Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 21 Dec 2020 13:03:57 -0500
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     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>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
Message-ID: <X+Djjd8dW12u+rSR@redhat.com>
References: <20201219043006.2206347-1-namit@vmware.com>
 <X95RRZ3hkebEmmaj@redhat.com>
 <EDC00345-B46E-4396-8379-98E943723809@gmail.com>
 <CALCETrVtsdeOtGWMUcmT1dzDBxRpecpZDe02L61qEmJmFxSvYw@mail.gmail.com>
 <X967yWAoaTejRk5y@redhat.com>
 <CALCETrVVa-cfogKZirRrP5tmy-gCDtb=jTpLk648BpBQsK9Z5A@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CALCETrVVa-cfogKZirRrP5tmy-gCDtb=jTpLk648BpBQsK9Z5A@mail.gmail.com>
User-Agent: Mutt/2.0.3 (2020-12-04)
Precedence: bulk

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.

Thanks,
Andrea