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

On Tue, Sep 15, 2026 at 2:13 PM Ilya Maximets [off-list ref] wrote:

On 9/15/26 9:55 PM, Norbert Szetei wrote:

quoted

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.

Hi, Norbert.

This looks like something that needs to be fixed in the ipv6_find_hdr().
IMO, this function should not return an offset that doesn't exist in the
packet, i.e. we should not be able to find what's not in there.

CC: David and Ido.

In general, we should be able to trust the skb_transport_offset(),
otherwise there could be other issues all over the place, not only in
openvswitch.

Totally agree.
We keep receiving defensive code instead of finding and fixing the
root causes :/

Fix ipv6_find_hdr() in net/ipv6/exthdrs_core.c

if (skb->len - start < hdrlen)
    return -EBADMSG;