[PATCH AUTOSEL 4.19 074/106] veth: Adjust hard_start offset on redirect XDP frames

[Sasha Levin]

From: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>

[ Upstream commit 5c8572251fabc5bb49fd623c064e95a9daf6a3e3 ]

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: 9fc8d518d9d5 ("veth: Handle xdp_frames in xdp napi ring")
Reported-by: Mao Wenan <maowenan@xxxxxxxxxx>
Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Acked-by: Toshiaki Makita <toshiaki.makita1@xxxxxxxxx>
Acked-by: Toke HÃiland-JÃrgensen <toke@xxxxxxxxxx>
Link: https://lore.kernel.org/bpf/158945338331.97035.5923525383710752178.stgit@firesoul
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 drivers/net/veth.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 41a00cd76955..2abbad1abaf2 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -377,13 +377,15 @@ static struct sk_buff *veth_xdp_rcv_one(struct veth_rq *rq,
 					unsigned int *xdp_xmit)
 {
 	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)) {
@@ -405,7 +407,6 @@ 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->dev, &xdp) < 0)) {
 				trace_xdp_exception(rq->dev, xdp_prog, act);
@@ -417,7 +418,6 @@ 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;
@@ -437,7 +437,7 @@ 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);
 		goto err;
-- 
2.25.1