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: f102d66b33 ("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>
cf52572ebb