Re: [PATCH net] net: openvswitch: fix use-after-free of the flow table mask array

[Eelco Chaudron]

On 6 Sep 2026, at 10:21, Norbert Szetei wrote:

> tbl_mask_array_realloc() retires the old mask_array before it stops being
> reachable:
>
> 	old = ovsl_dereference(tbl->mask_array);
> 	if (old) {
> 		...
> 		call_rcu(&old->rcu, mask_array_rcu_cb);
> 	}
>
> 	rcu_assign_pointer(tbl->mask_array, new);
>
> call_rcu() only waits for read-side critical sections already in flight.
> tbl->mask_array still points at old between the call_rcu() and the
> rcu_assign_pointer(), so a reader entering ovs_flow_tbl_lookup_stats() in
> that window picks up old in a fresh critical section that the pending
> grace period does not cover.
>
> tbl_mask_array_realloc() runs in process context under ovs_mutex, so the
> window is preemptible and can outlast the grace period. Then
> mask_array_rcu_cb() frees old before the swap runs:
>
>   BUG: KASAN: slab-use-after-free in flow_lookup.constprop.0+0x2bf/0x2f0
>   Read of size 8 at addr ffff888020b3e018 by task poc/741
>    flow_lookup.constprop.0+0x2bf/0x2f0
>    ovs_flow_tbl_lookup_stats+0x4a3/0x5c0
>    ovs_dp_process_packet+0x19c/0x710
>    ovs_vport_receive+0x243/0x390
>    internal_dev_xmit+0x81/0x170
>   Freed by task 728:
>    kfree+0x16a/0x4e0
>    rcu_core+0x853/0x1030
>
> Publish the new array before retiring the old one. The kfree_rcu() that
> call_rcu() replaced ran after the swap.
>
> Fixes: eac87c413bf9 ("net: openvswitch: reorder masks array based on usage")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-opus-5
> Signed-off-by: Norbert Szetei <norbert@xxxxxxxxxxxx>
> ---

Changes look good to me. Although not really necessary, I did run all the kernel userspace tests, and they pass.

Acked-by: Eelco Chaudron echaudro@xxxxxxxxxx