Re: [PATCH net] ipv6: ioam: fix type confusion of dst_entry

[Justin Iurman <justin.iurman@gmail.com>]

On 6/18/26 12:43, Jiayuan Chen wrote:
> IOAM uses a dummy dst_entry(null_dst) to mark that the destination should
> not be changed after the transformation. This dst is stored in the IOAM lwt
> state and may be passed to dst_cache_set_ip6().
> 
> However, the IPv6 dst cache path eventually calls rt6_get_cookie(), which
> treats the dst_entry as part of a struct rt6_info. Since the null_dst was
> embedded directly as a struct dst_entry in struct ioam6_lwt, this resulted
> in an invalid cast and rt6_get_cookie() reading fields from the wrong
> object.
> 
> In practice, the wrong cookie is not used while dst->obsolete is zero, but
> rt6_get_cookie() may also access per-cpu value when rt->sernum is
> zero. In this case, rt->sernum aliases ioam6_lwt::cache::reset_ts, which
> can become zero, making this a potential invalid pointer access.
> 
> Fix this by embedding a full struct rt6_info for the dummy IPv6 route and
> passing its dst member to the dst APIs.

Good catch, thanks!

> Fixes: 47ce7c854563 ("net: ipv6: ioam6: fix double reallocation")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

Reviewed-by: Justin Iurman <justin.iurman@gmail.com>