[Linux-kernel-mentees] [PATCH v3 net] rose: Fix Null pointer dereference in rose_send_frame()

[Anmol Karn]

rose_send_frame() dereferences `neigh->dev` when called from
rose_transmit_clear_request(), and the first occurrence 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().

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+a1c743815982d9496393 at syzkaller.appspotmail.com
Tested-by: syzbot+a1c743815982d9496393 at syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3
Signed-off-by: Anmol Karn <anmol.karan123 at gmail.com>
---
Changes in v3:
        - Corrected checkpatch warnings and errors (Suggested-by: Saeed Mahameed <saeed at kernel.org>)
	- Added "Fixes:" tag (Suggested-by: Saeed Mahameed <saeed at kernel.org>)
Changes in v2:
	- Added NULL check in rose_loopback_timer() (Suggested-by: Greg KH <gregkh at linuxfoundation.org>)

 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..2c51756ed7bf 100644
--- a/net/rose/rose_loopback.c
+++ b/net/rose/rose_loopback.c
@@ -96,7 +96,8 @@ static void rose_loopback_timer(struct timer_list *unused)
 		}

 		if (frametype == ROSE_CALL_REQUEST) {
-			if ((dev = rose_dev_get(dest)) != NULL) {
+			dev = rose_dev_get(dest);
+			if (rose_loopback_neigh->dev && dev) {
 				if (rose_rx_call_request(skb, dev, rose_loopback_neigh, lci_o) == 0)
 					kfree_skb(skb);
 			} else {
-
2.29.2