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Message-ID: <CAJuCfpHfJaowYqz9Ls-S_FwR1JUOsZ1XPwvj6m-CVUnFX_us6Q@mail.gmail.com>
Date:   Tue, 17 Jan 2023 12:57:47 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
        jglisse@...gle.com, mhocko@...e.com, vbabka@...e.cz,
        hannes@...xchg.org, mgorman@...hsingularity.net, dave@...olabs.net,
        willy@...radead.org, liam.howlett@...cle.com, peterz@...radead.org,
        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        paulmck@...nel.org, luto@...nel.org, songliubraving@...com,
        peterx@...hat.com, david@...hat.com, dhowells@...hat.com,
        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        peterjung1337@...il.com, rientjes@...gle.com,
        axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com,
        shakeelb@...gle.com, tatashin@...gle.com, edumazet@...gle.com,
        gthelen@...gle.com, gurua@...gle.com, arjunroy@...gle.com,
        soheil@...gle.com, hughlynch@...gle.com, leewalsh@...gle.com,
        posk@...gle.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock

On Tue, Jan 17, 2023 at 12:36 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Tue, Jan 17, 2023 at 8:51 PM Jann Horn <jannh@...gle.com> wrote:
> > On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb@...gle.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@...gle.com>
> > > Suggested-by: Peter Xu <peterx@...hat.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...

Yes, I think you are right. I'll move the check after locking the VMA. Thanks!

>
> > 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
> > >

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