Re: [PATCH bpf-next] xsk: proper socket state check in xsk_poll

[Daniel Borkmann <daniel@xxxxxxxxxxxxx>]

On 8/20/19 12:04 PM, Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@xxxxxxxxx>
>
> The poll() implementation for AF_XDP sockets did not perform the
> proper state checks, prior accessing the socket umem. This patch fixes
> that by performing a xsk_is_bound() check.
>
> Suggested-by: Hillf Danton <hdanton@xxxxxxxx>
> Reported-by: syzbot+c82697e3043781e08802@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 77cd0d7b3f25 ("xsk: add support for need_wakeup flag in AF_XDP rings")
> Signed-off-by: Björn Töpel <bjorn.topel@xxxxxxxxx>
> ---
>   net/xdp/xsk.c | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index ee4428a892fa..08bed5e92af4 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -356,13 +356,20 @@ static int xsk_generic_xmit(struct sock *sk, struct msghdr *m,
>   	return err;
>   }
>   
> +static bool xsk_is_bound(struct xdp_sock *xs)
> +{
> +	struct net_device *dev = READ_ONCE(xs->dev);
> +
> +	return dev && xs->state == XSK_BOUND;
> +}
> +
>   static int xsk_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
>   {
>   	bool need_wait = !(m->msg_flags & MSG_DONTWAIT);
>   	struct sock *sk = sock->sk;
>   	struct xdp_sock *xs = xdp_sk(sk);
>   
> -	if (unlikely(!xs->dev))
> +	if (unlikely(!xsk_is_bound(xs)))
>   		return -ENXIO;
>   	if (unlikely(!(xs->dev->flags & IFF_UP)))
>   		return -ENETDOWN;
> @@ -383,6 +390,9 @@ static unsigned int xsk_poll(struct file *file, struct socket *sock,
>   	struct net_device *dev = xs->dev;
>   	struct xdp_umem *umem = xs->umem;
>   
> +	if (unlikely(!xsk_is_bound(xs)))
> +		return mask;
> +
>   	if (umem->need_wakeup)
>   		dev->netdev_ops->ndo_xsk_wakeup(dev, xs->queue_id,
>   						umem->need_wakeup);
> @@ -417,7 +427,7 @@ static void xsk_unbind_dev(struct xdp_sock *xs)
>   {
>   	struct net_device *dev = xs->dev;
>   
> -	if (!dev || xs->state != XSK_BOUND)
> +	if (!xsk_is_bound(xs))
>   		return;

I think I'm a bit confused by your READ_ONCE() usage. ;-/ I can see why you're
using it in xsk_is_bound() above, but then at the same time all the other callbacks
like xsk_poll() or xsk_unbind_dev() above have a struct net_device *dev = xs->dev
right before the test. Could you elaborate?

Thanks,
Daniel