Bernard Pidoux reported a regression apparently caused by commit
c353e8983e0d ("net: introduce per netns packet chains").

skb->dev becomes NULL and we crash in __netif_receive_skb_core().

Before above commit, different kind of bugs or corruptions could happen
without a major crash.

But the root cause is that ax25_kiss_rcv() can queue/mangle input skb
without checking if this skb is shared or not.

Many thanks to Bernard Pidoux for his help, diagnosis and tests.

We had a similar issue years ago fixed with commit 7aaed57c5c28
("phonet: properly unshare skbs in phonet_rcv()").

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Bernard Pidoux <redacted>
Closes: https://lore.kernel.org/netdev/1713f383-c538-4918-bc64-13b3288cd542@free.fr/ (local)
Tested-by: Bernard Pidoux <redacted>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Joerg Reuter <redacted>
Cc: linux-hams@vger.kernel.org
Cc: David Ranch <redacted>
Cc: Dan Cross <redacted>
Cc: Folkert van Heusden <redacted>
---
 net/ax25/ax25_in.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ax25/ax25_in.c b/net/ax25/ax25_in.c
index 1cac25aca637..f2d66af86359 100644
--- a/net/ax25/ax25_in.c
+++ b/net/ax25/ax25_in.c

@@ -433,6 +433,10 @@ static int ax25_rcv(struct sk_buff *skb, struct net_device *dev,
 int ax25_kiss_rcv(struct sk_buff *skb, struct net_device *dev,
 		  struct packet_type *ptype, struct net_device *orig_dev)
 {
+	skb = skb_share_check(skb, GFP_ATOMIC);
+	if (!skb)
+		return NET_RX_DROP;
+
 	skb_orphan(skb);
 
 	if (!net_eq(dev_net(dev), &init_net)) {

-- 
2.51.0.318.gd7df087d1a-goog