linux-imx

netfilter: nft_compat: make lists per netns

There are two problems with nft_compat since the netlink config
plane uses a per-netns mutex:

1. Concurrent add/del accesses to the same list
2. accesses to a list element after it has been free'd already.

This patch fixes the first problem.

Freeing occurs from a work queue, after transaction mutexes have been
released, i.e., it still possible for a new transaction (even from
same net ns) to find the to-be-deleted expression in the list.

The ->destroy functions are not allowed to have any such side effects,
i.e. the list_del() in the destroy function is not allowed.

This part of the problem is solved in the next patch.
I tried to make this work by serializing list access via mutex
and by moving list_del() to a deactivate callback, but
Taehee spotted following race on this approach:

  NET #0                          NET #1
   >select_ops()
   ->init()
                                   ->select_ops()
   ->deactivate()
   ->destroy()
      nft_xt_put()
       kfree_rcu(xt, rcu_head);
                                   ->init() <-- use-after-free occurred.

Unfortunately, we can't increment reference count in
select_ops(), because we can't undo the refcount increase in
case a different expression fails in the same batch.

(The destroy hook will only be called in case the expression
 was initialized successfully).

Fixes: f102d66b335a ("netfilter: nf_tables: use dedicated mutex to guard transactions")
Reported-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>