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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 19 Dec 2020 21:08:55 -0800
Message-ID: <CALCETrVVa-cfogKZirRrP5tmy-gCDtb=jTpLk648BpBQsK9Z5A@mail.gmail.com>
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>,
        linux-mm <linux-mm@kvack.org>, Peter Xu <peterx@redhat.com>,
        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 Sat, Dec 19, 2020 at 6:49 PM Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> On Sat, Dec 19, 2020 at 06:01:39PM -0800, Andy Lutomirski wrote:
> > I missed the beginning of this thread, but it looks to me like
> > userfaultfd changes PTEs with not locking except mmap_read_lock().  It
>
> There's no mmap_read_lock, I assume you mean mmap_lock for reading.

Yes.

>
> 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?
I apparently missed it somewhere.

> Anyway to wait the wrprotect to do the deferred flush, before the
> unprotect can even start, one more mutex in the mm to take in all
> callers of change_protection_range with the mmap_lock for reading may
> be enough.

I'll read the code again tomorrow.