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

[Deepanshu Kartikey]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=431f9a9e3f5227fbb904
Fixes: 1f5be6b3b063 ("udp: Implement udp_bpf_recvmsg() for sockmap")
Signed-off-by: Deepanshu Kartikey <kartikey406@xxxxxxxxx>
---
 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;
-- 
2.43.0