[PATCH net] openvswitch: reject a negative transport length in the IPv6 L4 checksum update

update_ipv6_checksum() computes

	int transport_len = skb->len - skb_transport_offset(skb);

and gates the L4 access on

	if (likely(transport_len >= sizeof(struct tcphdr)))

sizeof yields size_t, so transport_len is converted to unsigned and every
negative value passes the guard.  inet_proto_csum_replace16() then reads
the transport checksum field out of bounds, and writes the updated value
back to the same address:

  BUG: KASAN: slab-use-after-free in inet_proto_csum_replace16+0x445/0x470
  Read of size 2 at addr ffff888131a4cb06 by task ovs_ipv6_oob/696
  CPU: 3 UID: 1000 PID: 696 Comm: ovs_ipv6_oob Tainted: G N 7.3.0-rc2+ #338
  Call Trace:
   inet_proto_csum_replace16+0x445/0x470
   set_ipv6_addr+0x3dd/0x460
   do_execute_actions+0x6a3d/0x7c40
   ovs_execute_actions+0xfd/0x480
   ovs_packet_cmd_execute+0xc38/0xf20
   genl_rcv_msg+0x59e/0x870
   netlink_rcv_skb+0x18b/0x450
   genl_rcv+0x2d/0x40
   netlink_unicast+0x6bc/0xa20

  The buggy address belongs to the object at ffff888131a4c980
   which belongs to the cache skbuff_small_head of size 704
  The buggy address is located 390 bytes inside of
   freed 704-byte region [ffff888131a4c980, ffff888131a4cc40)

ipv6_find_hdr() walks the extension header chain, skipping each header
by the length that header itself declares, and ipv6_optlen() returns up
to 2048.  The last skip is never checked against skb->len, and
parse_ipv6hdr() installs the result as the transport header.

Reject a negative length.

Fixes: ccb1352e76cf ("net: Add Open vSwitch kernel components.")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Norbert Szetei <redacted>
---
Reproducer available on request.

 net/openvswitch/actions.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
index dc5ff859f114..4fdc08cca058 100644
--- a/net/openvswitch/actions.c
+++ b/net/openvswitch/actions.c

@@ -360,6 +360,9 @@ static void update_ipv6_checksum(struct sk_buff *skb, u8 l4_proto,
 {
 	int transport_len = skb->len - skb_transport_offset(skb);
 
+	if (unlikely(transport_len < 0))
+		return;
+
 	if (l4_proto == NEXTHDR_TCP) {
 		if (likely(transport_len >= sizeof(struct tcphdr)))
 			inet_proto_csum_replace16(&tcp_hdr(skb)->check, skb,

-- 
2.55.0