[PATCH net 5/5] rose: guard rose_neigh_put() against NULL in timer expiry

[Bernard Pidoux <bernard.f6bvp@gmail.com>]

In rose_timer_expiry(), the ROSE_STATE_2 branch calls
rose_neigh_put(rose->neighbour) without first checking whether the
pointer is NULL.  After commit 5de7665e0a07 ("net: rose: fix timer
races against user threads") the timer is re-armed when the socket is
owned by a user thread; between the re-arm and the next firing, a
device-down event or concurrent teardown via rose_kill_by_device() can
set rose->neighbour to NULL, leading to a NULL-pointer dereference
inside rose_neigh_put().

Add a NULL check before the put and clear the pointer afterwards.

Fixes: 5de7665e0a07 ("net: rose: fix timer races against user threads")
Tested-by: Bernard Pidoux <bernard.f6bvp@gmail.com>
Signed-off-by: Bernard Pidoux <bernard.f6bvp@gmail.com>
---
 net/rose/rose_timer.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/rose/rose_timer.c b/net/rose/rose_timer.c
index bb60a1654d61..d997d24ab081 100644
--- a/net/rose/rose_timer.c
+++ b/net/rose/rose_timer.c
@@ -180,7 +180,10 @@ static void rose_timer_expiry(struct timer_list *t)
 		break;
 
 	case ROSE_STATE_2:	/* T3 */
-		rose_neigh_put(rose->neighbour);
+		if (rose->neighbour) {
+			rose_neigh_put(rose->neighbour);
+			rose->neighbour = NULL;
+		}
 		rose_disconnect(sk, ETIMEDOUT, -1, -1);
 		break;
 
-- 
2.51.0