On Wed, Jun 10, 2026 at 07:46:16PM -0700, Xiang Mei wrote:
> receive_big() bounds the device-announced length by
> (big_packets_num_skbfrags + 1) * PAGE_SIZE. That is still too loose:
> add_recvbuf_big() sets sg[1] to start at offset
> sizeof(struct padded_vnet_hdr) into the first page, so the chain
> actually carries hdr_len + (PAGE_SIZE - sizeof(padded_vnet_hdr)) +
> big_packets_num_skbfrags * PAGE_SIZE bytes -- 20 bytes less than the
> check allows for the common hdr_len == 12 case.
>
> A malicious virtio backend can announce a len in that gap. page_to_skb()
> then walks one frag past the page chain, storing a NULL page->private
> into skb_shinfo()->frags[MAX_SKB_FRAGS], which is both an out-of-bounds
> write past the static frag array and a NULL frag handed up the rx path.
>
> Bound len by the size add_recvbuf_big() actually advertised.
>
> Fixes: 0c716703965f ("virtio-net: fix received length check in big packets")
> Reported-by: Weiming Shi <[email protected]>
> Signed-off-by: Xiang Mei <[email protected]>
> Reviewed-by: Xuan Zhuo <[email protected]>
Thanks for the patch! Something small to improve:
> ---
> v3: revoke 2/2 and add Xuan Zhuo's Reviewed-by tag
>
> drivers/net/virtio_net.c | 8 +++++---
> 1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index f4adcfee7a80..afe73eda1491 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1999,15 +1999,17 @@ static struct sk_buff *receive_big(struct net_device
> *dev,
> struct virtnet_rq_stats *stats)
> {
> struct page *page = buf;
> + unsigned long max_len;
Assignment can happen here?
> struct sk_buff *skb;
>
> /* Make sure that len does not exceed the size allocated in
> * add_recvbuf_big.
> */
> - if (unlikely(len > (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE)) {
> + max_len = vi->hdr_len + (PAGE_SIZE - sizeof(struct padded_vnet_hdr)) +
> + vi->big_packets_num_skbfrags * PAGE_SIZE;
Took me a while to figure out what is going on, but I finally
understand:
Reducing
(vi->big_packets_num_skbfrags + 1) * PAGE_SIZE
(what we allocated)
by sizeof(struct padded_vnet_hdr) - vi->hdr_len
right?
So clearer as:
unsigned long max_len = (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE -
sizeof(struct padded_vnet_hdr) + vi->hdr_len;
> + if (unlikely(len > max_len)) {
> pr_debug("%s: rx error: len %u exceeds allocated size %lu\n",
> - dev->name, len,
> - (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE);
> + dev->name, len, max_len);
> goto err;
> }
>
> --
> 2.43.0
