On 9/15/26 9:55 PM, Norbert Szetei wrote:
> 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.
Best regards, Ilya Maximets.
>
> Reject a negative length.
>
> Fixes: ccb1352e76cf ("net: Add Open vSwitch kernel components.")
> Cc: [email protected]
> Assisted-by: LLM
> Signed-off-by: Norbert Szetei <[email protected]>
> ---
> 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,
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