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

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Date:   Fri, 23 Dec 2016 17:58:36 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
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, 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.