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

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

On 10/1/26 1:15 PM, Ilya Maximets wrote:
> On 10/1/26 10:52 AM, Fernando Fernandez Mancera wrote:
>> When executing IPv6 address rewrite actions on IPv6 fragments,
>> set_ipv6_addr() might attempt to recalculate the L4 header checksum
>> calling skb_transport_header() with an uninitialized transport header.
>> This can potentially lead to a OOB write.
> 
> Not really.  later fragments have l4_proto set to NEXTHDR_FRAGMENT
> and so no writes will be performed.  See parse_ipv6hdr().
> 

Oh, I didn't notice that. In that case, this is harmless. I will update 
the commit message, thanks!

>> A warning is triggered with DEBUG_NET=y.
> 
> Yeah, while the value is not actually used to access the packet data,
> the skbuff core generates a warning on read.
> 
>>
>> Fix this by recalculating the checksum only if there is a valid
>> transport header in the skb. Otherwise, assume this is a fragment and
>> skip it.
>>
>> 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 | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
>> index dc5ff859f114..a14668565ec4 100644
>> --- a/net/openvswitch/actions.c
>> +++ b/net/openvswitch/actions.c
>> @@ -395,7 +395,7 @@ static void set_ipv6_addr(struct sk_buff *skb, u8 l4_proto,
>>   			  __be32 addr[4], const __be32 new_addr[4],
>>   			  bool recalculate_csum)
>>   {
>> -	if (recalculate_csum)
>> +	if (recalculate_csum && skb_transport_header_was_set(skb))
>>   		update_ipv6_checksum(skb, l4_proto, addr, new_addr);
> I'd suggest moving the check into update_ipv6_checksum() and maybe add
> a small comment that the check is only to avoid warning on reading the
> offset that wasn't set, offset must always be set for any l4_proto for
> which the checksum is actually getting updated.
> 

Will do that. Thanks Ilya.

> Best regards, Ilya Maximets.