Re: [PATCH v2 net 5/7] virtio_net: Don't process redirected XDP frames when XDP is disabled

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 29, 2019 at 09:45:57AM +0900, Toshiaki Makita wrote:
> Commit 8dcc5b0ab0ec ("virtio_net: fix ndo_xdp_xmit crash towards dev not
> ready for XDP") tried to avoid access to unexpected sq while XDP is
> disabled, but was not complete.
> 
> There was a small window which causes out of bounds sq access in
> virtnet_xdp_xmit() while disabling XDP.
> 
> An example case of
>  - curr_queue_pairs = 6 (2 for SKB and 4 for XDP)
>  - online_cpu_num = xdp_queue_paris = 4
> when XDP is enabled:
> 
> CPU 0                         CPU 1
> (Disabling XDP)               (Processing redirected XDP frames)
> 
>                               virtnet_xdp_xmit()
> virtnet_xdp_set()
>  _virtnet_set_queues()
>   set curr_queue_pairs (2)
>                                check if rq->xdp_prog is not NULL
>                                virtnet_xdp_sq(vi)
>                                 qp = curr_queue_pairs -
>                                      xdp_queue_pairs +
>                                      smp_processor_id()
>                                    = 2 - 4 + 1 = -1
>                                 sq = &vi->sq[qp] // out of bounds access
>   set xdp_queue_pairs (0)
>   rq->xdp_prog = NULL
> 
> Basically we should not change curr_queue_pairs and xdp_queue_pairs
> while someone can read the values. Thus, when disabling XDP, assign NULL
> to rq->xdp_prog first, and wait for RCU grace period, then change
> xxx_queue_pairs.
> Note that we need to keep the current order when enabling XDP though.
> 
> - v2: Make rcu_assign_pointer/synchronize_net conditional instead of
>       _virtnet_set_queues.
> 
> Fixes: 186b3c998c50 ("virtio-net: support XDP_REDIRECT")
> Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  drivers/net/virtio_net.c | 33 ++++++++++++++++++++++++++-------
>  1 file changed, 26 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 669b65c..cea52e4 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2410,6 +2410,10 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>  		return -ENOMEM;
>  	}
>  
> +	old_prog = rtnl_dereference(vi->rq[0].xdp_prog);
> +	if (!prog && !old_prog)
> +		return 0;
> +
>  	if (prog) {
>  		prog = bpf_prog_add(prog, vi->max_queue_pairs - 1);
>  		if (IS_ERR(prog))
> @@ -2424,21 +2428,30 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>  		}
>  	}
>  
> +	if (!prog) {
> +		for (i = 0; i < vi->max_queue_pairs; i++) {
> +			rcu_assign_pointer(vi->rq[i].xdp_prog, prog);
> +			if (i == 0)
> +				virtnet_restore_guest_offloads(vi);
> +		}
> +		synchronize_net();
> +	}
> +
>  	err = _virtnet_set_queues(vi, curr_qp + xdp_qp);
>  	if (err)
>  		goto err;
>  	netif_set_real_num_rx_queues(dev, curr_qp + xdp_qp);
>  	vi->xdp_queue_pairs = xdp_qp;
>  
> -	for (i = 0; i < vi->max_queue_pairs; i++) {
> -		old_prog = rtnl_dereference(vi->rq[i].xdp_prog);
> -		rcu_assign_pointer(vi->rq[i].xdp_prog, prog);
> -		if (i == 0) {
> -			if (!old_prog)
> +	if (prog) {
> +		for (i = 0; i < vi->max_queue_pairs; i++) {
> +			rcu_assign_pointer(vi->rq[i].xdp_prog, prog);
> +			if (i == 0 && !old_prog)
>  				virtnet_clear_guest_offloads(vi);
> -			if (!prog)
> -				virtnet_restore_guest_offloads(vi);
>  		}
> +	}
> +
> +	for (i = 0; i < vi->max_queue_pairs; i++) {
>  		if (old_prog)
>  			bpf_prog_put(old_prog);
>  		if (netif_running(dev)) {
> @@ -2451,6 +2464,12 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>  	return 0;
>  
>  err:
> +	if (!prog) {
> +		virtnet_clear_guest_offloads(vi);
> +		for (i = 0; i < vi->max_queue_pairs; i++)
> +			rcu_assign_pointer(vi->rq[i].xdp_prog, old_prog);
> +	}
> +
>  	if (netif_running(dev)) {
>  		for (i = 0; i < vi->max_queue_pairs; i++) {
>  			virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
> -- 
> 1.8.3.1
>