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

[Ricardo Neri]

On Tue, 2017-01-03 at 08:41 -0800, Dave Hansen wrote:
> On 12/27/2016 02:33 PM, Ricardo Neri wrote:
> >>> 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?
> 
> Yes.  We don't let userspace spam the kernel, even once.  If we have a
> couple thousand "only once" places, then userspace can overwhelm the
> kernel log.

This makes sense. I was not looking at it this way.
> 
> Also, this needs a much better description of what's going on in the
> code.  Could you add a comment explaining what's going on, and why
> regno==5, etc...?

I will add more comments.

Thanks!
Ricardo