linux-kernel - Re: [PATCH 2/2] mm: Fix anon

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Date:   Wed, 26 Jul 2023 23:50:57 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linus Torvalds <torvalds@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <parri.andrea@...il.com>,
        Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 2/2] mm: Fix anon_vma memory ordering

On Wed, Jul 26, 2023 at 11:42 PM Jann Horn <jannh@...gle.com> wrote:
> A read of vma->anon_vma under mmap_lock in read mode (in particular in
> anon_vma_prepare()) can race with a concurrent update under mmap_lock
> in read mode plus pagetable lock (in __prepare_anon_vma()).
> However, the only allowed concurrent update is one that changes
> vma->anon_vma from NULL to a non-NULL pointer; once vma->anon_vma has
> been set to a non-NULL value, it will keep that value as long as the
> mmap lock is held in read mode.
[...]
> @@ -1072,7 +1071,15 @@ static int anon_vma_compatible(struct vm_area_struct *a, struct vm_area_struct *
>  static struct anon_vma *reusable_anon_vma(struct vm_area_struct *old, struct vm_area_struct *a, struct vm_area_struct *b)
>  {
>         if (anon_vma_compatible(a, b)) {
> -               struct anon_vma *anon_vma = READ_ONCE(old->anon_vma);
> +               /*
> +                * Pairs with smp_store_release() in __anon_vma_prepare().
> +                *
> +                * We could get away with a READ_ONCE() here, but
> +                * smp_load_acquire() ensures that the following
> +                * list_is_singular() check on old->anon_vma_chain doesn't race
> +                * with __anon_vma_prepare().

Of course I only realize directly after sending this patch that this
comment only holds...

> +                */
> +               struct anon_vma *anon_vma = smp_load_acquire(&old->anon_vma);
>
>                 if (anon_vma && list_is_singular(&old->anon_vma_chain))
>                         return anon_vma;
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 0c0d8857dfce..83bc4267269f 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -210,8 +210,9 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
>         anon_vma_lock_write(anon_vma);
>         /* page_table_lock to protect against threads */
>         spin_lock(&mm->page_table_lock);
> +       /* no need for smp_load_acquire() here, the lock prevents concurrency */
>         if (likely(!vma->anon_vma)) {
> -               vma->anon_vma = anon_vma;
> +               smp_store_release(&vma->anon_vma, anon_vma);
>                 anon_vma_chain_link(vma, avc, anon_vma);

... if we move the smp_store_release() down by one line here.

>                 anon_vma->num_active_vmas++;
>                 allocated = NULL;