[PATCH net v2 3/3] vsock/bpf: Fix bpf recvmsg() racing transport reassignment

[Michal Luczaj <mhal@xxxxxxx>]

Signal delivery during connect() may lead to a disconnect of an already
established socket. That involves removing socket from any sockmap and
resetting state to SS_UNCONNECTED. While it correctly restores socket's
proto, a call to vsock_bpf_recvmsg() might have been already under way in
another thread. If the connect()ing thread reassigns the vsock transport to
NULL, the recvmsg()ing thread may trigger a WARN_ON_ONCE.

connect
  / state = SS_CONNECTED /
                                sock_map_update_elem
                                vsock_bpf_recvmsg
                                  psock = sk_psock_get()
  lock sk
  if signal_pending
    unhash
      sock_map_remove_links
    state = SS_UNCONNECTED
  release sk

connect
  transport = NULL
                                  lock sk
                                  WARN_ON_ONCE(!vsk->transport)

Protect recvmsg() from racing against transport reassignment. Enforce the
sockmap invariant that psock implies transport: lock socket before getting
psock.

WARNING: CPU: 9 PID: 1222 at net/vmw_vsock/vsock_bpf.c:92 vsock_bpf_recvmsg+0xb55/0xe00
CPU: 9 UID: 0 PID: 1222 Comm: a.out Not tainted 6.14.0-rc5+
RIP: 0010:vsock_bpf_recvmsg+0xb55/0xe00
 sock_recvmsg+0x1b2/0x220
 __sys_recvfrom+0x190/0x270
 __x64_sys_recvfrom+0xdc/0x1b0
 do_syscall_64+0x93/0x1b0
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fixes: 634f1a7110b4 ("vsock: support sockmap")
Signed-off-by: Michal Luczaj <mhal@xxxxxxx>
---
 net/vmw_vsock/vsock_bpf.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/net/vmw_vsock/vsock_bpf.c b/net/vmw_vsock/vsock_bpf.c
index c68fdaf09046b68254dac3ea70ffbe73dfa45cef..5138195d91fb258d4bc09b48e80e13651d62863a 100644
--- a/net/vmw_vsock/vsock_bpf.c
+++ b/net/vmw_vsock/vsock_bpf.c
@@ -73,28 +73,35 @@ static int __vsock_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int
 	return err;
 }
 
-static int vsock_bpf_recvmsg(struct sock *sk, struct msghdr *msg,
-			     size_t len, int flags, int *addr_len)
+static int vsock_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
+			     int flags, int *addr_len)
 {
 	struct sk_psock *psock;
 	struct vsock_sock *vsk;
 	int copied;
 
+	/* Since signal delivery during connect() may reset the state of socket
+	 * that's already in a sockmap, take the lock before checking on psock.
+	 * This serializes a possible transport reassignment, protecting this
+	 * function from running with NULL transport.
+	 */
+	lock_sock(sk);
+
 	psock = sk_psock_get(sk);
-	if (unlikely(!psock))
+	if (unlikely(!psock)) {
+		release_sock(sk);
 		return __vsock_recvmsg(sk, msg, len, flags);
+	}
 
-	lock_sock(sk);
 	vsk = vsock_sk(sk);
-
 	if (WARN_ON_ONCE(!vsk->transport)) {
 		copied = -ENODEV;
 		goto out;
 	}
 
 	if (vsock_has_data(sk, psock) && sk_psock_queue_empty(psock)) {
-		release_sock(sk);
 		sk_psock_put(sk, psock);
+		release_sock(sk);
 		return __vsock_recvmsg(sk, msg, len, flags);
 	}
 
@@ -108,8 +115,8 @@ static int vsock_bpf_recvmsg(struct sock *sk, struct msghdr *msg,
 		}
 
 		if (sk_psock_queue_empty(psock)) {
-			release_sock(sk);
 			sk_psock_put(sk, psock);
+			release_sock(sk);
 			return __vsock_recvmsg(sk, msg, len, flags);
 		}
 
@@ -117,8 +124,8 @@ static int vsock_bpf_recvmsg(struct sock *sk, struct msghdr *msg,
 	}
 
 out:
-	release_sock(sk);
 	sk_psock_put(sk, psock);
+	release_sock(sk);
 
 	return copied;
 }

-- 
2.48.1