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

[Norbert Szetei <norbert@doyensec.com>]

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@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: Norbert Szetei <norbert@doyensec.com>
---
Reproducer available if needed. The issue was confirmed on a KASAN
build, where the crash reproduces in about a minute on a 2 vCPU guest.

 net/openvswitch/flow_table.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/openvswitch/flow_table.c b/net/openvswitch/flow_table.c
index 67d5b8c0fe79..1e0f9d193eb0 100644
--- a/net/openvswitch/flow_table.c
+++ b/net/openvswitch/flow_table.c
@@ -257,11 +257,13 @@ static int tbl_mask_array_realloc(struct flow_table *tbl, int size)
 			if (ovsl_dereference(old->masks[i]))
 				new->masks[new->count++] = old->masks[i];
 		}
-		call_rcu(&old->rcu, mask_array_rcu_cb);
 	}
 
 	rcu_assign_pointer(tbl->mask_array, new);
 
+	if (old)
+		call_rcu(&old->rcu, mask_array_rcu_cb);
+
 	return 0;
 }
 
-- 
2.55.0