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

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Message-ID: <1482878000.106950.10.camel@ranerica-desktop>
Date:   Tue, 27 Dec 2016 14:33:20 -0800
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>, linux-msdos@...r.kernel.org,
        wine-devel@...ehq.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Adam Buchbinder <adam.buchbinder@...il.com>,
        Colin Ian King <colin.king@...onical.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Qiaowei Ren <qiaowei.ren@...el.com>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>
Subject: 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
> <ricardo.neri-calderon@...ux.intel.com> 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 <dave.hansen@...ux.intel.com>
> > Cc: Adam Buchbinder <adam.buchbinder@...il.com>
> > Cc: Colin Ian King <colin.king@...onical.com>
> > Cc: Lorenzo Stoakes <lstoakes@...il.com>
> > Cc: Qiaowei Ren <qiaowei.ren@...el.com>
> > Cc: Ravi V. Shankar <ravi.v.shankar@...el.com>
> > Cc: x86@...nel.org
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> > ---
> >  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?