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

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Date:   Mon, 25 Oct 2021 12:13:54 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Nadav Amit <nadav.amit@...il.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Xu <peterx@...hat.com>,
        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 25/10/2021 11:59, Peter Zijlstra wrote:
> On Thu, Oct 21, 2021 at 05:21:10AM -0700, Nadav Amit wrote:
>> From: Nadav Amit <namit@...are.com>
>> Add a check to prevent access_error() from returning mistakenly that
>> page-faults due to instruction fetch are not allowed. Intel SDM does not
>> indicate whether "instruction fetch" and "write" in the hardware error
>> code are mutual exclusive, so check both before returning whether the
>> access is allowed.
> Dave, can we get that clarified? It seems a bit naf and leads to
> confusing code IMO.

There is no such thing as an instruction fetch (a read) causing a
modification to the mapping.  From this point of view, you'd never
expect to see them both set.

However, be aware that INSTR is only reported for NX || SMEP.  Without
either, instruction vs data accesses are distinguished internally (can
be demonstrated with SMAP) but not visible in the pagefault error code.

~Andrew