net/iucv: take a reference on the socket found in afiucv_hs_rcv()

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: 3881ac4 ("af_iucv: add HiperSockets transport")
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Reviewed-by: Hidayath Khan <hidayath@linux.ibm.com>
Link: https://patch.msgid.link/20260705-b4-disp-fc79c0dc-v1-1-d2cdcb57afa9@proton.me
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: bryamzxz

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;
 }