rose_send_frame() dereferences `neigh->dev` when called from
rose_transmit_clear_request(), and the first occurance of the `neigh`
is in rose_loopback_timer() as `rose_loopback_neigh`, and it is initialized
in rose_add_loopback_neigh() as NULL. i.e when `rose_loopback_neigh` used in 
rose_loopback_timer() its `->dev` was still NULL and rose_loopback_timer() 
was calling rose_rx_call_request() without checking for NULL.

- net/rose/rose_link.c
This bug seems to get triggered in this line:

rose_call = (ax25_address *)neigh->dev->dev_addr;

Fix it by adding NULL checking for `rose_loopback_neigh->dev` in rose_loopback_timer(). 

Reported-and-tested-by: syzbot+a1c743815982d9496393@syzkaller.appspotmail.com 
Link: https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3 
Signed-off-by: Anmol Karn <redacted>
---
 net/rose/rose_loopback.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/rose/rose_loopback.c b/net/rose/rose_loopback.c
index 7b094275ea8b..cd7774cb1d07 100644
--- a/net/rose/rose_loopback.c
+++ b/net/rose/rose_loopback.c

@@ -96,7 +96,7 @@ static void rose_loopback_timer(struct timer_list *unused)
 		}
 
 		if (frametype == ROSE_CALL_REQUEST) {
-			if ((dev = rose_dev_get(dest)) != NULL) {
+			if (rose_loopback_neigh->dev && (dev = rose_dev_get(dest)) != NULL) {
 				if (rose_rx_call_request(skb, dev, rose_loopback_neigh, lci_o) == 0)
 					kfree_skb(skb);
 			} else {

-- 
2.29.2