Re: [PATCH bpf] xdp: fix potential deadlock on socket mutex

["Jonathan Lemon" <jonathan.lemon@gmail.com>]

On 8 Jul 2019, at 4:03, Ilya Maximets wrote:

> There are 2 call chains:
>
>   a) xsk_bind --> xdp_umem_assign_dev
>   b) unregister_netdevice_queue --> xsk_notifier
>
> with the following locking order:
>
>   a) xs->mutex --> rtnl_lock
>   b) rtnl_lock --> xdp.lock --> xs->mutex
>
> Different order of taking 'xs->mutex' and 'rtnl_lock' could produce a
> deadlock here. Fix that by moving the 'rtnl_lock' before 'xs->lock' in
> the bind call chain (a).
>
> Reported-by: syzbot+bf64ec93de836d7f4c2c@syzkaller.appspotmail.com
> Fixes: 455302d1c9ae ("xdp: fix hang while unregistering device bound 
> to xdp socket")
> Signed-off-by: Ilya Maximets <i.maximets@samsung.com>

Thanks, Ilya!

I think in the long run the locking needs to be revisited,
but this should fix the deadlock for now.

Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>



> This patch is a fix for patch that is not yet in mainline, but
> already in 'net' tree. I'm not sure what is the correct process
> for applying such fixes.
>
>  net/xdp/xdp_umem.c | 16 ++++++----------
>  net/xdp/xsk.c      |  2 ++
>  2 files changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/net/xdp/xdp_umem.c b/net/xdp/xdp_umem.c
> index 20c91f02d3d8..83de74ca729a 100644
> --- a/net/xdp/xdp_umem.c
> +++ b/net/xdp/xdp_umem.c
> @@ -87,21 +87,20 @@ int xdp_umem_assign_dev(struct xdp_umem *umem, 
> struct net_device *dev,
>  	struct netdev_bpf bpf;
>  	int err = 0;
>
> +	ASSERT_RTNL();
> +
>  	force_zc = flags & XDP_ZEROCOPY;
>  	force_copy = flags & XDP_COPY;
>
>  	if (force_zc && force_copy)
>  		return -EINVAL;
>
> -	rtnl_lock();
> -	if (xdp_get_umem_from_qid(dev, queue_id)) {
> -		err = -EBUSY;
> -		goto out_rtnl_unlock;
> -	}
> +	if (xdp_get_umem_from_qid(dev, queue_id))
> +		return -EBUSY;
>
>  	err = xdp_reg_umem_at_qid(dev, umem, queue_id);
>  	if (err)
> -		goto out_rtnl_unlock;
> +		return err;
>
>  	umem->dev = dev;
>  	umem->queue_id = queue_id;
> @@ -110,7 +109,7 @@ int xdp_umem_assign_dev(struct xdp_umem *umem, 
> struct net_device *dev,
>
>  	if (force_copy)
>  		/* For copy-mode, we are done. */
> -		goto out_rtnl_unlock;
> +		return 0;
>
>  	if (!dev->netdev_ops->ndo_bpf ||
>  	    !dev->netdev_ops->ndo_xsk_async_xmit) {
> @@ -125,7 +124,6 @@ int xdp_umem_assign_dev(struct xdp_umem *umem, 
> struct net_device *dev,
>  	err = dev->netdev_ops->ndo_bpf(dev, &bpf);
>  	if (err)
>  		goto err_unreg_umem;
> -	rtnl_unlock();
>
>  	umem->zc = true;
>  	return 0;
> @@ -135,8 +133,6 @@ int xdp_umem_assign_dev(struct xdp_umem *umem, 
> struct net_device *dev,
>  		err = 0; /* fallback to copy mode */
>  	if (err)
>  		xdp_clear_umem_at_qid(dev, queue_id);
> -out_rtnl_unlock:
> -	rtnl_unlock();
>  	return err;
>  }
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 703cf5ea448b..2aa6072a3e55 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -416,6 +416,7 @@ static int xsk_bind(struct socket *sock, struct 
> sockaddr *addr, int addr_len)
>  	if (flags & ~(XDP_SHARED_UMEM | XDP_COPY | XDP_ZEROCOPY))
>  		return -EINVAL;
>
> +	rtnl_lock();
>  	mutex_lock(&xs->mutex);
>  	if (xs->state != XSK_READY) {
>  		err = -EBUSY;
> @@ -501,6 +502,7 @@ static int xsk_bind(struct socket *sock, struct 
> sockaddr *addr, int addr_len)
>  		xs->state = XSK_BOUND;
>  out_release:
>  	mutex_unlock(&xs->mutex);
> +	rtnl_unlock();
>  	return err;
>  }
>
> -- 
> 2.17.1