Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-33-surenb@google.com>
 <CAG48ez0Z-wnBLzCNDHgTviV0Ws+s4grX-sFRZ-43dxhJg+GzfA@mail.gmail.com>
In-Reply-To: <CAG48ez0Z-wnBLzCNDHgTviV0Ws+s4grX-sFRZ-43dxhJg+GzfA@mail.gmail.com>
From:   Jann Horn <jannh@google.com>
Date:   Tue, 17 Jan 2023 21:36:05 +0100
Message-ID: <CAG48ez1J2YC=_wu82p1BCgR+igd3aF_rwi5WmLJX9MjYM-BUsw@mail.gmail.com>
Subject: Re: [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, michel@lespinasse.org,
        jglisse@google.com, mhocko@suse.com, vbabka@suse.cz,
        hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net,
        willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org,
        ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com,
        paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com,
        peterx@redhat.com, david@redhat.com, dhowells@redhat.com,
        hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev,
        punit.agrawal@bytedance.com, lstoakes@gmail.com,
        peterjung1337@gmail.com, rientjes@google.com,
        axelrasmussen@google.com, joelaf@google.com, minchan@google.com,
        shakeelb@google.com, tatashin@google.com, edumazet@google.com,
        gthelen@google.com, gurua@google.com, arjunroy@google.com,
        soheil@google.com, hughlynch@google.com, leewalsh@google.com,
        posk@google.com, linux-mm@kvack.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Jan 17, 2023 at 8:51 PM Jann Horn <jannh@google.com> wrote:
> On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb@google.com> wrote:
> > Due to the possibility of handle_userfault dropping mmap_lock, avoid fault
> > handling under VMA lock and retry holding mmap_lock. This can be handled
> > more gracefully in the future.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > Suggested-by: Peter Xu <peterx@redhat.com>
> > ---
> >  mm/memory.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 20806bc8b4eb..12508f4d845a 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -5273,6 +5273,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> >         if (!vma->anon_vma)
> >                 goto inval;
> >
> > +       /*
> > +        * Due to the possibility of userfault handler dropping mmap_lock, avoid
> > +        * it for now and fall back to page fault handling under mmap_lock.
> > +        */
> > +       if (userfaultfd_armed(vma))
> > +               goto inval;
>
> This looks racy wrt concurrent userfaultfd_register(). I think you'll
> want to do the userfaultfd_armed(vma) check _after_ locking the VMA,

I still think this change is needed...

> and ensure that the userfaultfd code write-locks the VMA before
> changing the __VM_UFFD_FLAGS in vma->vm_flags.

Ah, but now I see you already took care of this half of the issue with
the reset_vm_flags() change in
https://lore.kernel.org/linux-mm/20230109205336.3665937-16-surenb@google.com/
.


> >         if (!vma_read_trylock(vma))
> >                 goto inval;
> >
> > --
> > 2.39.0
> >