[PATCH net v2] virtio-net: fix received length check in big packets

[PATCH net v2] virtio-net: fix received length check in big packets

[Bui Quang Minh]

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@gmail.com>
---
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

RE: [PATCH net v2] virtio-net: fix received length check in big packets

[Parav Pandit]

> From: Bui Quang Minh <minhquangbui99@gmail.com>
> Sent: 08 July 2025 08:12 PM
> 
> 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.
Do you mean a device has copied X bytes in receive buffer but device reports X+Y bytes in the ring?

> 
> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big
> packets")
> Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
> ---
> 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

Re: [PATCH net v2] virtio-net: fix received length check in big packets

[Bui Quang Minh]

On 7/8/25 21:52, Parav Pandit wrote:
>> From: Bui Quang Minh <minhquangbui99@gmail.com>
>> Sent: 08 July 2025 08:12 PM
>>
>> 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.
> Do you mean a device has copied X bytes in receive buffer but device reports X+Y bytes in the ring?

Yes, that's what I mean. AFAIK, it's not checked in the ring level.


Thanks,
Quang Minh.

>
>> Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big
>> packets")
>> Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
>> ---
>> 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

Re: [PATCH net v2] virtio-net: fix received length check in big packets

[Paolo Abeni]

On 7/8/25 4:42 PM, 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@gmail.com>
> ---
> 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);

Minor nit (not intended to block this patch): since you are touching
this, you could consider replacing the BUG_ON() with a:

 if (WARN_ON_ONCE()) <goto error path>.

/P

Re: [PATCH net v2] virtio-net: fix received length check in big packets

[Jason Wang]

On Thu, Jul 10, 2025 at 5:57 PM Paolo Abeni <pabeni@redhat.com> wrote:
>
> On 7/8/25 4:42 PM, 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@gmail.com>
> > ---
> > 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);
>
> Minor nit (not intended to block this patch): since you are touching
> this, you could consider replacing the BUG_ON() with a:
>
>  if (WARN_ON_ONCE()) <goto error path>.

I'm not sure I get this, but using BUG_ON() can help to prevent bugs
from being explored.

Thanks

>
> /P
>

Re: [PATCH net v2] virtio-net: fix received length check in big packets

[Lei Yang]

Tested this patch with virtio-net regression tests, everything works fine.

Tested-by: Lei Yang <leiyang@redhat.com>

On Thu, Jul 10, 2025 at 6:44 PM Jason Wang <jasowang@redhat.com> wrote:
>
> On Thu, Jul 10, 2025 at 5:57 PM Paolo Abeni <pabeni@redhat.com> wrote:
> >
> > On 7/8/25 4:42 PM, 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@gmail.com>
> > > ---
> > > 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);
> >
> > Minor nit (not intended to block this patch): since you are touching
> > this, you could consider replacing the BUG_ON() with a:
> >
> >  if (WARN_ON_ONCE()) <goto error path>.
>
> I'm not sure I get this, but using BUG_ON() can help to prevent bugs
> from being explored.
>
> Thanks
>
> >
> > /P
> >
>
>

Re: [PATCH net v2] virtio-net: fix received length check in big packets

[Michael S. Tsirkin]

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@gmail.com>


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

> ---
> 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

Re: [PATCH net v2] virtio-net: fix received length check in big packets

[Michael S. Tsirkin]

On Thu, Jul 10, 2025 at 06:44:03PM +0800, Jason Wang wrote:
> On Thu, Jul 10, 2025 at 5:57 PM Paolo Abeni <pabeni@redhat.com> wrote:
> >
> > On 7/8/25 4:42 PM, 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@gmail.com>
> > > ---
> > > 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);
> >
> > Minor nit (not intended to block this patch): since you are touching
> > this, you could consider replacing the BUG_ON() with a:
> >
> >  if (WARN_ON_ONCE()) <goto error path>.
> 
> I'm not sure I get this, but using BUG_ON() can help to prevent bugs
> from being explored.
> 
> Thanks


You mean exploited.

Paolo what's your thought here? Why do you want to work around this
one, specifically? I don't see how we can get offset >= PAGE_SIZE.


> >
> > /P
> >