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: af9b028e270fd ("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>

diff --git a/net/tipc/link.c b/net/tipc/link.c
index 6ae2140eb4f7498f262bcb722f5d365188b20794..a6a694b789274532bc95d753158f3da533a51892 100644 (file)
--- a/net/tipc/link.c
+++ b/net/tipc/link.c
@@ -1030,7 +1030,6 @@ 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;
@@ -1038,13 +1037,18 @@ int tipc_link_xmit(struct tipc_link *l, struct sk_buff_head *list,
        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) {