net: tipc: fix refcount warning in tipc_aead_encrypt

syzbot reported a refcount warning [1] caused by calling get_net() on
a network namespace that is being destroyed (refcount=0). This happens
when a TIPC discovery timer fires during network namespace cleanup.

The recently added get_net() call in commit e279024617134 ("net/tipc:
fix slab-use-after-free Read in tipc_aead_encrypt_done") attempts to
hold a reference to the network namespace. However, if the namespace
is already being destroyed, its refcount might be zero, leading to the
use-after-free warning.

Replace get_net() with maybe_get_net(), which safely checks if the
refcount is non-zero before incrementing it. If the namespace is being
destroyed, return -ENODEV early, after releasing the bearer reference.

[1]: https://lore.kernel.org/all/[email protected]/T/#m12019cf9ae77e1954f666914640efa36d52704a2

Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/T/#m12019cf9ae77e1954f666914640efa36d52704a2
Fixes: e27902461713 ("net/tipc: fix slab-use-after-free Read in tipc_aead_encrypt_done")
Signed-off-by: Charalampos Mitrodimas <[email protected]>
Reviewed-by: Tung Nguyen <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/tipc/crypto.c b/net/tipc/crypto.c
index f4cfe88670f5..ea5bb131ebd0 100644
--- a/net/tipc/crypto.c
+++ b/net/tipc/crypto.c
@@ -818,7 +818,11 @@ static int tipc_aead_encrypt(struct tipc_aead *aead, struct sk_buff *skb,
 	}
 
 	/* Get net to avoid freed tipc_crypto when delete namespace */
-	get_net(aead->crypto->net);
+	if (!maybe_get_net(aead->crypto->net)) {
+		tipc_bearer_put(b);
+		rc = -ENODEV;
+		goto exit;
+	}
 
 	/* Now, do encrypt */
 	rc = crypto_aead_encrypt(req);