Re: [PATCH] udp_bpf: fix use-after-free in udp_bpf_recvmsg()

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 4/2/26 1:09 PM, Deepanshu Kartikey wrote:
> udp_bpf_recvmsg() calls sk_msg_recvmsg() without holding lock_sock(),
> unlike tcp_bpf_recvmsg() which properly acquires lock_sock() before
> calling __sk_msg_recvmsg(). This allows concurrent tasks to race inside
> sk_msg_recvmsg() on the same psock ingress queue, where one task can
> free msg_rx via kfree_sk_msg() while another task is still reading it
> via sk_msg_elem(), causing a slab-use-after-free.
>
> Fix this by adding lock_sock()/release_sock() around the sk_msg_recvmsg()
> path in udp_bpf_recvmsg(), consistent with tcp_bpf_recvmsg(). Also make
> udp_msg_wait_data() release lock_sock() before sleeping and reacquire it
> after waking, so it can be called with the socket lock held, consistent
> with how tcp_msg_wait_data() uses sk_wait_event() which does the same
> internally.
>
> Note: syzbot testing shows a separate pre-existing warning:
>    sk->sk_forward_alloc
>    WARNING: net/ipv4/af_inet.c:162 inet_sock_destruct
> This warning triggers from the idle CPU path (pv_native_safe_halt)
> and is unrelated to this patch. It appears to be a pre-existing
> memory accounting issue in the UDP sockmap path that requires
> separate investigation.
>
>
> Reported-by: syzbot+431f9a9e3f5227fbb904@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=431f9a9e3f5227fbb904
> Fixes: 1f5be6b3b063 ("udp: Implement udp_bpf_recvmsg() for sockmap")
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
> ---
>   net/ipv4/udp_bpf.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/net/ipv4/udp_bpf.c b/net/ipv4/udp_bpf.c
> index 9f33b07b1481..f924b255cee6 100644
> --- a/net/ipv4/udp_bpf.c
> +++ b/net/ipv4/udp_bpf.c
> @@ -50,7 +50,9 @@ static int udp_msg_wait_data(struct sock *sk, struct sk_psock *psock,
>   	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
>   	ret = udp_msg_has_data(sk, psock);
>   	if (!ret) {
> +		release_sock(sk);
>   		wait_woken(&wait, TASK_INTERRUPTIBLE, timeo);
> +		lock_sock(sk);
>   		ret = udp_msg_has_data(sk, psock);
>   	}
>   	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> @@ -79,6 +81,7 @@ static int udp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>   		goto out;
>   	}
>   
> +	lock_sock(sk);
>   msg_bytes_ready:
>   	copied = sk_msg_recvmsg(sk, psock, msg, len, flags);
>   	if (!copied) {
> @@ -90,12 +93,14 @@ static int udp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>   		if (data) {
>   			if (psock_has_data(psock))
>   				goto msg_bytes_ready;
> +			release_sock(sk);
>   			ret = sk_udp_recvmsg(sk, msg, len, flags);
>   			goto out;
>   		}
>   		copied = -EAGAIN;
>   	}
>   	ret = copied;
> +	release_sock(sk);
>   out:
>   	sk_psock_put(sk, psock);
>   	return ret;

Kuniyuki  is already working on this. Please see the
existing discussion.

https://lore.kernel.org/bpf/20260221233234.3814768-4-kuniyu@google.com/