Re: [PATCH net v2] net: openvswitch: validate transport header presence in set_ipv6_addr

[Fernando Fernandez Mancera <fmancera@suse.de>]

On 10/7/26 10:53 PM, Ilya Maximets wrote:
> On 10/2/26 9:52 AM, Fernando Fernandez Mancera wrote:
>> When executing IPv6 address rewrite actions on IPv6 fragments,
>> set_ipv6_addr() calls update_ipv6_checksum(). If parse_ipv6hdr()
>> processes a non-first IPv6 fragment, it sets key->ip.proto to
>> NEXTHDR_FRAGMENT and returns early without calling
>> skb_set_transport_header().
>>
>> update_ipv6_checksum() unconditionally evaluates skb_transport_offset()
>> on entry before checking l4_proto.  Because skb->transport_header is
>> uninitialized, this triggers a warning under CONFIG_DEBUG_NET=y although
>> it is completely harmless.
>>
>> Fix this by returning early in update_ipv6_checksum() if l4_proto is
>> NEXTHDR_FRAGMENT. This avoids reading the uninitialized transport offset
>> for fragments while preserving the debug warning for any other protocol
>> where the transport header is unexpectedly missing.
>>
>> See the syzbot trace:
>>
>> !skb_transport_header_was_set(skb)
>> WARNING: ./include/linux/skbuff.h:3075 at skb_transport_header include/linux/skbuff.h:3075 [inline], CPU#1: syz-executor463/5635
>> WARNING: ./include/linux/skbuff.h:3075 at skb_transport_offset include/linux/skbuff.h:3250 [inline], CPU#1: syz-executor463/5635
>> WARNING: ./include/linux/skbuff.h:3075 at update_ipv6_checksum net/openvswitch/actions.c:361 [inline], CPU#1: syz-executor463/5635
>> WARNING: ./include/linux/skbuff.h:3075 at set_ipv6_addr+0x462/0x660 net/openvswitch/actions.c:399, CPU#1: syz-executor463/5635
>> [...]
>> RIP: 0010:skb_transport_header include/linux/skbuff.h:3075 [inline]
>> RIP: 0010:skb_transport_offset include/linux/skbuff.h:3250 [inline]
>> RIP: 0010:update_ipv6_checksum net/openvswitch/actions.c:361 [inline]
>> RIP: 0010:set_ipv6_addr+0x462/0x660 net/openvswitch/actions.c:399
>> [...]
>> Call Trace:
>>   <TASK>
>>   set_ipv6 net/openvswitch/actions.c:531 [inline]
>>   do_execute_actions+0x557e/0x8600 net/openvswitch/actions.c:1366
>>   ovs_execute_actions+0xde/0x520 net/openvswitch/actions.c:1592
>>   ovs_packet_cmd_execute+0xb4f/0xf10 net/openvswitch/datapath.c:705
>>   genl_family_rcv_msg_doit+0x233/0x340 net/netlink/genetlink.c:1114
>>   genl_rcv_msg+0x614/0x7a0 net/netlink/genetlink.c:1209
>>   netlink_rcv_skb+0x226/0x4a0 net/netlink/af_netlink.c:2572
>>   genl_rcv+0x28/0x40 net/netlink/genetlink.c:1218
>>   netlink_unicast+0x7bd/0x940 net/netlink/af_netlink.c:1361
>>   netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1916
>>   sock_sendmsg_nosec+0x14e/0x190 net/socket.c:800
>>
>> Reported-by: syzbot+4cc63fcfb3845e149969@syzkaller.appspotmail.com
>> Closes: https://syzkaller.appspot.com/bug?extid=4cc63fcfb3845e149969
>> Fixes: 3fdbd1ce11e5 ("openvswitch: add ipv6 'set' action")
>> Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
>> ---
>>   net/openvswitch/actions.c | 10 +++++++++-
>>   1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
>> index dc5ff859f114..68b42e900c73 100644
>> --- a/net/openvswitch/actions.c
>> +++ b/net/openvswitch/actions.c
>> @@ -358,7 +358,15 @@ static void set_ip_addr(struct sk_buff *skb, struct iphdr *nh,
>>   static void update_ipv6_checksum(struct sk_buff *skb, u8 l4_proto,
>>   				 __be32 addr[4], const __be32 new_addr[4])
>>   {
>> -	int transport_len = skb->len - skb_transport_offset(skb);
>> +	int transport_len;
>> +
>> +	/* avoid reading the transport header offset if it isn't set,
>> +	 * as it triggers a warning
>> +	 */
> 
> nit: We prefer full sentences, i.e. start with a capital and end with a dot.
> 
> But also, I think, since the switch to NEXTHDR_FRAGMENT check, the comment
> lost it's intended purpose as the code is pretty much self-documenting now.
> It's clear that the fragment doesn't have the transport header.  The comment
> made sense if we needed to explain in which case we can get here without
> having the offset initialized.  I'd suggest we drop the comment.
> 
> You're also not adding such comments in the other patch for ipv4.
> 

Fair, I thought that in IPv4 case it was more obvious than here. Anyway, 
let me just drop it in a v3. Thanks a lot for the reviews :-)

> Otherwise, LGTM.
> 
>> +	if (l4_proto == NEXTHDR_FRAGMENT)
>> +		return;
>> +
>> +	transport_len = skb->len - skb_transport_offset(skb);
>>   
>>   	if (l4_proto == NEXTHDR_TCP) {
>>   		if (likely(transport_len >= sizeof(struct tcphdr)))
> 
> Best regards, Ilya Maximets.
>