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

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

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().

> 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.

Best regards, Ilya Maximets.