From: Johan Almbladh <johan.almbladh@xxxxxxxxxxxxxxxxx> [ Upstream commit 1fd6e5675336daf4747940b4285e84b0c114ae32 ] The byte queue limits (BQL) mechanism is intended to move queuing from the driver to the network stack in order to reduce latency caused by excessive queuing in hardware. However, when transmitting or redirecting a packet using generic XDP, the qdisc layer is bypassed and there are no additional queues. Since netif_xmit_stopped() also takes BQL limits into account, but without having any alternative queuing, packets are silently dropped. This patch modifies the drop condition to only consider cases when the driver itself cannot accept any more packets. This is analogous to the condition in __dev_direct_xmit(). Dropped packets are also counted on the device. Bypassing the qdisc layer in the generic XDP TX path means that XDP packets are able to starve other packets going through a qdisc, and DDOS attacks will be more effective. In-driver-XDP use dedicated TX queues, so they do not have this starvation issue. Signed-off-by: Johan Almbladh <johan.almbladh@xxxxxxxxxxxxxxxxx> Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx> Link: https://lore.kernel.org/bpf/20220705082345.2494312-1-johan.almbladh@xxxxxxxxxxxxxxxxx Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- net/core/dev.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/net/core/dev.c b/net/core/dev.c index 842917883adb..c908d1b3e3bd 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -4792,7 +4792,10 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb, } /* When doing generic XDP we have to bypass the qdisc layer and the - * network taps in order to match in-driver-XDP behavior. + * network taps in order to match in-driver-XDP behavior. This also means + * that XDP packets are able to starve other packets going through a qdisc, + * and DDOS attacks will be more effective. In-driver-XDP use dedicated TX + * queues, so they do not have this starvation issue. */ void generic_xdp_tx(struct sk_buff *skb, struct bpf_prog *xdp_prog) { @@ -4804,7 +4807,7 @@ void generic_xdp_tx(struct sk_buff *skb, struct bpf_prog *xdp_prog) txq = netdev_core_pick_tx(dev, skb, NULL); cpu = smp_processor_id(); HARD_TX_LOCK(dev, txq, cpu); - if (!netif_xmit_stopped(txq)) { + if (!netif_xmit_frozen_or_drv_stopped(txq)) { rc = netdev_start_xmit(skb, dev, txq, 0); if (dev_xmit_complete(rc)) free_skb = false; @@ -4812,6 +4815,7 @@ void generic_xdp_tx(struct sk_buff *skb, struct bpf_prog *xdp_prog) HARD_TX_UNLOCK(dev, txq); if (free_skb) { trace_xdp_exception(dev, xdp_prog, XDP_TX); + dev_core_stats_tx_dropped_inc(dev); kfree_skb(skb); } } -- 2.35.1
