Re: [PATCH] bpf, sockmap: Fix sk_redir use-after-free in send verdict

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Sun Jul 19, 2026 at 11:22 AM EDT, Chengfeng Ye wrote:
> sk_psock_msg_verdict() takes a socket reference for psock->sk_redir.
> tcp_bpf_send_verdict() copies that pointer while holding the source socket
> lock, but does not take a reference for the local copy before dropping the
> lock around tcp_bpf_sendmsg_redir().
>
> When apply_bytes keeps the cached verdict active, another sendmsg() on the
> same source socket can consume the remaining bytes and release the cached
> reference while the first thread still holds only the raw local pointer:
>
>   CPU 0                                  CPU 1
>   sk_redir = psock->sk_redir
>   apply_bytes remains nonzero
>   release_sock(sk)
>                                          lock_sock(sk)
>                                          apply_bytes reaches zero
>                                          psock->sk_redir = NULL
>                                          release_sock(sk)
>                                          tcp_bpf_sendmsg_redir(sk_redir)
>                                          sock_put(sk_redir)
>   tcp_bpf_sendmsg_redir(sk_redir)
>
> The final sock_put() can free sk_redir before CPU 0 dereferences it.
>
> KASAN reported:
>
>   BUG: KASAN: slab-use-after-free in tcp_bpf_sendmsg_redir+0xf39/0x1020
>   Read of size 8 at addr ffff888108537090 by task poc/87
>   Call Trace:
>    tcp_bpf_sendmsg_redir+0xf39/0x1020
>    tcp_bpf_sendmsg+0x977/0x1a50
>    __sys_sendto+0x32c/0x3a0
>    __x64_sys_sendto+0xdb/0x1b0
>   Allocated by task 85:
>    sk_prot_alloc+0x56/0x210
>    sk_clone+0x6f/0x14b0
>    inet_csk_clone_lock+0x24/0x740
>    tcp_create_openreq_child+0x25/0x2710
>    tcp_v4_syn_recv_sock+0x10a/0xe00
>   Freed by task 0:
>    __kasan_slab_free+0x43/0x70
>    slab_free_after_rcu_debug+0xa6/0x1e0
>    rcu_core+0x50a/0x1850
>   Last potentially related work creation:
>    __sk_destruct+0x3da/0x540
>    sk_psock_destroy+0x81e/0xab0
>    process_one_work+0x63a/0x1070
>
> Take a temporary socket reference while the source socket lock still
> protects psock->sk_redir, and drop it after tcp_bpf_sendmsg_redir()
> returns.  This keeps each unlocked use independent of cached-verdict
> ownership.
>
> Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

> ---
>  net/ipv4/tcp_bpf.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> index 8e905b50dead..69cc8bc33bcd 100644
> --- a/net/ipv4/tcp_bpf.c
> +++ b/net/ipv4/tcp_bpf.c
> @@ -469,6 +469,7 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
>  	case __SK_REDIRECT:
>  		redir_ingress = psock->redir_ingress;
>  		sk_redir = psock->sk_redir;
> +		sock_hold(sk_redir);
>  		sk_msg_apply_bytes(psock, tosend);
>  		if (!psock->apply_bytes) {
>  			/* Clean up before releasing the sock lock. */
> @@ -489,6 +490,7 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
>  
>  		if (eval == __SK_REDIRECT)
>  			sock_put(sk_redir);
> +		sock_put(sk_redir);
>  
>  		lock_sock(sk);
>  		sk_mem_uncharge(sk, sent);