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Subject: Re: [PATCH v2 3/5] x86/mm: check exec permissions on fault
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <37d0b84c-f6ca-15cb-8d78-b6b5a9496da7@intel.com>
Date:   Mon, 25 Oct 2021 10:51:21 -0700
Cc:     Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Nick Piggin <npiggin@gmail.com>, x86@kernel.org
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References: <20211021122112.592634-1-namit@vmware.com>
 <20211021122112.592634-4-namit@vmware.com>
 <e55875fa-1264-7e08-3bb8-ed984f6ea5b3@intel.com>
 <00C2DC4B-A77D-4B32-B7F7-2291830BC2D2@gmail.com>
 <37d0b84c-f6ca-15cb-8d78-b6b5a9496da7@intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
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> On Oct 25, 2021, at 10:45 AM, Dave Hansen <dave.hansen@intel.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.