[PATCH net v4] virtio-net: fix len check in receive_big()

[PATCH net v4] virtio-net: fix len check in receive_big()

[Xiang Mei]

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 <bestswngs@gmail.com>
Signed-off-by: Xiang Mei <xmei5@asu.edu>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
v4: use easy to understand math to compute the max_len
v3: revoke 2/2 and add Xuan Zhuo's Reviewed-by tag
v2: add additiona check as 2/2

 drivers/net/virtio_net.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index f4adcfee7a80..8f4562316aaa 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1999,15 +1999,16 @@ static struct sk_buff *receive_big(struct net_device *dev,
 				   struct virtnet_rq_stats *stats)
 {
 	struct page *page = buf;
+	unsigned long max_len = (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE -
+				sizeof(struct padded_vnet_hdr) + vi->hdr_len;
 	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)) {
+	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

Re: [PATCH net v4] virtio-net: fix len check in receive_big()

[Michael S. Tsirkin]

On Mon, Jun 15, 2026 at 09:28:37PM -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 <bestswngs@gmail.com>
> Signed-off-by: Xiang Mei <xmei5@asu.edu>
> Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> v4: use easy to understand math to compute the max_len
> v3: revoke 2/2 and add Xuan Zhuo's Reviewed-by tag

I still feel 2/2 is good defence in depth but it can be
pursued separately.

> v2: add additiona check as 2/2
> 
>  drivers/net/virtio_net.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index f4adcfee7a80..8f4562316aaa 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1999,15 +1999,16 @@ static struct sk_buff *receive_big(struct net_device *dev,
>  				   struct virtnet_rq_stats *stats)
>  {
>  	struct page *page = buf;
> +	unsigned long max_len = (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE -
> +				sizeof(struct padded_vnet_hdr) + vi->hdr_len;
>  	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)) {
> +	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

Re: [PATCH net v4] virtio-net: fix len check in receive_big()

[Xiang Mei]

On Mon, Jun 15, 2026 at 9:40 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Jun 15, 2026 at 09:28:37PM -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 <bestswngs@gmail.com>
> > Signed-off-by: Xiang Mei <xmei5@asu.edu>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
>
> > ---
> > v4: use easy to understand math to compute the max_len
> > v3: revoke 2/2 and add Xuan Zhuo's Reviewed-by tag
>
> I still feel 2/2 is good defence in depth but it can be
> pursued separately.
Thanks, Michael. I'll leave 2/2 out of this series.
Appreciate the review.

Xiang
>
> > v2: add additiona check as 2/2
> >
> >  drivers/net/virtio_net.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index f4adcfee7a80..8f4562316aaa 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1999,15 +1999,16 @@ static struct sk_buff *receive_big(struct net_device *dev,
> >                                  struct virtnet_rq_stats *stats)
> >  {
> >       struct page *page = buf;
> > +     unsigned long max_len = (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE -
> > +                             sizeof(struct padded_vnet_hdr) + vi->hdr_len;
> >       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)) {
> > +     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
>