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

Ricardo Neri
2017-01-04T01:31:10Z

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