On Wed, Sep 16, 2026 at 09:57:53PM +0200, Norbert Szetei wrote:
> ipv6_find_hdr() walks the extension header chain, skipping each header by
> the length that header itself declares. ipv6_optlen() returns up to 2048,
> and the skip is never checked against skb->len, so the offset stored in
> *offset can point past the end of the packet.
>
> openvswitch installs that offset as the transport header, and
> update_ipv6_checksum() then reads and writes the transport checksum field
> out of bounds:
>
> BUG: KASAN: slab-use-after-free in inet_proto_csum_replace16+0x445/0x470
> Read of size 2 at addr ffff88810b754b06 by task ovs_ipv6_oob/629
> CPU: 4 UID: 1000 PID: 629 Comm: ovs_ipv6_oob Tainted: G N 7.3.0-rc3+ #348
> 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 ffff88810b754980
> which belongs to the cache skbuff_small_head of size 704
> The buggy address is located 390 bytes inside of
> freed 704-byte region [ffff88810b754980, ffff88810b754c40)
>
> Other callers use that offset too, so bound it here rather than in one
> caller.
>
> Reject a header whose declared length does not fit in the packet.
> ipv6_find_hdr() already fails with -EBADMSG on a malformed chain, so this
> adds no new failure mode.
>
> Fixes: f8f626754ebe ("ipv6: Move ipv6_find_hdr() out of Netfilter code.")
Blaming b777e0ce7437 would be more accurate, but it doesn't matter in
practice given that f8f626754ebe is from 2012.
> Suggested-by: Ilya Maximets <[email protected]>
> Suggested-by: Eric Dumazet <[email protected]>
> Cc: [email protected]
> Assisted-by: LLM
> Signed-off-by: Norbert Szetei <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
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