ax25: properly unshare skbs in ax25_kiss_rcv()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: ax25: properly unshare skbs in ax25_kiss_rcv() 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()").

References

• https://git.kernel.org/stable/c/42b46684e2c78ee052d8c2ee8d9c2089233c9094
• https://git.kernel.org/stable/c/5b079be1b9da49ad88fc304c874d4be7085f7883
• https://git.kernel.org/stable/c/2bd0f67212908243ce88e35bf69fa77155b47b14
• https://git.kernel.org/stable/c/01a2984cb803f2d487b7074f9718db2bf3531f69
• https://git.kernel.org/stable/c/7d449b7a6c8ee434d10a483feed7c5c50108cf56
• https://git.kernel.org/stable/c/89064cf534bea4bb28c83fe6bbb26657b19dd5fe
• https://git.kernel.org/stable/c/b1c71d674a308d2fbc83efcf88bfc4217a86aa17
• https://git.kernel.org/stable/c/8156210d36a43e76372312c87eb5ea3dbb405a85