Re: [PATCH v1] futex: Use-after-free between futex_key_to_node_opt and vma_replace_policy

[Eric Dumazet]

On Fri, Mar 13, 2026 at 1:08 PM Hao-Yu Yang <naup96721@xxxxxxxxx> wrote:
>
> During futex_key_to_node_opt() execution, vma->vm_policy is read under
> speculative mmap lock and RCU. Concurrently, mbind() may call
> vma_replace_policy() which frees the old mempolicy immediately via
> kmem_cache_free().
>
> This creates a race where __futex_key_to_node() dereferences a freed
> mempolicy pointer, causing a use-after-free read of mpol->mode.
>
> patch just initial attempt to patch and need more discussion.
> I think we need to remove this path or add some lock.
>
> Fixes: c042c505210d ("futex: Implement FUTEX2_MPOL")
> Reported-by: Hao-Yu Yang <naup96721@xxxxxxxxx>
> Signed-off-by: Hao-Yu Yang <naup96721@xxxxxxxxx>
> ---
>  kernel/futex/core.c | 23 -----------------------
>  1 file changed, 23 deletions(-)

Good catch !

Adding rcu to __mpol_put() is really a no brainer.

 Thanks.

diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
index 0fe96f3ab3ef02e902e1676e750c2006ecd6147f..65c732d440d2f4e4566204429f1e5e7487ab8f91
100644
--- a/include/linux/mempolicy.h
+++ b/include/linux/mempolicy.h
@@ -55,6 +55,7 @@ struct mempolicy {
                nodemask_t cpuset_mems_allowed; /* relative to these nodes */
                nodemask_t user_nodemask;       /* nodemask passed by user */
        } w;
+       struct rcu_head rcu;
 };

 /*
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 0e5175f1c767d81394276559b9610c24d854f5bc..6dc61a3d4a32f74a06bf005acfd82d4a43112348
100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -487,7 +487,7 @@ void __mpol_put(struct mempolicy *pol)
 {
        if (!atomic_dec_and_test(&pol->refcnt))
                return;
-       kmem_cache_free(policy_cache, pol);
+       kfree_rcu(pol, rcu);
 }
 EXPORT_SYMBOL_FOR_MODULES(__mpol_put, "kvm");