On Tue, Jul 08, 2025 at 09:42:06PM +0700, Bui Quang Minh wrote:
> Since commit 4959aebba8c0 ("virtio-net: use mtu size as buffer length
> for big packets"), the allocated size for big packets is not
> MAX_SKB_FRAGS * PAGE_SIZE anymore but depends on negotiated MTU. The
> number of allocated frags for big packets is stored in
> vi->big_packets_num_skbfrags. This commit fixes the received length
> check corresponding to that change. The current incorrect check can lead
> to NULL page pointer dereference in the below while loop when erroneous
> length is received.
>
> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big packets")
> Signed-off-by: Bui Quang Minh <minhquangbui99@xxxxxxxxx>
Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
> Changes in v2:
> - Remove incorrect give_pages call
> ---
> drivers/net/virtio_net.c | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 5d674eb9a0f2..3a7f435c95ae 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -823,7 +823,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> {
> struct sk_buff *skb;
> struct virtio_net_common_hdr *hdr;
> - unsigned int copy, hdr_len, hdr_padded_len;
> + unsigned int copy, hdr_len, hdr_padded_len, max_remaining_len;
> struct page *page_to_free = NULL;
> int tailroom, shinfo_size;
> char *p, *hdr_p, *buf;
> @@ -887,12 +887,15 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> * tries to receive more than is possible. This is usually
> * the case of a broken device.
> */
> - if (unlikely(len > MAX_SKB_FRAGS * PAGE_SIZE)) {
> + BUG_ON(offset >= PAGE_SIZE);
> + max_remaining_len = (unsigned int)PAGE_SIZE - offset;
> + max_remaining_len += vi->big_packets_num_skbfrags * PAGE_SIZE;
> + if (unlikely(len > max_remaining_len)) {
> net_dbg_ratelimited("%s: too much data\n", skb->dev->name);
> dev_kfree_skb(skb);
> return NULL;
> }
> - BUG_ON(offset >= PAGE_SIZE);
> +
> while (len) {
> unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
> skb_add_rx_frag(skb, skb_shinfo(skb)->nr_frags, page, offset,
> --
> 2.43.0
