Re: [PATCH net-next 16/24] net: netkit, veth, tun, virt*: Use nested-BH locking for XDP redirect.

[Daniel Borkmann]

Hi Sebastian,

On 12/15/23 6:07 PM, Sebastian Andrzej Siewior wrote:
> The per-CPU variables used during bpf_prog_run_xdp() invocation and
> later during xdp_do_redirect() rely on disabled BH for their protection.
> Without locking in local_bh_disable() on PREEMPT_RT these data structure
> require explicit locking.
>
> This is a follow-up on the previous change which introduced
> bpf_run_lock.redirect_lock and uses it now within drivers.
>
> The simple way is to acquire the lock before bpf_prog_run_xdp() is
> invoked and hold it until the end of function.
> This does not always work because some drivers (cpsw, atlantic) invoke
> xdp_do_flush() in the same context.
> Acquiring the lock in bpf_prog_run_xdp() and dropping in
> xdp_do_redirect() (without touching drivers) does not work because not
> all driver, which use bpf_prog_run_xdp(), do support XDP_REDIRECT (and
> invoke xdp_do_redirect()).
>
> Ideally the minimal locking scope would be bpf_prog_run_xdp() +
> xdp_do_redirect() and everything else (error recovery, DMA unmapping,
> free/ alloc of memory, …) would happen outside of the locked section.
[...]

>   drivers/net/hyperv/netvsc_bpf.c |  1 +
>   drivers/net/netkit.c            | 13 +++++++----
>   drivers/net/tun.c               | 28 +++++++++++++----------
>   drivers/net/veth.c              | 40 ++++++++++++++++++++-------------
>   drivers/net/virtio_net.c        |  1 +
>   drivers/net/xen-netfront.c      |  1 +
>   6 files changed, 52 insertions(+), 32 deletions(-)
[...]

Please exclude netkit from this set given it does not support XDP, but
instead only accepts tc BPF typed programs.

Thanks,
Daniel

> diff --git a/drivers/net/netkit.c b/drivers/net/netkit.c
> index 39171380ccf29..fbcf78477bda8 100644
> --- a/drivers/net/netkit.c
> +++ b/drivers/net/netkit.c
> @@ -80,8 +80,15 @@ static netdev_tx_t netkit_xmit(struct sk_buff *skb, struct net_device *dev)
>   	netkit_prep_forward(skb, !net_eq(dev_net(dev), dev_net(peer)));
>   	skb->dev = peer;
>   	entry = rcu_dereference(nk->active);
> -	if (entry)
> -		ret = netkit_run(entry, skb, ret);
> +	if (entry) {
> +		scoped_guard(local_lock_nested_bh, &bpf_run_lock.redirect_lock) {
> +			ret = netkit_run(entry, skb, ret);
> +			if (ret == NETKIT_REDIRECT) {
> +				dev_sw_netstats_tx_add(dev, 1, len);
> +				skb_do_redirect(skb);
> +			}
> +		}
> +	}
>   	switch (ret) {
>   	case NETKIT_NEXT:
>   	case NETKIT_PASS:
> @@ -95,8 +102,6 @@ static netdev_tx_t netkit_xmit(struct sk_buff *skb, struct net_device *dev)
>   		}
>   		break;
>   	case NETKIT_REDIRECT:
> -		dev_sw_netstats_tx_add(dev, 1, len);
> -		skb_do_redirect(skb);
>   		break;
>   	case NETKIT_DROP:
>   	default: