linux-kernel - Re: [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock

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Message-ID: <CAG48ez1J2YC=_wu82p1BCgR+igd3aF_rwi5WmLJX9MjYM-BUsw@mail.gmail.com>
Date:   Tue, 17 Jan 2023 21:36:05 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Suren Baghdasaryan <surenb@...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,
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        ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
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        hughd@...gle.com, bigeasy@...utronix.de, kent.overstreet@...ux.dev,
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        linux-arm-kernel@...ts.infradead.org,
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Subject: Re: [PATCH 32/41] mm: prevent userfaults to be handled under per-vma lock

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...

> 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
> >