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

[syzbot <syzbot+431f9a9e3f5227fbb904@syzkaller.appspotmail.com>]

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Subject: [PATCH] udp_bpf: fix use-after-free in udp_bpf_recvmsg()
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git main


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() pairs around the recvmsg
path. Since udp_msg_wait_data() does not release lock_sock() internally
before sleeping (unlike tcp_msg_wait_data()), we must release the socket
lock manually before calling udp_msg_wait_data() and reacquire it after
to avoid a deadlock with the ingress path delivering new data.

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 | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/ipv4/udp_bpf.c b/net/ipv4/udp_bpf.c
index 9f33b07b1481..5e6d82816103 100644
--- a/net/ipv4/udp_bpf.c
+++ b/net/ipv4/udp_bpf.c
@@ -74,7 +74,9 @@ static int udp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 	if (unlikely(!psock))
 		return sk_udp_recvmsg(sk, msg, len, flags);
 
+	lock_sock(sk);
 	if (!psock_has_data(psock)) {
+		release_sock(sk);
 		ret = sk_udp_recvmsg(sk, msg, len, flags);
 		goto out;
 	}
@@ -86,16 +88,20 @@ static int udp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 		int data;
 
 		timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
+		release_sock(sk);
 		data = udp_msg_wait_data(sk, psock, timeo);
+		lock_sock(sk);
 		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