[PATCH net v2 2/2] virtio-net: harden page_to_skb() big-packet frag loop

[Xiang Mei]

This is a robustness hardening patch. The slow-path frag loop in
page_to_skb() walks the page chain via page->private until the
device-reported len is consumed, implicitly trusting that len fits the
chain. It does not stop when the chain is exhausted (page becomes NULL
at the tail), nor when nr_frags reaches the end of the static
skb_shinfo()->frags[MAX_SKB_FRAGS] array.

Both bounds are needed: the chain length is big_packets_num_skbfrags + 1
pages, which for an MTU-driven configuration can be well below
MAX_SKB_FRAGS, so neither guard implies the other.

Make the loop self-defending so it no longer relies on the caller having
validated len: stop once the chain is exhausted, and never index past
MAX_SKB_FRAGS. No functional change for well-formed input.

Signed-off-by: Xiang Mei <xmei5@xxxxxxx>
---
v2: robustness patch

 drivers/net/virtio_net.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index afe73eda1491..518c22fa1b68 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -906,8 +906,11 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
 	}
 
 	BUG_ON(offset >= PAGE_SIZE);
-	while (len) {
+	while (len && page) {
 		unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
+
+		if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS))
+			break;
 		skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page, offset,
 				frag_size, truesize);
 		len -= frag_size;
-- 
2.43.0