[RFC PATCH net] udp: check sk for UDP GRO fraglist

It is a workaround patch.

UDP/IP header of UDP GROed frag_skbs are not updated even after NAT
forwarding. Only the header of head_skb from ip_finish_output_gso ->
skb_gso_segment is updated but following frag_skbs are not updated.

A call path skb_mac_gso_segment -> inet_gso_segment ->
udp4_ufo_fragment -> __udp_gso_segment -> __udp_gso_segment_list
does not try to update any UDP/IP header of the segment list.

It might make sense because each skb of frag_skbs is converted to a
list of regular packets. Header update with checksum calculation may
be not needed for UDP GROed frag_skbs.

But UDP GRO frag_list is started from udp_gro_receive, we don't know
whether the skb will be NAT forwarded at that time. For workaround,
try to get sock always when call udp4_gro_receive -> udp_gro_receive
to check if the skb is for local.

I'm still not sure if UDP GRO frag_list is really designed for local
session only. Can kernel support NAT forward for UDP GRO frag_list?
What am I missing?

Fixes: 9fd1ff5d2ac7 (udp: Support UDP fraglist GRO/GSO.)
Signed-off-by: Dongseok Yi <redacted>
---
 net/ipv4/udp_offload.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index ff39e94..d476216 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c

@@ -457,7 +457,7 @@ struct sk_buff *udp_gro_receive(struct list_head *head, struct sk_buff *skb,
 	int flush = 1;
 
 	NAPI_GRO_CB(skb)->is_flist = 0;
-	if (skb->dev->features & NETIF_F_GRO_FRAGLIST)
+	if (sk && (skb->dev->features & NETIF_F_GRO_FRAGLIST))
 		NAPI_GRO_CB(skb)->is_flist = sk ? !udp_sk(sk)->gro_enabled: 1;
 
 	if ((sk && udp_sk(sk)->gro_enabled) || NAPI_GRO_CB(skb)->is_flist) {

@@ -537,8 +537,7 @@ struct sk_buff *udp4_gro_receive(struct list_head *head, struct sk_buff *skb)
 	NAPI_GRO_CB(skb)->is_ipv6 = 0;
 	rcu_read_lock();
 
-	if (static_branch_unlikely(&udp_encap_needed_key))
-		sk = udp4_gro_lookup_skb(skb, uh->source, uh->dest);
+	sk = udp4_gro_lookup_skb(skb, uh->source, uh->dest);
 
 	pp = udp_gro_receive(head, skb, uh, sk);
 	rcu_read_unlock();

-- 
2.7.4

On Fri, Jan 08, 2021 at 09:52:28PM +0900, Dongseok Yi wrote:

It is a workaround patch.

UDP/IP header of UDP GROed frag_skbs are not updated even after NAT
forwarding. Only the header of head_skb from ip_finish_output_gso ->
skb_gso_segment is updated but following frag_skbs are not updated.

A call path skb_mac_gso_segment -> inet_gso_segment ->
udp4_ufo_fragment -> __udp_gso_segment -> __udp_gso_segment_list
does not try to update any UDP/IP header of the segment list.

It might make sense because each skb of frag_skbs is converted to a
list of regular packets. Header update with checksum calculation may
be not needed for UDP GROed frag_skbs.

But UDP GRO frag_list is started from udp_gro_receive, we don't know
whether the skb will be NAT forwarded at that time. For workaround,
try to get sock always when call udp4_gro_receive -> udp_gro_receive
to check if the skb is for local.

I'm still not sure if UDP GRO frag_list is really designed for local
session only. Can kernel support NAT forward for UDP GRO frag_list?
What am I missing?

The initial idea when I implemented this was to have a fast
forwarding path for UDP. So forwarding is a usecase, but NAT
is a problem, indeed. A quick fix could be to segment the
skb before it gets NAT forwarded. Alternatively we could
check for a header change in __udp_gso_segment_list and
update the header of the frag_skbs accordingly in that case.

On 2021-01-08 22:35, Steffen Klassert wrote:

On Fri, Jan 08, 2021 at 09:52:28PM +0900, Dongseok Yi wrote:

quoted

It is a workaround patch.

UDP/IP header of UDP GROed frag_skbs are not updated even after NAT
forwarding. Only the header of head_skb from ip_finish_output_gso ->
skb_gso_segment is updated but following frag_skbs are not updated.

A call path skb_mac_gso_segment -> inet_gso_segment ->
udp4_ufo_fragment -> __udp_gso_segment -> __udp_gso_segment_list
does not try to update any UDP/IP header of the segment list.

It might make sense because each skb of frag_skbs is converted to a
list of regular packets. Header update with checksum calculation may
be not needed for UDP GROed frag_skbs.

But UDP GRO frag_list is started from udp_gro_receive, we don't know
whether the skb will be NAT forwarded at that time. For workaround,
try to get sock always when call udp4_gro_receive -> udp_gro_receive
to check if the skb is for local.

I'm still not sure if UDP GRO frag_list is really designed for local
session only. Can kernel support NAT forward for UDP GRO frag_list?
What am I missing?

The initial idea when I implemented this was to have a fast
forwarding path for UDP. So forwarding is a usecase, but NAT
is a problem, indeed. A quick fix could be to segment the
skb before it gets NAT forwarded. Alternatively we could
check for a header change in __udp_gso_segment_list and
update the header of the frag_skbs accordingly in that case.

Thank you for explaining.
Can I think of it as a known issue? I think we should have a fix
because NAT can be triggered by user. Can I check the current status?
Already planning a patch or a new patch should be written?

On Mon, Jan 11, 2021 at 11:02:42AM +0900, Dongseok Yi wrote:

On 2021-01-08 22:35, Steffen Klassert wrote:

quoted

On Fri, Jan 08, 2021 at 09:52:28PM +0900, Dongseok Yi wrote:

quoted

It is a workaround patch.

UDP/IP header of UDP GROed frag_skbs are not updated even after NAT
forwarding. Only the header of head_skb from ip_finish_output_gso ->
skb_gso_segment is updated but following frag_skbs are not updated.

A call path skb_mac_gso_segment -> inet_gso_segment ->
udp4_ufo_fragment -> __udp_gso_segment -> __udp_gso_segment_list
does not try to update any UDP/IP header of the segment list.

It might make sense because each skb of frag_skbs is converted to a
list of regular packets. Header update with checksum calculation may
be not needed for UDP GROed frag_skbs.

But UDP GRO frag_list is started from udp_gro_receive, we don't know
whether the skb will be NAT forwarded at that time. For workaround,
try to get sock always when call udp4_gro_receive -> udp_gro_receive
to check if the skb is for local.

I'm still not sure if UDP GRO frag_list is really designed for local
session only. Can kernel support NAT forward for UDP GRO frag_list?
What am I missing?

The initial idea when I implemented this was to have a fast
forwarding path for UDP. So forwarding is a usecase, but NAT
is a problem, indeed. A quick fix could be to segment the
skb before it gets NAT forwarded. Alternatively we could
check for a header change in __udp_gso_segment_list and
update the header of the frag_skbs accordingly in that case.

Thank you for explaining.
Can I think of it as a known issue?

No, it was not known before you reported it.

I think we should have a fix
because NAT can be triggered by user. Can I check the current status?
Already planning a patch or a new patch should be written?

We have to do a new patch to fix that issue. If you want do
do so, go ahead.