[PATCH net] amt: do not offer software GSO on the amt device

[PATCH net] amt: do not offer software GSO on the amt device

[Omar Ramadan]

amt_link_setup() puts NETIF_F_GSO_SOFTWARE in the device features, so
the core hands amt_dev_xmit() a GSO skb unsegmented: a UDP_SEGMENT
sender, or a UDP packet GRO merged on a link with UDP GRO forwarding
or fraglist GRO, that is routed to the amt device. The relay copies
the skb for each tunnel in amt_send_multicast_data() and pushes the AMT
and outer headers, but it never marks the copy for UDP tunnel
segmentation, so skb->encapsulation stays clear and gso_type is still
the inner SKB_GSO_UDP_L4. When the outer IP layer segments it,
__udp_gso_segment() cuts the outer UDP payload at the inner gso_size:
the first segment carries the AMT header, the inner headers and part
of the data, and every later one carries raw inner payload behind a
new outer UDP header, which a gateway takes for a malformed AMT
message.

The amt device cannot offload segmentation to anything, so stop
offering it and let validate_xmit_skb() segment a GSO skb before
amt_dev_xmit() replicates it. Each gateway then gets one AMT message
per inner packet. Neither mode needs GSO: a gateway only sends IGMP
and MLD reports.

Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface")
Signed-off-by: Omar Ramadan <omar@blockcast.net>
---
 drivers/net/amt.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/net/amt.c b/drivers/net/amt.c
index bddc24e18..6eb148c9f 100644
--- a/drivers/net/amt.c
+++ b/drivers/net/amt.c
@@ -3155,10 +3155,8 @@ static void amt_link_setup(struct net_device *dev)
 	dev->priv_flags		|= IFF_NO_QUEUE;
 	dev->lltx		= true;
 	dev->netns_immutable	= true;
-	dev->features		|= NETIF_F_GSO_SOFTWARE;
 	dev->hw_features	|= NETIF_F_SG | NETIF_F_HW_CSUM;
 	dev->hw_features	|= NETIF_F_FRAGLIST | NETIF_F_RXCSUM;
-	dev->hw_features	|= NETIF_F_GSO_SOFTWARE;
 	dev->pcpu_stat_type	= NETDEV_PCPU_STAT_TSTATS;
 	eth_hw_addr_random(dev);
 	eth_zero_addr(dev->broadcast);

base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7
-- 
2.47.3

Re: [PATCH net] amt: do not offer software GSO on the amt device

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - How the issue was discovered, e.g. hit in production, hit during
   development, syzbot report, manual code inspection, LLM or static
   analysis tool scan.

 - Whether the issue was actually triggered, or is only theoretical
   (e.g. found by code inspection). If it was triggered please include
   the symptoms, like the stack trace or error messages.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
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then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.