Re: [PATCH] virtio_vsock: Fix race condition in virtio_transport_recv_pkt

[Stefano Garzarella <sgarzare@redhat.com>]

Hi Jia,
thanks for the patch! I have some comments.

On Fri, May 29, 2020 at 09:31:23PM +0800, Jia He wrote:
> When client tries to connect(SOCK_STREAM) the server in the guest with NONBLOCK
> mode, there will be a panic on a ThunderX2 (armv8a server):
> [  463.718844][ T5040] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
> [  463.718848][ T5040] Mem abort info:
> [  463.718849][ T5040]   ESR = 0x96000044
> [  463.718852][ T5040]   EC = 0x25: DABT (current EL), IL = 32 bits
> [  463.718853][ T5040]   SET = 0, FnV = 0
> [  463.718854][ T5040]   EA = 0, S1PTW = 0
> [  463.718855][ T5040] Data abort info:
> [  463.718856][ T5040]   ISV = 0, ISS = 0x00000044
> [  463.718857][ T5040]   CM = 0, WnR = 1
> [  463.718859][ T5040] user pgtable: 4k pages, 48-bit VAs, pgdp=0000008f6f6e9000
> [  463.718861][ T5040] [0000000000000000] pgd=0000000000000000
> [  463.718866][ T5040] Internal error: Oops: 96000044 [#1] SMP
> [...]
> [  463.718977][ T5040] CPU: 213 PID: 5040 Comm: vhost-5032 Tainted: G           O      5.7.0-rc7+ #139
> [  463.718980][ T5040] Hardware name: GIGABYTE R281-T91-00/MT91-FS1-00, BIOS F06 09/25/2018
> [  463.718982][ T5040] pstate: 60400009 (nZCv daif +PAN -UAO)
> [  463.718995][ T5040] pc : virtio_transport_recv_pkt+0x4c8/0xd40 [vmw_vsock_virtio_transport_common]
> [  463.718999][ T5040] lr : virtio_transport_recv_pkt+0x1fc/0xd40 [vmw_vsock_virtio_transport_common]
> [  463.719000][ T5040] sp : ffff80002dbe3c40
> [...]
> [  463.719025][ T5040] Call trace:
> [  463.719030][ T5040]  virtio_transport_recv_pkt+0x4c8/0xd40 [vmw_vsock_virtio_transport_common]
> [  463.719034][ T5040]  vhost_vsock_handle_tx_kick+0x360/0x408 [vhost_vsock]
> [  463.719041][ T5040]  vhost_worker+0x100/0x1a0 [vhost]
> [  463.719048][ T5040]  kthread+0x128/0x130
> [  463.719052][ T5040]  ret_from_fork+0x10/0x18
> 
> The race condition as follows:
> Task1                            Task2
> =====                            =====
> __sock_release                   virtio_transport_recv_pkt
>   __vsock_release                  vsock_find_bound_socket (found)
>     lock_sock_nested
>     vsock_remove_sock
>     sock_orphan
>       sk_set_socket(sk, NULL)
>     ...
>     release_sock
>                                 lock_sock
>                                    virtio_transport_recv_connecting
>                                      sk->sk_socket->state (panic)
> 
> This fixes it by checking vsk again whether it is in bound/connected table.
> 
> Signed-off-by: Jia He <justin.he@arm.com>
> Cc: stable@vger.kernel.org
> ---
>  net/vmw_vsock/virtio_transport_common.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
> index 69efc891885f..0dbd6a45f0ed 100644
> --- a/net/vmw_vsock/virtio_transport_common.c
> +++ b/net/vmw_vsock/virtio_transport_common.c
> @@ -1132,6 +1132,17 @@ void virtio_transport_recv_pkt(struct virtio_transport *t,
>  
>  	lock_sock(sk);
>  
> +	/* Check it again if vsk is removed by vsock_remove_sock */
> +	spin_lock_bh(&vsock_table_lock);
> +	if (!__vsock_in_bound_table(vsk) && !__vsock_in_connected_table(vsk)) {
> +		spin_unlock_bh(&vsock_table_lock);
> +		(void)virtio_transport_reset_no_sock(t, pkt);
> +		release_sock(sk);
> +		sock_put(sk);
> +		goto free_pkt;
> +	}
> +	spin_unlock_bh(&vsock_table_lock);
> +

As an a simpler alternative, can we check the sk_shutdown or the socket
state without check again both bound and connected tables?

This is a data path, so we should take it faster.

I mean something like this:

	if (sk->sk_shutdown == SHUTDOWN_MASK) {
		...
	}

or

	if (sock_flag(sk, SOCK_DEAD)) {
		...
	}

I prefer the first option, but I think also the second option should
work.

Thanks,
Stefano

>  	/* Update CID in case it has changed after a transport reset event */
>  	vsk->local_addr.svm_cid = dst.svm_cid;
>  
> -- 
> 2.17.1
>