rose: Fix NULL pointer dereference in rose_send_frame()

The syzkaller reported an issue:

KASAN: null-ptr-deref in range [0x0000000000000380-0x0000000000000387]
CPU: 0 PID: 4069 Comm: kworker/0:15 Not tainted 6.0.0-syzkaller-02734-g0326074f #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/22/2022
Workqueue: rcu_gp srcu_invoke_callbacks
RIP: 0010:rose_send_frame+0x1dd/0x2f0 net/rose/rose_link.c:101
Call Trace:
 <IRQ>
 rose_transmit_clear_request+0x1d5/0x290 net/rose/rose_link.c:255
 rose_rx_call_request+0x4c0/0x1bc0 net/rose/af_rose.c:1009
 rose_loopback_timer+0x19e/0x590 net/rose/rose_loopback.c:111
 call_timer_fn+0x1a0/0x6b0 kernel/time/timer.c:1474
 expire_timers kernel/time/timer.c:1519 [inline]
 __run_timers.part.0+0x674/0xa80 kernel/time/timer.c:1790
 __run_timers kernel/time/timer.c:1768 [inline]
 run_timer_softirq+0xb3/0x1d0 kernel/time/timer.c:1803
 __do_softirq+0x1d0/0x9c8 kernel/softirq.c:571
 [...]
 </IRQ>

It triggers NULL pointer dereference when 'neigh->dev->dev_addr' is
called in the rose_send_frame(). It's the first occurrence of the
`neigh` is in rose_loopback_timer() as `rose_loopback_neigh', and
the 'dev' in 'rose_loopback_neigh' is initialized sa nullptr.

It had been fixed by commit 3b3fd068
("rose: Fix Null pointer dereference in rose_send_frame()") ever.
But it's introduced by commit 3c53cd65
("rose: check NULL rose_loopback_neigh->loopback") again.

We fix it by add NULL check in rose_transmit_clear_request(). When
the 'dev' in 'neigh' is NULL, we don't reply the request and just
clear it.

syzkaller don't provide repro, and I provide a syz repro like:
r0 = syz_init_net_socket$bt_sco(0x1f, 0x5, 0x2)
ioctl$sock_inet_SIOCSIFFLAGS(r0, 0x8914, &(0x7f0000000180)={'rose0\x00', 0x201})
r1 = syz_init_net_socket$rose(0xb, 0x5, 0x0)
bind$rose(r1, &(0x7f00000000c0)=@full={0xb, @dev, @null, 0x0, [@null, @null, @netrom, @netrom, @default, @null]}, 0x40)
connect$rose(r1, &(0x7f0000000240)=@short={0xb, @dev={0xbb, 0xbb, 0xbb, 0x1, 0x0}, @remote={0xcc, 0xcc, 0xcc, 0xcc, 0xcc, 0xcc, 0x1}, 0x1, @netrom={0xbb, 0xbb, 0xbb, 0xbb, 0xbb, 0x0, 0x0}}, 0x1c)

Fixes: 3c53cd65 ("rose: check NULL rose_loopback_neigh->loopback")
Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/rose/rose_link.c

@@ -236,6 +236,9 @@ void rose_transmit_clear_request(struct rose_neigh *neigh, unsigned int lci, uns
 	unsigned char *dptr;
 	int len;
 
+	if (!neigh->dev)
+		return;
+
 	len = AX25_BPQ_HEADER_LEN + AX25_MAX_HEADER_LEN + ROSE_MIN_LEN + 3;
 
 	if ((skb = alloc_skb(len, GFP_ATOMIC)) == NULL)