[PATCH bpf] veth: convert frag_list skbs before running XDP

[Matt Fleming <matt@readmodwrite.com>]

From: Matt Fleming <mfleming@cloudflare.com>

A frag_list skb can reach veth with data_len set but nr_frags zero.
veth_convert_skb_to_xdp_buff() only converts skbs that are shared,
locked, have frags[], or do not have enough headroom. It later uses
skb_is_nonlinear() to decide whether to set XDP_FLAGS_HAS_FRAGS and
xdp_frags_size.

That exposes frag_list data to XDP as if it were stored in frags[], but
frags[] is empty. AF_XDP copy mode can then trust the bogus XDP fragment
metadata, walk an empty fragment entry, and crash in memcpy() from
__xsk_rcv().

Route frag_list skbs through skb_pp_cow_data() before exposing them to
XDP, and only advertise XDP frags when the resulting skb has frags[].
skb_copy_bits() already handles frag_list input, and skb_pp_cow_data()
builds frags[] output with skb_add_rx_frag(), which is the representation
XDP multi-buffer expects.

Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb")
Cc: stable@vger.kernel.org
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
---
 drivers/net/veth.c | 4 ++--
 net/core/skbuff.c  | 9 ++++-----
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 1c5142149175..efb24aae1f26 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -756,7 +756,7 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
 	u32 frame_sz;
 
 	if (skb_shared(skb) || skb_head_is_locked(skb) ||
-	    skb_shinfo(skb)->nr_frags ||
+	    skb_shinfo(skb)->nr_frags || skb_has_frag_list(skb) ||
 	    skb_headroom(skb) < XDP_PACKET_HEADROOM) {
 		if (skb_pp_cow_data(rq->page_pool, pskb, XDP_PACKET_HEADROOM))
 			goto drop;
@@ -771,7 +771,7 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
 	xdp_prepare_buff(xdp, skb->head, skb_headroom(skb),
 			 skb_headlen(skb), true);
 
-	if (skb_is_nonlinear(skb)) {
+	if (skb_shinfo(skb)->nr_frags) {
 		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
 		xdp_buff_set_frags_flag(xdp);
 	} else {
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 18dabb4e9cfa..1e837d01a908 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -936,12 +936,11 @@ int skb_pp_cow_data(struct page_pool *pool, struct sk_buff **pskb,
 	int err, i, head_off;
 	void *data;
 
-	/* XDP does not support fraglist so we need to linearize
-	 * the skb.
+	/*
+	 * skb_copy_bits() handles both frags[] and frag_list input. If the
+	 * copied skb remains non-linear, it uses frags[], which is the
+	 * representation used by XDP multi-buffer.
 	 */
-	if (skb_has_frag_list(skb))
-		return -EOPNOTSUPP;
-
 	max_head_size = SKB_WITH_OVERHEAD(PAGE_SIZE - headroom);
 	if (skb->len > max_head_size + MAX_SKB_FRAGS * PAGE_SIZE)
 		return -ENOMEM;
-- 
2.43.0