Re: [PATCH net v3] net: openvswitch: validate transport header presence in set_ipv6_addr

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

On 10/8/26 12:41 PM, 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>
> ---
> v3: drop the redundant comment introduced in v2
> ---
>  net/openvswitch/actions.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
> index dc5ff859f114..27152bca8a8c 100644
> --- a/net/openvswitch/actions.c
> +++ b/net/openvswitch/actions.c
> @@ -358,7 +358,12 @@ 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;
> +
> +	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)))

Thanks!

Reviewed-by: Ilya Maximets <i.maximets@ovn.org>