Re: [PATCH net] netlink: avoid hashing the network namespace pointer

[Eric Dumazet <edumazet@kernel.org>]

On 10/7/26 00:41, Kyle Zeng wrote:
> The netlink rhashtable key includes a raw struct net pointer and a
> user-controlled port ID.  Both /proc/net/netlink and socket diagnostics
> expose the table's bucket order.  By binding and rebinding chosen
> NETLINK_USERSOCK port IDs, an unprivileged reader can distinguish equal
> buckets and recover the low bits of the Jenkins hash.  Its 32-bit seed
> and the limited set of kernel-image slides can then be searched offline
> to recover the address of init_net.
> 
> Use the namespace's unique, non-address ID in the comparison key
> instead.  This ID is assigned before the per-net initializers run and
> remains unchanged for the namespace's lifetime.  The lookup key and
> object hash are still built by netlink_compare_arg_init(), keeping
> lookup, insertion, removal and rehashing consistent while preserving
> namespace separation.  Neither public table walker needs to change.
> 

Please use net->net_cookie instead.

It has the same value in current trees (net->net_cookie = 
ns_tree_gen_id(net)),
but ns.ns_id only appeared in 6.18, while your Fixes: tag points to
a 2015 commit.

net_cookie is set at the top of setup_net() in stable kernels >= 5.15,
so backports would be trivial.

 >  struct netlink_compare_arg
 >  {
 > -       possible_net_t pnet;
 > +       u64 netns_id;

'netns_id' is confusing, we already have NETNSA_NSID and net->netns_ids.

 > -              !net_eq(sock_net(&nlk->sk), read_pnet(&x->pnet));
 > +              sock_net(&nlk->sk)->ns.ns_id != x->netns_id;

We now dereference sock_net() from netlink_compare(), under RCU,
possibly for a socket of a dismantling netns.

I think this is fine (sockets are freed after call_rcu(), and
cleanup_net() has an rcu_barrier() before freeing the netns),
but please mention it in the changelog.

Thanks.