Linux Virtualization - May 2020 - [PATCH v3] virtio_vsock: Fix race condition in virtio_transport_recv_pkt

From: Jia He <justin.he at arm.com>
Date: Sat, 30 May 2020 09:38:28 +0800
> When client on the host tries to connect(SOCK_STREAM, O_NONBLOCK) to the
> server on the guest, there will be a panic on a ThunderX2 (armv8a server):
 ...
> The race condition is as follows:
> Task1                                Task2
> =====                                ====> __sock_release               
virtio_transport_recv_pkt
>   __vsock_release                      vsock_find_bound_socket (found sk)
>     lock_sock_nested
>     vsock_remove_sock
>     sock_orphan
>       sk_set_socket(sk, NULL)
>     sk->sk_shutdown = SHUTDOWN_MASK
>     ...
>     release_sock
>                                     lock_sock
>                                        virtio_transport_recv_connecting
>                                          sk->sk_socket->state
(panic!)
> 
> The root cause is that vsock_find_bound_socket can't hold the
lock_sock,
> so there is a small race window between vsock_find_bound_socket() and
> lock_sock(). If __vsock_release() is running in another task,
> sk->sk_socket will be set to NULL inadvertently.
> 
> This fixes it by checking sk->sk_shutdown(suggested by Stefano) after
> lock_sock since sk->sk_shutdown is set to SHUTDOWN_MASK under the
> protection of lock_sock_nested.
> 
> Signed-off-by: Jia He <justin.he at arm.com>
> Reviewed-by: Stefano Garzarella <sgarzare at redhat.com>
Applied and queued up for -stable, thank you.