From: Bryam Vargas <redacted>

afiucv_hs_rcv() looks up the destination socket under iucv_sk_list.lock,
drops the lock, and then passes the socket to the afiucv_hs_callback_*()
handlers without holding a reference. AF_IUCV sockets are not
RCU-protected and are freed synchronously by iucv_sock_kill() ->
sock_put(), so a concurrent close can free the socket in the window
between read_unlock() and the handler, which then dereferences freed
memory (for example sk->sk_data_ready() in afiucv_hs_callback_syn()).

Take a reference with sock_hold() while the socket is still on the list
and release it with sock_put() once the handler has run.

Fixes: 3881ac441f64 ("af_iucv: add HiperSockets transport")
Signed-off-by: Bryam Vargas <redacted>
---
afiucv_netdev_event() has the same iucv_sk_list use-after-free and is
being fixed separately by Nagamani PV:
  https://lore.kernel.org/all/20260508170534.2208812-1-nagamani@linux.ibm.com/ (local)
This patch covers the receive path, afiucv_hs_rcv(), which that fix does
not touch.

Verified with an LKMM/herd7 litmus (the missing sock_hold() is the only
delta between a flagged data race and none) and with an in-kernel KASAN
model that reproduces the afiucv_hs_rcv() lookup-without-hold path (the
freed socket's sk_data_ready pointer is read after the callback runs);
adding the reference clears it. af_iucv is s390-only, so this is the
model rather than the driver. Reproducer available on request.
---
 net/iucv/af_iucv.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
index fed240b453bd..b85fb9767dec 100644
--- a/net/iucv/af_iucv.c
+++ b/net/iucv/af_iucv.c

@@ -2089,6 +2089,8 @@ static int afiucv_hs_rcv(struct sk_buff *skb, struct net_device *dev,
 			}
 		}
 	}
+	if (sk)
+		sock_hold(sk);
 	read_unlock(&iucv_sk_list.lock);
 	if (!iucv)
 		sk = NULL;

@@ -2138,6 +2140,8 @@ static int afiucv_hs_rcv(struct sk_buff *skb, struct net_device *dev,
 		kfree_skb(skb);
 	}
 
+	if (sk)
+		sock_put(sk);
 	return err;
 }
 

---
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
change-id: 20260705-b4-disp-fc79c0dc-019670262472

Best regards,
-- 
Bryam Vargas [off-list ref]