Re: [PATCH 2/2 net-next] net: openvswitch: drop IP fragments on SCTP set action

[Ilya Maximets <i.maximets@ovn.org>]

On 10/18/26 7:15 PM, Fernando Fernandez Mancera wrote:
> On 10/17/26 6:38 AM, netdev-bot+sashiko@kernel.org wrote:
>> Thank you for your contribution! Sashiko AI review found 4 potential
>> issue(s) to consider.
>>
>> Critical: 0 · High: 1 · Medium: 1 · Low: 2
>>
>> If you believe none of the issues are legitimate, please reply to each
>> issue of Medium severity or higher and explain why the AI is wrong.
>> If any of them are legitimate and you plan to respin, please let patchwork
>> know by including "pw-bot: cr" as a separate line at the end of your reply
>> (one such reply per series is enough).
>>
>> - [High] net/openvswitch: set_sctp corrupts SCTP checksum for padded
>>    packets
>> - [Medium] In net/openvswitch/actions.c, set_sctp() now returns -EINVAL
>>    whenever `flow_key->ip.frag != OVS_FRAG_TYPE_NONE`.
>> - [Low] The commit message says first and later fragments failed the same
>>    way: the skb holds part of the SCTP packet and 'the CRC delta of a…
>> - [Low] set_tcp() and set_udp() (actions.c:644-645, 598-599) skip
>>    OVS_FRAG_TYPE_LATER packets: they return 0 and the action list…
>>
>> --
>>
>> net: openvswitch: drop IP fragments on SCTP set action
>>
>> This patch makes set_sctp() return -EINVAL for any packet whose flow key
>> has ip.frag set to anything other than OVS_FRAG_TYPE_NONE. The goal is to
>> drop SCTP fragments with OVS_DROP_ACTION_ERROR instead of forwarding them
>> with rewritten ports and an invalid CRC32c checksum.
>>
>> Link: https://lore.kernel.org/netdev/d4fc14f3-4308-4560-9012-43c09c1b3845@ovn.org/
>>
>>> For a first fragment, and for
>>> a later fragment, it holds just a part of it, and the CRC delta of a
>>> change at the start of the packet depends on how many bytes follow, so
>>> the checksum written is wrong.
>>
> [...]>
>> [Severity: High]
>> This patch doesn't introduce it, but does the same CRC delta problem
>> also hit unfragmented packets that carry trailing link layer padding?
>>
>> sctp_compute_cksum() covers skb->len - sctphoff bytes. As far as I can
>> see, nothing on the OVS receive path trims skb->len down to the IPv4
>> total length or the IPv6 payload length before actions run. The only
>> trim I can find is ovs_skb_network_trim() in
>> net/openvswitch/conntrack.c, and that only runs for ct actions. So a
>> small SCTP packet that was padded to the 60 byte Ethernet minimum
>> reaches set_sctp() with the pad bytes still in the skb.
>>
>> In that case old_correct_csum and new_csum are both computed over the
>> SCTP packet plus the padding. As the commit message says, the CRC delta
>> of a change at the start of the packet depends on how many bytes follow
>> it. That makes old_correct_csum ^ new_csum the wrong delta for the
>> receiver, and old_correct_csum also no longer matches a correct
>> old_csum. The value written to sh->checksum is wrong even for a packet
>> that arrived with a valid checksum. ip_rcv() or ipv6_rcv() on the
>> receiver trims the padding, sctp_rcv() then sees a CRC32c mismatch, and
>> the packet is dropped.
>>
>> This looks easy to hit. Over IPv4, an SCTP packet that carries only a
>> COOKIE ACK, SHUTDOWN ACK or SHUTDOWN COMPLETE chunk is 36 bytes of IP,
>> well under the Ethernet minimum. A flow that rewrites SCTP ports, for
>> example for NAT, would then corrupt those chunks. I think losing COOKIE
>> ACK would keep an association from ever being established.
>>
>> Could set_sctp() limit the CRC computation to the length taken from the
>> IP header? Another option would be to trim the skb first, the way
>> ovs_skb_network_trim() does. Do you want to handle this in this series,
>> or as a separate fix?
>>
> 
> This might be right. I need to look better into SCTP and openvswitch 
> flows, I will write this down and look into it later. I think it should 
> be a separated patch

