> From: Bui Quang Minh <[email protected]>
> Sent: 24 October 2025 08:37 PM
>
> Since commit 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big
> packets"), when guest gso is off, 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.
>
> Because the host announced buffer length can be malicious (e.g. the host
> vhost_net driver's get_rx_bufs is modified to announce incorrect length), we
> need a check in virtio_net receive path. Currently, the check is not adapted
> to
> the new change which can lead to NULL page pointer dereference in the below
> while loop when receiving length that is larger than the allocated one.
>
This looks wrong.
A device DMAed N bytes, and it reports N + M bytes in the completion?
Such devices should be fixed.
If driver allocated X bytes, and device copied X + Y bytes on receive packet,
it will crash the driver host anyway.
The fixes tag in this patch is incorrect because this is not a driver bug.
It is just adding resiliency in driver for broken device. So driver cannot have
fixes tag here.
> This commit fixes the received length check corresponding to the new change.
>
> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big
> packets")
> Cc: [email protected]
> Signed-off-by: Bui Quang Minh <[email protected]>
> ---
> Changes in v5:
> - Move the length check to receive_big
> - Link to v4: https://lore.kernel.org/netdev/20251022160623.51191-1-
> [email protected]/
> Changes in v4:
> - Remove unrelated changes, add more comments
> - Link to v3: https://lore.kernel.org/netdev/20251021154534.53045-1-
> [email protected]/
> Changes in v3:
> - Convert BUG_ON to WARN_ON_ONCE
> - Link to v2: https://lore.kernel.org/netdev/20250708144206.95091-1-
> [email protected]/
> Changes in v2:
> - Remove incorrect give_pages call
> - Link to v1: https://lore.kernel.org/netdev/20250706141150.25344-1-
> [email protected]/
> ---
> drivers/net/virtio_net.c | 25 ++++++++++++-------------
> 1 file changed, 12 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c index
> a757cbcab87f..2c3f544add5e 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -910,17 +910,6 @@ static struct sk_buff *page_to_skb(struct
> virtnet_info *vi,
> goto ok;
> }
>
> - /*
> - * Verify that we can indeed put this data into a skb.
> - * This is here to handle cases when the device erroneously
> - * tries to receive more than is possible. This is usually
> - * the case of a broken device.
> - */
> - if (unlikely(len > MAX_SKB_FRAGS * PAGE_SIZE)) {
> - 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); @@ -2107,9 +2096,19 @@ static struct sk_buff *receive_big(struct
> net_device *dev,
> struct virtnet_rq_stats *stats)
> {
> struct page *page = buf;
> - struct sk_buff *skb =
> - page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, 0);
> + struct sk_buff *skb;
> +
> + /* Make sure that len does not exceed the allocated size in
> + * add_recvbuf_big.
> + */
> + if (unlikely(len > vi->big_packets_num_skbfrags * PAGE_SIZE)) {
> + pr_debug("%s: rx error: len %u exceeds allocate size %lu\n",
> + dev->name, len,
> + vi->big_packets_num_skbfrags * PAGE_SIZE);
> + goto err;
> + }
>
> + skb = page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, 0);
> u64_stats_add(&stats->bytes, len - vi->hdr_len);
> if (unlikely(!skb))
> goto err;
> --
> 2.43.0
