[PATCH net-next v2 0/3] net: gso: fix MTU validation of BIG TCP

[PATCH net-next v2 0/3] net: gso: fix MTU validation of BIG TCP

[Mariusz Klimek]

This series fixes the MTU validation of BIG TCP jumbograms and removes the
existing IP6SKB_FAKEJUMBO work-around that only fixes the issue in one
location.

For GSO packets, the length that matters for MTU validation is the segment
length, not the total length of the packet. skb_gso_network_seglen is used
by skb_gso_validate_network_len to calculate the segment length including
the network and transport headers and to then verify that the segment
length is below the MTU.

skb_gso_network_seglen assumes that the headers of the segments are
identical to those of the unsegmented packet, but that assumption is
incorrect for BIG TCP jumbograms which have an added HBH header that is
removed upon segmentation. The calculated segment length ends up being 8
bytes more than the actual segment length.

The actual segment length is set according to the MSS, so the segment
length calculated by skb_gso_network_seglen is greater than the MTU,
causing the skb_gso_validate_network_len check to fail despite the fact
that the actual segment length is lower than the MTU.

There is currently a work-around that fixes this bug in some cases:
ip6_xmit sets the IP6SKB_FAKEJUMBO flag for BIG TCP jumbograms, which
causes the MTU validation in ip6_finish_output_gso to be skipped
(intentionally). However, this work-around doesn't apply to MTU validations
performed in other places such as in ip6_forward. BIG TCP jumbograms don't
pass the MTU validation when forwarded locally and are therefore dropped,
unless the MTU of the originating interface is lower than the MTUs of the
rest of the interfaces the packets are forwarded through.

v2:
  fix jumbogram check in skb_gso_network_seglen
  add jumbogram check to skb_gso_mac_seglen as well

v1:
  Link: https://lore.kernel.org/netdev/20251127091325.7248-1-maklimek97@gmail.com/

Mariusz Klimek (3):
  net: gso: do not include jumbogram HBH header in seglen calculation
  ipv6: remove IP6SKB_FAKEJUMBO flag
  selftests/net: remove unnecessary MTU config in big_tcp.sh

 include/linux/ipv6.h                   |  1 -
 net/core/gso.c                         | 14 +++++++++-----
 net/ipv6/ip6_output.c                  |  4 +---
 tools/testing/selftests/net/big_tcp.sh |  1 -
 4 files changed, 10 insertions(+), 10 deletions(-)

-- 
2.47.3

[PATCH net-next v2 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

[Mariusz Klimek]

This patch fixes an issue in skb_gso_network_seglen and similar functions
where the calculated segment length includes the HBH headers of BIG TCP
jumbograms despite these headers being removed before segmentation. These
headers are added by GRO or by ip6_xmit for BIG TCP packets and are later
removed by GSO. This bug causes MTU validation of BIG TCP jumbograms to
fail.

Signed-off-by: Mariusz Klimek <maklimek97@gmail.com>
---
 net/core/gso.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/net/core/gso.c b/net/core/gso.c
index bcd156372f4d..deacd32f644d 100644
--- a/net/core/gso.c
+++ b/net/core/gso.c
@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
+#include <linux/if_vlan.h>
 #include <linux/skbuff.h>
 #include <linux/sctp.h>
 #include <net/gso.h>
@@ -177,8 +178,13 @@ static unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
  */
 static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
 {
-	unsigned int hdr_len = skb_transport_header(skb) -
-			       skb_network_header(skb);
+	unsigned int off = skb_network_offset(skb) + sizeof(struct ipv6hdr);
+	unsigned int hdr_len = skb_network_header_len(skb);
+
+	/* Jumbogram HBH header is removed upon segmentation. */
+	if (skb_protocol(skb, true) == htons(ETH_P_IPV6) &&
+	    skb->len - off > IPV6_MAXPLEN)
+		hdr_len -= sizeof(struct hop_jumbo_hdr);
 
 	return hdr_len + skb_gso_transport_seglen(skb);
 }
@@ -194,9 +200,7 @@ static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
  */
 static unsigned int skb_gso_mac_seglen(const struct sk_buff *skb)
 {
-	unsigned int hdr_len = skb_transport_header(skb) - skb_mac_header(skb);
-
-	return hdr_len + skb_gso_transport_seglen(skb);
+	return skb_mac_header_len(skb) + skb_gso_network_seglen(skb);
 }
 
 /**
-- 
2.47.3

[PATCH net-next v2 2/3] ipv6: remove IP6SKB_FAKEJUMBO flag

[Mariusz Klimek]

This patch removes the IP6SKB_FAKEJUMBO flag that is used as a work-around
to bypass MTU validation of BIG TCP jumbograms due to a bug in
skb_gso_network_seglen. This work-around is no longer required now that the
bug is fixed.

