In the Linux kernel, the following vulnerability has been resolved:

net/x25: Fix potential double free of skb

When allocskb fails in x25queuerxframe it calls kfreeskb(skb) at line 48 and returns 1 (error). This error propagates back through the call chain:

x25queuerxframe returns 1 | v x25state3machine receives the return value 1 and takes the else branch at line 278, setting queued=0 and returning 0 | v x25processrxframe returns queued=0 | v x25backlogrcv at line 452 sees queued=0 and calls kfreeskb(skb) again

This would free the same skb twice. Looking at x25backlogrcv:

net/x25/x25in.c:x25backlogrcv() { ... queued = x25processrxframe(sk, skb); ... if (!queued) kfreeskb(skb); }