tipc: fix NULL deref in tipc_link_xmit()

The buffer list can have zero skb as following path:
tipc_named_node_up()->tipc_node_xmit()->tipc_link_xmit(), so
we need to check the list before casting an &sk_buff.

Fault report:
 [] tipc: Bulk publication failure
 [] general protection fault, probably for non-canonical [#1] PREEMPT [...]
 [] KASAN: null-ptr-deref in range [0x00000000000000c8-0x00000000000000cf]
 [] CPU: 0 PID: 0 Comm: swapper/0 Kdump: loaded Not tainted 5.10.0-rc4+ #2
 [] Hardware name: Bochs ..., BIOS Bochs 01/01/2011
 [] RIP: 0010:tipc_link_xmit+0xc1/0x2180
 [] Code: 24 b8 00 00 00 00 4d 39 ec 4c 0f 44 e8 e8 d7 0a 10 f9 48 [...]
 [] RSP: 0018:ffffc90000006ea0 EFLAGS: 00010202
 [] RAX: dffffc0000000000 RBX: ffff8880224da000 RCX: 1ffff11003d3cc0d
 [] RDX: 0000000000000019 RSI: ffffffff886007b9 RDI: 00000000000000c8
 [] RBP: ffffc90000007018 R08: 0000000000000001 R09: fffff52000000ded
 [] R10: 0000000000000003 R11: fffff52000000dec R12: ffffc90000007148
 [] R13: 0000000000000000 R14: 0000000000000000 R15: ffffc90000007018
 [] FS:  0000000000000000(0000) GS:ffff888037400000(0000) knlGS:000[...]
 [] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 [] CR2: 00007fffd2db5000 CR3: 000000002b08f000 CR4: 00000000000006f0

Fixes: af9b028 ("tipc: make media xmit call outside node spinlock context")
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
Link: https://lore.kernel.org/r/20210108071337.3598-1-hoang.h.le@dektech.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/tipc/link.c

@@ -1030,21 +1030,25 @@ void tipc_link_reset(struct tipc_link *l)
 int tipc_link_xmit(struct tipc_link *l, struct sk_buff_head *list,
 		   struct sk_buff_head *xmitq)
 {
-	struct tipc_msg *hdr = buf_msg(skb_peek(list));
 	struct sk_buff_head *backlogq = &l->backlogq;
 	struct sk_buff_head *transmq = &l->transmq;
 	struct sk_buff *skb, *_skb;
 	u16 bc_ack = l->bc_rcvlink->rcv_nxt - 1;
 	u16 ack = l->rcv_nxt - 1;
 	u16 seqno = l->snd_nxt;
 	int pkt_cnt = skb_queue_len(list);
-	int imp = msg_importance(hdr);
 	unsigned int mss = tipc_link_mss(l);
 	unsigned int cwin = l->window;
 	unsigned int mtu = l->mtu;
+	struct tipc_msg *hdr;
 	bool new_bundle;
 	int rc = 0;
+	int imp;
+
+	if (pkt_cnt <= 0)
+		return 0;
 
+	hdr = buf_msg(skb_peek(list));
 	if (unlikely(msg_size(hdr) > mtu)) {
 		pr_warn("Too large msg, purging xmit list %d %d %d %d %d!\n",
 			skb_queue_len(list), msg_user(hdr),
@@ -1053,6 +1057,7 @@ int tipc_link_xmit(struct tipc_link *l, struct sk_buff_head *list,
 		return -EMSGSIZE;
 	}
 
+	imp = msg_importance(hdr);
 	/* Allow oversubscription of one data msg per source at congestion */
 	if (unlikely(l->backlog[imp].len >= l->backlog[imp].limit)) {
 		if (imp == TIPC_SYSTEM_IMPORTANCE) {