netdev - [Linux-kernel-mentees] [PATCH v2 net] rose: Fix Null pointer dereference in rose_send

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Date:   Thu,  5 Nov 2020 21:26:00 +0530
From:   Anmol Karn <anmol.karan123@...il.com>
To:     ralf@...ux-mips.org, davem@...emloft.net, kuba@...nel.org
Cc:     gregkh@...uxfoundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hams@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzkaller-bugs@...glegroups.com, anmol.karan123@...il.com,
        syzbot+a1c743815982d9496393@...kaller.appspotmail.com
Subject: [Linux-kernel-mentees] [PATCH v2 net] rose: Fix Null pointer dereference in rose_send_frame()

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@...kaller.appspotmail.com 
Link: https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3 
Signed-off-by: Anmol Karn <anmol.karan123@...il.com>
---
 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