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From:   Andy Lutomirski <luto@kernel.org>
Date:   Mon, 21 Dec 2020 10:22:38 -0800
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Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Nadav Amit <nadav.amit@gmail.com>,
        Dave Hansen <dave.hansen@intel.com>,
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        lkml <linux-kernel@vger.kernel.org>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        stable <stable@vger.kernel.org>,
        Minchan Kim <minchan@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
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On Mon, Dec 21, 2020 at 10:04 AM Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> Hello,
>
> On Sat, Dec 19, 2020 at 09:08:55PM -0800, Andy Lutomirski wrote:
> > On Sat, Dec 19, 2020 at 6:49 PM Andrea Arcangeli <aarcange@redhat.com> wrote:
> > > The ptes are changed always with the PT lock, in fact there's no
> > > problem with the PTE updates. The only difference with mprotect
> > > runtime is that the mmap_lock is taken for reading. And the effect
> > > contested for this change doesn't affect the PTE, but supposedly the
> > > tlb flushing deferral.
> >
> > Can you point me at where the lock ends up being taken in this path?
>
> pte_offset_map_lock in change_pte_range, as in mprotect, no difference.
>
> As I suspected on my follow up, the bug described wasn't there, but
> I'll look at the new theory posted.

Indeed.