Re: [PATCH v2] futex: Use-after-free between futex_key_to_node_opt and vma_replace_policy

[Eric Dumazet <edumazet@google.com>]

On Wed, Mar 25, 2026 at 8:14 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Mar 24, 2026 at 09:27:41PM +0100, David Hildenbrand (Arm) wrote:
> > So IIUC, futex_key_to_node_opt() looks up a VMA under RCU, without
> > holding the mmap lock. Concurrent mmap-write lock is detected by using
> > the mmap_lock_speculate_try_begin()/mmap_lock_speculate_retry() seqcount.
> >
> > After looking up the VMA, we access the VMA policy.
> >
> > vma_policy() does a straight vma->vm_policy.
> >
> > What prevents the compiler here to do some load tearing while it is
> > getting modified by mbind()? Or what stops the writer side to to some
> > store tearing?
> >
> > Shouldn't we be using at least READ_ONCE/WRITE_ONCE() etc?
>
> Bah, at that point we might as well RCU the thing like so, I suppose.
>
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1026,7 +1026,7 @@ static int vma_replace_policy(struct vm_
>         }
>
>         old = vma->vm_policy;
> -       vma->vm_policy = new; /* protected by mmap_lock */
> +       rcu_assign_pointer(vma->vm_policy, new); /* protected by mmap_lock */
>         mpol_put(old);
>
>         return 0;
> diff --git a/kernel/futex/core.c b/kernel/futex/core.c
> index 4bacf5565368..6336a80e3dca 100644
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -342,7 +342,7 @@ static int __futex_key_to_node(struct mm_struct *mm, unsigned long addr)
>         if (!vma)
>                 return FUTEX_NO_NODE;
>
> -       mpol = vma_policy(vma);
> +       mpol = rcu_dereference_raw(vma->vm_policy);
>         if (!mpol)
>                 return FUTEX_NO_NODE;

Yes, but sparse will bite :)

READ_ONCE()/WRITE_ONCE() on these two locations seems acceptable.