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Subject: Re: [v2 2/7] x86/mpx: Fail when implicit zero-displacement is used
 along with R/EBP
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        Andy Lutomirski <luto@amacapital.net>
References: <20161224013745.108716-1-ricardo.neri-calderon@linux.intel.com>
 <20161224013745.108716-3-ricardo.neri-calderon@linux.intel.com>
 <CALCETrVbeB1o76rYSzyCTuKyM_gqxONV79m7G3kDhXQfEJun_w@mail.gmail.com>
 <1482878000.106950.10.camel@ranerica-desktop>
Cc: Ingo Molnar <mingo@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@suse.de>, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, linux-msdos@vger.kernel.org,
        wine-devel@winehq.org, Adam Buchbinder <adam.buchbinder@gmail.com>,
        Colin Ian King <colin.king@canonical.com>,
        Lorenzo Stoakes <lstoakes@gmail.com>,
        Qiaowei Ren <qiaowei.ren@intel.com>,
        "Ravi V . Shankar" <ravi.v.shankar@intel.com>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <2ef4ec41-b8f7-d775-aaff-02656b79a5dd@linux.intel.com>
Date: Tue, 3 Jan 2017 08:41:50 -0800
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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.

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...?