Linux kernel BPF bpf_skb_check_mtu transport_header validation flaw
CVE-2025-68363 is a flaw in the Linux kernel BPF subsystem, in net/core/filter.c, where the bpf_skb_check_mtu helper may use skb->transport_header when invoked with the BPF_MTU_CHK_SEGS flag without first ensuring that the transport header has been set. The issue was introduced in Linux kernel 5.12 by commit 34b2021cc61642d61c3cf943d9e71925b827941b. Under the documented conditions, skb->transport_header can be unset, and the helper's use of it can trigger a WARN_ON_ONCE condition via skb_gso_validate_network_len. The reported reproducer involves CONFIG_DEBUG_NET enabled, skb->gso_size set, and use of bpf_prog_test_run. The upstream fix adds a skb_transport_header_was_set() check immediately before using skb->transport_header, preserving compatibility with existing BPF programs.
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