linux-kernel - Re: [RESEND PATCH v3 2/5] x86/mm: check exec permissions on fault

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Date:   Fri, 11 Mar 2022 13:23:26 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        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" <x86@...nel.org>
Subject: Re: [RESEND PATCH v3 2/5] x86/mm: check exec permissions on fault

On 3/11/22 13:16, Nadav Amit wrote:
>> This is really about checking the sanity of the "hardware"-provided
>> error code.  Let's just do it in  handle_page_fault(), maybe hidden in a
>> function like:
>>
>> void check_error_code_sanity(unsigned long error_code)
>> {
>> 	WARN_ON_ONCE(...);
>> }
>>
>> You can leave the X86_PF_PK check in place for now.  It's probably going
>> away soon anyway.
> Done. Thanks. But note that removing the check from access_error() means
> that if the assertion is broken, userspace might crash inadvertently
> (in contrast to the version I sent, which would have potentially led to
> infinite stream of page-faults). I don’t know which behavior is better,
> so let’s go with your version and just hope it doesn’t happen.

Actually, crashing sounds much nicer to me than infinite page faults.
It's a lot easier to debug, *especially* with a warning on dmesg.