Signed-off-by: Mariusz Klimek <maklimek97@gmail.com>
---
 include/linux/ipv6.h  | 1 -
 net/ipv6/ip6_output.c | 4 +---
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 7294e4e89b79..9f076171106e 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -155,7 +155,6 @@ struct inet6_skb_parm {
 #define IP6SKB_L3SLAVE         64
 #define IP6SKB_JUMBOGRAM      128
 #define IP6SKB_SEG6	      256
-#define IP6SKB_FAKEJUMBO      512
 #define IP6SKB_MULTIPATH      1024
 #define IP6SKB_MCROUTE        2048
 };
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index f904739e99b9..9af9ec6bdb8c 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -179,8 +179,7 @@ ip6_finish_output_gso_slowpath_drop(struct net *net, struct sock *sk,
 static int ip6_finish_output_gso(struct net *net, struct sock *sk,
 				 struct sk_buff *skb, unsigned int mtu)
 {
-	if (!(IP6CB(skb)->flags & IP6SKB_FAKEJUMBO) &&
-	    !skb_gso_validate_network_len(skb, mtu))
+	if (!skb_gso_validate_network_len(skb, mtu))
 		return ip6_finish_output_gso_slowpath_drop(net, sk, skb, mtu);
 
 	return ip6_finish_output2(net, sk, skb);
@@ -323,7 +322,6 @@ int ip6_xmit(const struct sock *sk, struct sk_buff *skb, struct flowi6 *fl6,
 
 		proto = IPPROTO_HOPOPTS;
 		seg_len = 0;
-		IP6CB(skb)->flags |= IP6SKB_FAKEJUMBO;
 	}
 
 	skb_push(skb, sizeof(struct ipv6hdr));
-- 
2.47.3

[PATCH net-next v2 3/3] selftests/net: remove unnecessary MTU config in big_tcp.sh

[Mariusz Klimek]

This patch removes the manual lowering of the client MTU in big_tcp.sh. The
MTU lowering was previously required as a work-around due to a bug in the
MTU validation of BIG TCP jumbograms. The MTU was lowered to 1442, but note
that 1492 (1500 - 8) would of worked just as well. Now that the bug has
been fixed, the manual client MTU modification can be removed entirely.

Signed-off-by: Mariusz Klimek <maklimek97@gmail.com>
---
 tools/testing/selftests/net/big_tcp.sh | 1 -
 1 file changed, 1 deletion(-)

diff --git a/tools/testing/selftests/net/big_tcp.sh b/tools/testing/selftests/net/big_tcp.sh
index 2db9d15cd45f..b5d9145296d3 100755
--- a/tools/testing/selftests/net/big_tcp.sh
+++ b/tools/testing/selftests/net/big_tcp.sh
@@ -32,7 +32,6 @@ setup() {
 	ip -net $ROUTER_NS link add link2 type veth peer name link3 netns $SERVER_NS
 
 	ip -net $CLIENT_NS link set link0 up
-	ip -net $CLIENT_NS link set link0 mtu 1442
 	ip -net $CLIENT_NS addr add $CLIENT_IP4/24 dev link0
 	ip -net $CLIENT_NS addr add $CLIENT_IP6/64 dev link0 nodad
 	ip -net $CLIENT_NS route add $SERVER_IP4 dev link0 via $CLIENT_GW4
-- 
2.47.3

Re: [PATCH net-next v2 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

[Paolo Abeni]

On 1/6/26 10:52 AM, Mariusz Klimek wrote:
> @@ -177,8 +178,13 @@ static unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
>   */
>  static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
>  {
> -	unsigned int hdr_len = skb_transport_header(skb) -
> -			       skb_network_header(skb);
> +	unsigned int off = skb_network_offset(skb) + sizeof(struct ipv6hdr);
> +	unsigned int hdr_len = skb_network_header_len(skb);
> +
> +	/* Jumbogram HBH header is removed upon segmentation. */
> +	if (skb_protocol(skb, true) == htons(ETH_P_IPV6) &&
> +	    skb->len - off > IPV6_MAXPLEN)
> +		hdr_len -= sizeof(struct hop_jumbo_hdr);

IIRC there is some ongoing discussion about introducing big tcp support
for virtio. Perhaps a DEBUG_NET_WARN_ON_ONCE(SKB_GSO_DODGY) could help
keeping this check updated at due time?

