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Message-ID: <CAG48ez00FhfjTReO8+B43tEXuzFcat1mQs3jxu_eGFzEp2Vsag@mail.gmail.com>
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;
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