[PATCH net] vsock: Fix memory leak in vmci_transport_recv_dgram_cb()

During the closure of a datagram socket, the vmci_transport_recv_dgram_cb()
function may be called, which will add the packets
to the socket's backlog; then, after the receive queue is cleared,
the __release_sock() function will move the packet back from the socket's
backlog to the receive queue, which will lead to a memory leak.

sock_close
  __sock_release
    __vsock_release
      // take ownership by user-space
      lock_sock_nested
      sock_set_flag(sk, SOCK_DEAD)
      vmci_transport_release
                                      vmci_dispatch_dgs
                                        vmci_datagram_invoke_guest_handler
                                          vmci_transport_recv_dgram_cb
                                            sk_receive_skb
                                              if (!sock_owned_by_user(sk))
                                                  [...]
                                         (!)  else if sk_add_backlog()
                                                  [...]
  (!) skb_queue_purge(&sk->sk_receive_queue)

      release_sock
        __release_sock
          // move all packets from the backlog
          // to the receive queue
   (!)    sk_backlog_rcv
            vsock_queue_rcv_skb
            [...]
              __sock_queue_rcv_skb
                if (!sock_flag(sk, SOCK_DEAD))
                  // Since the socket is "dead", the function will not
                  // be called
                  sk->sk_data_ready(sk)
        sock_release_ownership(sk);
        sock_put

Add a receive queue cleanup in the socket destructor to fix this.

syzkaller report:

unreferenced object 0xffff8880117bfb80 (size 240):
  comm "irq/56-vmw_vmci", pid 205, jiffies 4295064448 (age 73.992s)
  hex dump (first 32 bytes):
    78 dc 26 1e 80 88 ff ff 78 dc 26 1e 80 88 ff ff  x.&.....x.&.....
    00 00 00 00 00 00 00 00 c0 da 26 1e 80 88 ff ff  ..........&.....
  backtrace:
    [<ffffffff82be8d97>] __alloc_skb+0x287/0x330 net/core/skbuff.c:505
    [<ffffffffa114bcbd>] vmci_transport_recv_dgram_cb+0xbd/0x210 [vmw_vsock_vmci_transport]
    [<ffffffffa05072c6>] vmci_datagram_invoke_guest_handler+0x366/0x450 [vmw_vmci]
    [<ffffffffa050ad75>] vmci_dispatch_dgs+0x235/0x490 [vmw_vmci]
    [<ffffffffa050b025>] vmci_interrupt+0x55/0x290 [vmw_vmci]
    [<ffffffff8137ad2b>] irq_thread_fn+0x8b/0x1a0 kernel/irq/manage.c:1205
    [<ffffffff8137c7ae>] irq_thread+0x28e/0x530 kernel/irq/manage.c:1314
    [<ffffffff81253dd6>] kthread+0x2e6/0x3a0 kernel/kthread.c:376
    [<ffffffff81004392>] ret_from_fork+0x22/0x30 arch/x86/entry/entry_64.S:295

Found by InfoTeCS on behalf of Linux Verification Center
(linuxtesting.org) with Syzkaller.

Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")
Signed-off-by: Ilia Gavrilov <redacted>
---
 net/vmw_vsock/af_vsock.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 9b71479a2b29..fbafd882a12c 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c

@@ -1048,6 +1048,7 @@ static void vsock_sk_destruct(struct sock *sk)
 {
 	struct vsock_sock *vsk = vsock_sk(sk);
 
+	__skb_queue_purge(&sk->sk_receive_queue);
 	/* Flush MSG_ZEROCOPY leftovers. */
 	__skb_queue_purge(&sk->sk_error_queue);
 

-- 
2.47.3