@Jason: could you please double check if I'm off WRT virtio support for
big TCP?

/P

Re: [PATCH net-next v2 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

[Paolo Abeni]

On 1/13/26 9:40 AM, Paolo Abeni wrote:
> On 1/6/26 10:52 AM, Mariusz Klimek wrote:
>> @@ -177,8 +178,13 @@ static unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
>>   */
>>  static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
>>  {
>> -	unsigned int hdr_len = skb_transport_header(skb) -
>> -			       skb_network_header(skb);
>> +	unsigned int off = skb_network_offset(skb) + sizeof(struct ipv6hdr);
>> +	unsigned int hdr_len = skb_network_header_len(skb);
>> +
>> +	/* Jumbogram HBH header is removed upon segmentation. */
>> +	if (skb_protocol(skb, true) == htons(ETH_P_IPV6) &&
>> +	    skb->len - off > IPV6_MAXPLEN)
>> +		hdr_len -= sizeof(struct hop_jumbo_hdr);
> 
> IIRC there is some ongoing discussion about introducing big tcp support
> for virtio. Perhaps a DEBUG_NET_WARN_ON_ONCE(SKB_GSO_DODGY) could help
> keeping this check updated at due time?
> 
> @Jason: could you please double check if I'm off WRT virtio support for
> big TCP?

Reconsidering the above, I think the mentioned check could be added
separately, if and when virtio big TCP will come to life.

/P

Re: [PATCH net-next v2 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

[Paolo Abeni]

On 1/6/26 10:52 AM, Mariusz Klimek wrote:
> @@ -177,8 +178,13 @@ static unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
>   */
>  static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
>  {
> -	unsigned int hdr_len = skb_transport_header(skb) -
> -			       skb_network_header(skb);
> +	unsigned int off = skb_network_offset(skb) + sizeof(struct ipv6hdr);
> +	unsigned int hdr_len = skb_network_header_len(skb);
> +
> +	/* Jumbogram HBH header is removed upon segmentation. */
> +	if (skb_protocol(skb, true) == htons(ETH_P_IPV6) &&
> +	    skb->len - off > IPV6_MAXPLEN)
> +		hdr_len -= sizeof(struct hop_jumbo_hdr);

I'm sorry for splitting the feedback in multiple replies.

I think the concern I expressed on v1:

https://lore.kernel.org/netdev/a7b90a3a-79ed-42a4-a782-17cde1b9a2d6@redhat.com/

is still not addressed here. What I fear is:

- TCP cooks a plain GSO packet just below the 64K limit.
- Such packet goes trough UDP (or gre) encapsulation, the skb->len size
(including outer network header) grows above the 64K limit.
- the above check is satisfied, but no jumbo hop option is present.

I think you could use the `ipv6_has_hopopt_jumbo()` helper to be on the
safe side.

/P

Re: [PATCH net-next v2 0/3] net: gso: fix MTU validation of BIG TCP

[Alice Mikityanska]

On Tue, 06 Jan 2026 at 10:52:40 +0100, Mariusz Klimek wrote:
> This series fixes the MTU validation of BIG TCP jumbograms and removes the
> existing IP6SKB_FAKEJUMBO work-around that only fixes the issue in one
> location.

My series removes IPv6 HBH in BIG TCP entirely:

https://lore.kernel.org/netdev/20260113212655.116122-1-alice.kernel@fastmail.im/T/

I believe, it makes this fix no longer necessary.

