CVE-2025-39847: Missing Release of Memory after Effective Lifetime

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

ppp: fix memory leak in pad_compress_skb

If alloc_skb() fails in pad_compress_skb(), it returns NULL without releasing the old skb. The caller does:

skb = pad_compress_skb(ppp, skb); if (!skb) goto drop;

drop: kfree_skb(skb);

When pad_compress_skb() returns NULL, the reference to the old skb is lost and kfree_skb(skb) ends up doing nothing, leading to a memory leak.

Align pad_compress_skb() semantics with realloc(): only free the old skb if allocation and compression succeed. At the call site, use the new_skb variable so the original skb is not lost when pad_compress_skb() fails.