ax25: properly unshare skbs in ax25_kiss_rcv()

[ Upstream commit 8156210 ]

Bernard Pidoux reported a regression apparently caused by commit
c353e89 ("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 7aaed57
("phonet: properly unshare skbs in phonet_rcv()").

Fixes: 1da177e ("Linux-2.6.12-rc2")
Reported-by: Bernard Pidoux <f6bvp@free.fr>
Closes: https://lore.kernel.org/netdev/1713f383-c538-4918-bc64-13b3288cd542@free.fr/
Tested-by: Bernard Pidoux <f6bvp@free.fr>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Joerg Reuter <jreuter@yaina.de>
Cc: David Ranch <dranch@trinnet.net>
Cc: Folkert van Heusden <folkert@vanheusden.com>
Reviewed-by: Dan Cross <crossd@gmail.com>
Link: https://patch.msgid.link/20250902124642.212705-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 89064cf534bea4bb28c83fe6bbb26657b19dd5fe)

Author: edumazet

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)) {