[PATCH net] net: airoha: Fix potential use-after-free in airoha_ppe_deinit()

[Wayen Yan <win847@gmail.com>]

airoha_ppe_deinit() replaces the NPU pointer with NULL via
rcu_replace_pointer() but does not wait for existing RCU readers
to exit before calling ppe_deinit() and airoha_npu_put(). This can
cause a use-after-free if a reader in an RCU read-side critical
section still holds a reference to the NPU when it is freed.

The init path (airoha_ppe_init) already calls synchronize_rcu()
after rcu_assign_pointer(), but the deinit path introduced in
commit 6abcf751bc08 ("net: airoha: Fix schedule while atomic in
airoha_ppe_deinit()") omitted the matching barrier when switching
from rcu_read_lock()/rcu_dereference() to rcu_replace_pointer().

Add synchronize_rcu() before ppe_deinit() to ensure all existing
RCU readers have completed before the NPU resources are released.

Fixes: 6abcf751bc084804a9e5b3051442e8a2ce67f48a ("net: airoha: Fix schedule while atomic in airoha_ppe_deinit()")
Signed-off-by: Wayen Yan <win847@gmail.com>
---
 drivers/net/ethernet/airoha/airoha_ppe.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/net/ethernet/airoha/airoha_ppe.c b/drivers/net/ethernet/airoha/airoha_ppe.c
index e7c78293002a..f6396925722d 100644
--- a/drivers/net/ethernet/airoha/airoha_ppe.c
+++ b/drivers/net/ethernet/airoha/airoha_ppe.c
@@ -1659,6 +1659,7 @@ void airoha_ppe_deinit(struct airoha_eth *eth)
 	npu = rcu_replace_pointer(eth->npu, NULL,
 				  lockdep_is_held(&flow_offload_mutex));
 	if (npu) {
+		synchronize_rcu();
 		npu->ops.ppe_deinit(npu);
 		airoha_npu_put(npu);
 	}
-- 
2.51.0