+1

> 
>> [Severity: Medium]
>> Does this also drop IPv6 atomic fragments? These are packets with a
>> Fragment header whose offset is 0 and whose M bit is clear, so the skb
>> holds the complete SCTP packet.
>>
>> parse_ipv6hdr() in net/openvswitch/flow.c marks any Fragment header with
>> a zero offset as OVS_FRAG_TYPE_FIRST:
>>
>> 	nexthdr = ipv6_find_hdr(skb, &payload_ofs, -1, &frag_off, &flags);
>> 	if (flags & IP6_FH_F_FRAG) {
>> 		if (frag_off) {
>> 			...
>> 		}
>> 		key->ip.frag = OVS_FRAG_TYPE_FIRST;
>>
>> ipv6_find_hdr() in net/ipv6/exthdrs_core.c sets IP6_FH_F_FRAG whenever a
>> Fragment header is present, and it masks off the M bit:
>>
>> 	_frag_off = ntohs(*fp) & ~0x7;
>>
>> As a result, set_sctp() cannot tell an atomic fragment from a real first
>> fragment.
>>
>> Before this patch, sctp_compute_cksum() covered the whole SCTP packet in
>> this case, so the old_csum ^ old_correct_csum ^ new_csum update was
>> correct and the packet was forwarded with its ports rewritten. Now
>> do_execute_actions() frees it with OVS_DROP_ACTION_ERROR and skips the
>> rest of the action list.
>>
>> RFC 8021 deprecates generating atomic fragments, but RFC 6946 still says
>> receivers must process them.
>>
>> Could the check look at the M bit, or confirm that the skb covers the
>> full IPv6 payload, rather than rejecting every OVS_FRAG_TYPE_FIRST packet?
>> IPv4 SCTP looks fine here, because key_extract() only sets
>> OVS_FRAG_TYPE_FIRST for it when IP_MF is set.
>>
> 
> Yes, it does that and AFAIU, we do not care about it. Indeed, the next 
> step is to reject the flow directly for fragments.

Atomic fragments are not actually fragments, so we could handle them
more gracefully, e.g. by not setting ip.frag in parse_ipv6hdr() when
there are no more fragments.  But that would be a separate change and
it should not be necessary in real world cases.

> 
>> [Severity: Low]
>> This isn't a bug, but should later fragments be handled the same way here
>> as in set_tcp() and set_udp()? Both of those treat OVS_FRAG_TYPE_LATER as
>> a successful no-op:
>>
>> 	if (flow_key->ip.frag == OVS_FRAG_TYPE_LATER)
>> 		return 0;
>>
>> This check returns -EINVAL for both FIRST and LATER. Take a flow that
>> wildcards ip.frag and rewrites L4 ports. IPv4 TCP/UDP later fragments are
>> forwarded unchanged and the action list continues. IPv4 SCTP later
>> fragments are dropped with OVS_DROP_ACTION_ERROR, and any actions after
>> the set are skipped.
>>
>> The commit message says the drop is intended. The same flow drops the
>> first fragment anyway, so the visible difference is mostly drop
>> accounting and whether later actions run.
>>
>> The uAPI description of OVS_ACTION_ATTR_SET in
>> include/uapi/linux/openvswitch.h also doesn't mention that a validated
>> SCTP set can fail at runtime on fragments while TCP/UDP sets are skipped.
>>
> 
> Based on the discussions with the maintainers, we do not care about it 
> and in the future we are rejecting it directly.

Also, if we're dropping the first fragment it doesn't make much sense
to allow the later ones through.

Best regards, Ilya Maximets.