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

[Anmol Karn]

In rose_send_frame(), when comparing two ax.25 addresses, it assigns rose_call to 
either global ROSE callsign or default port, but when the former block triggers and 
rose_call is assigned by (ax25_address *)neigh->dev->dev_addr, a NULL pointer is 
dereferenced by 'neigh' when dereferencing 'dev'.

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

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

Prevent it by checking NULL condition for neigh->dev before comparing addressed for 
rose_call initialization.

Reported-by: syzbot+a1c743815982d9496393@xxxxxxxxxxxxxxxxxxxxxxxxx 
Link: https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3 
Signed-off-by: Anmol Karn <anmol.karan123@xxxxxxxxx>
---
I am bit sceptical about the error return code, please suggest if anything else is 
appropriate in place of '-ENODEV'.

 net/rose/rose_link.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/rose/rose_link.c b/net/rose/rose_link.c
index f6102e6f5161..92ea6a31d575 100644
--- a/net/rose/rose_link.c
+++ b/net/rose/rose_link.c
@@ -97,6 +97,9 @@ static int rose_send_frame(struct sk_buff *skb, struct rose_neigh *neigh)
 	ax25_address *rose_call;
 	ax25_cb *ax25s;
 
+	if (!neigh->dev)
+		return -ENODEV;
+
 	if (ax25cmp(&rose_callsign, &null_ax25_address) == 0)
 		rose_call = (ax25_address *)neigh->dev->dev_addr;
 	else
-- 
2.28.0