linux-kernel - Re: [PATCH v2 3/5] x86/mm: check exec permissions on fault

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Message-Id: <C30DF7E3-98B9-4985-B83B-E4FCD86A1663@gmail.com>
Date:   Mon, 25 Oct 2021 10:51:21 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>, x86@...nel.org
Subject: Re: [PATCH v2 3/5] x86/mm: check exec permissions on fault



> On Oct 25, 2021, at 10:45 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 10/25/21 9:19 AM, Nadav Amit wrote:
>> That was my first version, but I was concerned that perhaps there is
>> some strange scenario in which both X86_PF_WRITE and X86_PF_INSN can
>> be set. That is the reason that Peter asked you whether this is
>> something that might happen.
>> 
>> If you confirm they cannot be both set, I would the version you just
>> mentioned.
> 
> I'm pretty sure they can't be set together on any sane hardware.  A
> bonkers hypervisor or CPU could do it of course, but they'd be crazy.
> 
> BTW, feel free to add a WARN_ON_ONCE() if WRITE and INSN are both set.
> That would be a nice place to talk about the assumption.
> 

I can do that. But be aware that if the assumption is broken, it might
lead to the application getting stuck in an infinite loop of
page-faults instead of receiving SIGSEGV.