> For GSO packets, the length that matters for MTU validation is the segment
> length, not the total length of the packet. skb_gso_network_seglen is used
> by skb_gso_validate_network_len to calculate the segment length including
> the network and transport headers and to then verify that the segment
> length is below the MTU.
> 
> skb_gso_network_seglen assumes that the headers of the segments are
> identical to those of the unsegmented packet, but that assumption is
> incorrect for BIG TCP jumbograms which have an added HBH header that is
> removed upon segmentation. The calculated segment length ends up being 8
> bytes more than the actual segment length.
> 
> The actual segment length is set according to the MSS, so the segment
> length calculated by skb_gso_network_seglen is greater than the MTU,
> causing the skb_gso_validate_network_len check to fail despite the fact
> that the actual segment length is lower than the MTU.
> 
> There is currently a work-around that fixes this bug in some cases:
> ip6_xmit sets the IP6SKB_FAKEJUMBO flag for BIG TCP jumbograms, which
> causes the MTU validation in ip6_finish_output_gso to be skipped
> (intentionally). However, this work-around doesn't apply to MTU validations
> performed in other places such as in ip6_forward. BIG TCP jumbograms don't
> pass the MTU validation when forwarded locally and are therefore dropped,
> unless the MTU of the originating interface is lower than the MTUs of the
> rest of the interfaces the packets are forwarded through.
> 
> v2:
>   fix jumbogram check in skb_gso_network_seglen
>   add jumbogram check to skb_gso_mac_seglen as well
> 
> v1:
>   Link: https://lore.kernel.org/netdev/20251127091325.7248-1-maklimek97@gmail.com/
> 
> Mariusz Klimek (3):
>   net: gso: do not include jumbogram HBH header in seglen calculation
>   ipv6: remove IP6SKB_FAKEJUMBO flag
>   selftests/net: remove unnecessary MTU config in big_tcp.sh
> 
>  include/linux/ipv6.h                   |  1 -
>  net/core/gso.c                         | 14 +++++++++-----
>  net/ipv6/ip6_output.c                  |  4 +---
>  tools/testing/selftests/net/big_tcp.sh |  1 -
>  4 files changed, 10 insertions(+), 10 deletions(-)
> 
> -- 
> 2.47.3
> 
> 

Re: [PATCH net-next v2 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

[Jason Wang]

On Tue, Jan 13, 2026 at 9:51 PM Paolo Abeni <pabeni@redhat.com> wrote:
>
> On 1/13/26 9:40 AM, Paolo Abeni wrote:
> > On 1/6/26 10:52 AM, Mariusz Klimek wrote:
> >> @@ -177,8 +178,13 @@ static unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
> >>   */
> >>  static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
> >>  {
> >> -    unsigned int hdr_len = skb_transport_header(skb) -
> >> -                           skb_network_header(skb);
> >> +    unsigned int off = skb_network_offset(skb) + sizeof(struct ipv6hdr);
> >> +    unsigned int hdr_len = skb_network_header_len(skb);
> >> +
> >> +    /* Jumbogram HBH header is removed upon segmentation. */
> >> +    if (skb_protocol(skb, true) == htons(ETH_P_IPV6) &&
> >> +        skb->len - off > IPV6_MAXPLEN)
> >> +            hdr_len -= sizeof(struct hop_jumbo_hdr);
> >
> > IIRC there is some ongoing discussion about introducing big tcp support
> > for virtio. Perhaps a DEBUG_NET_WARN_ON_ONCE(SKB_GSO_DODGY) could help
> > keeping this check updated at due time?
> >
> > @Jason: could you please double check if I'm off WRT virtio support for
> > big TCP?
>
> Reconsidering the above, I think the mentioned check could be added
> separately, if and when virtio big TCP will come to life.

I agree. BIG TCP support for virtio requires spec features like
tso_max_size/segs advertisement, jumbogram head flags. And if I am not
wrong, HBH header is probably not needed at that time.

