CVE-2025-39848 — ax25: properly unshare skbs in ax25_kiss_rcv()

Affected: Linux, Siemens SIMATIC CN 4100

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()").

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