Linux kernel tcp_bpf improper cleanup on psock->cork allocation failure
CVE-2025-39913 is a vulnerability in the Linux kernel tcp_bpf subsystem. In the affected code path, tcp_bpf_send_verdict() attempts to allocate psock->cork to retain data when a sk_msg BPF program uses bpf_msg_cork_bytes() and the transmitted data is smaller than the configured cork threshold, causing the data to be carried over to a subsequent sendmsg(). If allocation of psock->cork fails, the failure could occur silently due to fault injection combined with __GFP_NOWARN, and the code did not properly unwind state established by sk_msg_alloc(). Specifically, the sk->sk_forward_alloc accounting change was not reverted because sk_msg_free() was not called. The fix adds a call to sk_msg_free() when tcp_bpf_send_verdict() cannot allocate psock->cork and resets *copied to 0 so sendmsg() can return a proper error. The bug was reported by syzbot and manifested as a kernel warning during socket destruction at inet_sock_destruct().
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