[PATCH net v2] net: tipc: fix refcount warning in tipc_aead_encrypt

[Charalampos Mitrodimas <charmitro@posteo.net>]

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/68342b55.a70a0220.253bc2.0091.GAE@google.com/T/#m12019cf9ae77e1954f666914640efa36d52704a2

Reported-by: syzbot+f0c4a4aba757549ae26c@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68342b55.a70a0220.253bc2.0091.GAE@google.com/T/#m12019cf9ae77e1954f666914640efa36d52704a2
Fixes: e27902461713 ("net/tipc: fix slab-use-after-free Read in tipc_aead_encrypt_done")
Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>
---
Changes in v2:
- Return "-ENODEV" instead of "-ENXIO".
- Link to v1: https://lore.kernel.org/r/20250526-net-tipc-warning-v1-1-472f3aa9dd9f@posteo.net
---
 net/tipc/crypto.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/tipc/crypto.c b/net/tipc/crypto.c
index 8584893b478510dc1ddda321ed06054de327609b..79f91b6ca8c8477208f13d41a37af24e7aa94577 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);

---
base-commit: 49fffac983ac52aea0ab94914be3f56bcf92d5dc
change-id: 20250526-net-tipc-warning-bda0aa9c5422

Best regards,
-- 
Charalampos Mitrodimas <charmitro@posteo.net>