along with R/EBP

On Fri, 2016-12-23 at 17:58 -0800, Andy Lutomirski wrote:
> On Fri, Dec 23, 2016 at 5:37 PM, Ricardo Neri
> <[email protected]> wrote:
> > Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
> > Developer's Manual volume 2A states that when memory addressing with no
> > explicit displacement (i.e, mod part of ModR/M is 0), a SIB byte is used
> > and the base of the SIB byte points to (R/EBP) (i.e., base = 5), an
> > explicit displacement of 0 must be used.
> >
> > Make the address decoder to return -EINVAL in such a case.
> >
> > Cc: Dave Hansen <[email protected]>
> > Cc: Adam Buchbinder <[email protected]>
> > Cc: Colin Ian King <[email protected]>
> > Cc: Lorenzo Stoakes <[email protected]>
> > Cc: Qiaowei Ren <[email protected]>
> > Cc: Ravi V. Shankar <[email protected]>
> > Cc: [email protected]
> > Signed-off-by: Ricardo Neri <[email protected]>
> > ---
> >  arch/x86/mm/mpx.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/arch/x86/mm/mpx.c b/arch/x86/mm/mpx.c
> > index 6a75a75..71681d0 100644
> > --- a/arch/x86/mm/mpx.c
> > +++ b/arch/x86/mm/mpx.c
> > @@ -120,6 +120,13 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
> >
> >         case REG_TYPE_BASE:
> >                 regno = X86_SIB_BASE(insn->sib.value);
> > +               if (regno == 5 && X86_MODRM_RM(insn->modrm.value) == 0) {
> > +                       WARN_ONCE(1, "An explicit displacement is required when %sBP used as SIB base.",
> > +                                 (IS_ENABLED(CONFIG_X86_64) && insn->x86_64) ?
> > +                                 "R13 or R" : "E");
> > +                       return -EINVAL;
> > +               }
> > +
> 
> Now that I've read the cover letter, I see what's going on.  This
> should not warn -- user code can easily trigger this deliberately.

OK, I'll remove it. Are you concerned about the warning printing the
calltrace, even only once?