Thanks

>
> /P
>

Re: [PATCH net-next v2 1/3] net: gso: do not include jumbogram HBH header in seglen calculation

[Mariusz Klimek]

On 1/13/26 15:14, Paolo Abeni wrote:
> On 1/6/26 10:52 AM, Mariusz Klimek wrote:
>> @@ -177,8 +178,13 @@ static unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
>>   */
>>  static unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
>>  {
>> -	unsigned int hdr_len = skb_transport_header(skb) -
>> -			       skb_network_header(skb);
>> +	unsigned int off = skb_network_offset(skb) + sizeof(struct ipv6hdr);
>> +	unsigned int hdr_len = skb_network_header_len(skb);
>> +
>> +	/* Jumbogram HBH header is removed upon segmentation. */
>> +	if (skb_protocol(skb, true) == htons(ETH_P_IPV6) &&
>> +	    skb->len - off > IPV6_MAXPLEN)
>> +		hdr_len -= sizeof(struct hop_jumbo_hdr);
> 
> I'm sorry for splitting the feedback in multiple replies.
> 
> I think the concern I expressed on v1:
> 
> https://lore.kernel.org/netdev/a7b90a3a-79ed-42a4-a782-17cde1b9a2d6@redhat.com/
> 
> is still not addressed here. What I fear is:
> 
> - TCP cooks a plain GSO packet just below the 64K limit.
> - Such packet goes trough UDP (or gre) encapsulation, the skb->len size
> (including outer network header) grows above the 64K limit.
> - the above check is satisfied, but no jumbo hop option is present.
> 

Could you maybe clarify what you mean?

This check is for the outer IPv6 header. skb->len - off is the IPv6 payload
length so the packet contains a hop-by-hop header if and only if
skb->len - off > IPV6_MAXPLEN. It doesn't matter what comes after the IPv6
header. If the TCP payload is smaller than 65535 and it is the encapsulation
headers that cause the IPv6 payload length to be above 65535, a jumbogram
hop-by-hop header is still added (in ip6_xmit).

> I think you could use the `ipv6_has_hopopt_jumbo()` helper to be on the
> safe side.

Sure, I could submit another revision with this change if my explanation is
unsatisfactory.

> 
> /P
> 

-- 
Mariusz K.

Re: [PATCH net-next v2 0/3] net: gso: fix MTU validation of BIG TCP

[Mariusz Klimek]

On 1/13/26 22:42, Alice Mikityanska wrote:
> On Tue, 06 Jan 2026 at 10:52:40 +0100, Mariusz Klimek wrote:
>> This series fixes the MTU validation of BIG TCP jumbograms and removes the
>> existing IP6SKB_FAKEJUMBO work-around that only fixes the issue in one
>> location.
> 
> My series removes IPv6 HBH in BIG TCP entirely:
> 
> https://lore.kernel.org/netdev/20260113212655.116122-1-alice.kernel@fastmail.im/T/
> 
> I believe, it makes this fix no longer necessary.
> 

You're right, your patch also fixes this issue by removing HBH headers
altogether. Thanks for bringing this up :)

Heads up: I'm working on another patch series that uses some of the code
you're removing...

Paolo: should we still merge this or should we just wait for Alice's patch to
be accepted? Also, should I already submit my next patch series or should I
wait until we're finished with this one?

-- 
Mariusz K.

Re: [PATCH net-next v2 0/3] net: gso: fix MTU validation of BIG TCP

[Alice Mikityanska]

On Fri, 30 Jan 2026 at 11:00, Mariusz Klimek <maklimek97@gmail.com> wrote:
>
> On 1/13/26 22:42, Alice Mikityanska wrote:
> > On Tue, 06 Jan 2026 at 10:52:40 +0100, Mariusz Klimek wrote:
> >> This series fixes the MTU validation of BIG TCP jumbograms and removes the
> >> existing IP6SKB_FAKEJUMBO work-around that only fixes the issue in one
> >> location.
> >
> > My series removes IPv6 HBH in BIG TCP entirely:
> >
> > https://lore.kernel.org/netdev/20260113212655.116122-1-alice.kernel@fastmail.im/T/
> >
> > I believe, it makes this fix no longer necessary.
> >
>
> You're right, your patch also fixes this issue by removing HBH headers
> altogether. Thanks for bringing this up :)
>
> Heads up: I'm working on another patch series that uses some of the code
> you're removing...
>
> Paolo: should we still merge this or should we just wait for Alice's patch to
> be accepted? Also, should I already submit my next patch series or should I
> wait until we're finished with this one?

CCed the tcpdump folks: I would appreciate if you could take forward
either of these pull requests:

https://github.com/the-tcpdump-group/tcpdump/pull/1329
https://github.com/the-tcpdump-group/tcpdump/pull/1396

That would unblock the kernel series and allow Paolo to merge it, and
also unblock Mariusz's work.

Thanks,
Alice

> --
> Mariusz K.