net/iucv: fix use after free in iucv_sock_close()

iucv_sever_path() is called from process context and from bh context.
iucv->path is used as indicator whether somebody else is taking care of
severing the path (or it is already removed / never existed).
This needs to be done with atomic compare and swap, otherwise there is a
small window where iucv_sock_close() will try to work with a path that has
already been severed and freed by iucv_callback_connrej() called by
iucv_tasklet_fn().

Example:
[452744.123844] Call Trace:
[452744.123845] ([<0000001e87f03880>] 0x1e87f03880)
[452744.123966]  [<00000000d593001e>] iucv_path_sever+0x96/0x138
[452744.124330]  [<000003ff801ddbca>] iucv_sever_path+0xc2/0xd0 [af_iucv]
[452744.124336]  [<000003ff801e01b6>] iucv_sock_close+0xa6/0x310 [af_iucv]
[452744.124341]  [<000003ff801e08cc>] iucv_sock_release+0x3c/0xd0 [af_iucv]
[452744.124345]  [<00000000d574794e>] __sock_release+0x5e/0xe8
[452744.124815]  [<00000000d5747a0c>] sock_close+0x34/0x48
[452744.124820]  [<00000000d5421642>] __fput+0xba/0x268
[452744.124826]  [<00000000d51b382c>] task_work_run+0xbc/0xf0
[452744.124832]  [<00000000d5145710>] do_notify_resume+0x88/0x90
[452744.124841]  [<00000000d5978096>] system_call+0xe2/0x2c8
[452744.125319] Last Breaking-Event-Address:
[452744.125321]  [<00000000d5930018>] iucv_path_sever+0x90/0x138
[452744.125324]
[452744.125325] Kernel panic - not syncing: Fatal exception in interrupt

Note that bh_lock_sock() is not serializing the tasklet context against
process context, because the check for sock_owned_by_user() and
corresponding handling is missing.

Ideas for a future clean-up patch:
A) Correct usage of bh_lock_sock() in tasklet context, as described in
Link: https://lore.kernel.org/netdev/1280155406.2899.407.camel@edumazet-laptop/
Re-enqueue, if needed. This may require adding return values to the
tasklet functions and thus changes to all users of iucv.

B) Change iucv tasklet into worker and use only lock_sock() in af_iucv.

Fixes: 7d316b94 ("af_iucv: remove IUCV-pathes completely")
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://patch.msgid.link/20240729122818.947756-1-wintera@linux.ibm.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/iucv/af_iucv.c

@@ -335,8 +335,8 @@ static void iucv_sever_path(struct sock *sk, int with_user_data)
 	struct iucv_sock *iucv = iucv_sk(sk);
 	struct iucv_path *path = iucv->path;
 
-	if (iucv->path) {
-		iucv->path = NULL;
+	/* Whoever resets the path pointer, must sever and free it. */
+	if (xchg(&iucv->path, NULL)) {
 		if (with_user_data) {
 			low_nmcpy(user_data, iucv->src_name);
 			high_nmcpy(user_data, iucv->dst_name);