veth: Adjust hard_start offset on redirect XDP frames (5c857225) · Commits · 戴 / test

When native XDP redirect into a veth device, the frame arrives in the xdp_frame structure. It is then processed in veth_xdp_rcv_one(), which can run a new XDP bpf_prog on the packet. Doing so requires converting xdp_frame to xdp_buff, but the tricky part is that xdp_frame memory area is located in the top (data_hard_start) memory area that xdp_buff will point into. The current code tried to protect the xdp_frame area, by assigning xdp_buff.data_hard_start past this memory. This results in 32 bytes less headroom to expand into via BPF-helper bpf_xdp_adjust_head(). This protect step is actually not needed, because BPF-helper bpf_xdp_adjust_head() already reserve this area, and don't allow BPF-prog to expand into it. Thus, it is safe to point data_hard_start directly at xdp_frame memory area. Fixes: ("veth: Handle xdp_frames in xdp napi ring") Reported-by:

Mao Wenan <maowenan@huawei.com> Signed-off-by:

Jesper Dangaard Brouer <brouer@redhat.com> Signed-off-by:

Alexei Starovoitov <ast@kernel.org> Acked-by:

Toshiaki Makita <toshiaki.makita1@gmail.com> Acked-by:

Toke Høiland-Jørgensen <toke@redhat.com> Link: https://lore.kernel.org/bpf/158945338331.97035.5923525383710752178.stgit@firesoul

drivers/net/veth.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -564,13 +564,15 @@ static struct sk_buff veth_xdp_rcv_one(struct veth_rq rq,
		struct veth_stats *stats)
		{
		void *hard_start = frame->data - frame->headroom;
		void *head = hard_start - sizeof(struct xdp_frame);
		int len = frame->len, delta = 0;
		struct xdp_frame orig_frame;
		struct bpf_prog *xdp_prog;
		unsigned int headroom;
		struct sk_buff *skb;

		/* bpf_xdp_adjust_head() assures BPF cannot access xdp_frame area */
		hard_start -= sizeof(struct xdp_frame);

		rcu_read_lock();
		xdp_prog = rcu_dereference(rq->xdp_prog);
		if (likely(xdp_prog)) {
		@@ -592,7 +594,6 @@ static struct sk_buff veth_xdp_rcv_one(struct veth_rq rq,
		break;
		case XDP_TX:
		orig_frame = *frame;
		xdp.data_hard_start = head;
		xdp.rxq->mem = frame->mem;
		if (unlikely(veth_xdp_tx(rq, &xdp, bq) < 0)) {
		trace_xdp_exception(rq->dev, xdp_prog, act);
		@@ -605,7 +606,6 @@ static struct sk_buff veth_xdp_rcv_one(struct veth_rq rq,
		goto xdp_xmit;
		case XDP_REDIRECT:
		orig_frame = *frame;
		xdp.data_hard_start = head;
		xdp.rxq->mem = frame->mem;
		if (xdp_do_redirect(rq->dev, &xdp, xdp_prog)) {
		frame = &orig_frame;
		@@ -629,7 +629,7 @@ static struct sk_buff veth_xdp_rcv_one(struct veth_rq rq,
		rcu_read_unlock();

		headroom = sizeof(struct xdp_frame) + frame->headroom - delta;
		skb = veth_build_skb(head, headroom, len, 0);
		skb = veth_build_skb(hard_start, headroom, len, 0);
		if (!skb) {
		xdp_return_frame(frame);
		stats->rx_drops++;

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