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

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Date:   Tue, 17 Jan 2023 20:51:01 +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,
        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,
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        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 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,
and ensure that the userfaultfd code write-locks the VMA before
changing the __VM_UFFD_FLAGS in vma->vm_flags.

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