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

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Date:   Mon, 25 Oct 2021 07:23:52 -0700
From:   Dave Hansen <dave.hansen@...el.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 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>,
        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 10/25/21 3:59 AM, Peter Zijlstra wrote:
>> 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.

We can, but there are quite a few implicit relationships in those bits.
 PF_INSN and PF_PK can't ever be set together, for instance.  It's
pretty clear as long as you have fetch==read in your head.