Michal Kubecek <mkubecek@xxxxxxx> writes:
> There is a race condition between nf_{,un}register_hook() and
> cleanup_net() which can either trigger WARN check or cause a memory
> leak. The scenario is like this (2a and 2b are alternatives):
>
> 1. cleanup_net() removes one or more struct net from net_namespace_list
> 2a. nf_register_hook() adds per-netns hooks to all netns (but not those
> removed in step 1) and adds the hook to global nf_hook_list
> 2b. nf_unregister_hook() deletes per-netns hooks from all netns (but not
> those removed in step 1) and removes the hook from nf_hook_list
> 3. cleanup_net() calls pernet subsystem exit functions for netns being
> removed; one of them is netfilter_net_exit() which (among others)
> calls nf_unregister_net_hook() to unregister per-netns hooks for all
> hooks in nf_hook_list.
>
> In case (a), per-netns hooks are never added as the namespace was
> already invisible to for_each_net() in step 2a but an attempt to remove
> them in step 3 (the hook is already in nf_hook_list) triggers a WARN
> check in nf_unregister_net_hook() (no real harm done, however). In case
> (b), the per-netns hook is removed neither in step 2b (netns is already
> invisible to for_each_net()) nor in step 3 (the hook is already removed
> from nf_hook_list), causing a memory leak.
>
> Prevent the race by protecting the for_each_net() loop in
> nf_{,un}register_hook() (also) by net_mutex. There is already a
> precendens for this in rtnl_link_unregister() which addresses similar
> race.
So this analysis of a problem appears to be spot on.
Reviewed-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
I really really want there to be a better way to do this, but it is
really not ok for a hook to continue it's life past
nf_unregister_net_hook as after that point the code may be removed
from the kernel (sigh).
Although keeping with the precedent and minimizing net_mutex
we could remove the WARN and keep nf_register_hook as it is.
But that sounds entirely too clever for a fix that will
probably be backported.
But that sounds entirely too clever for a fix that likely needs to be
backported.
Eric
> Fixes: 085db2c04557 ("netfilter: Per network namespace netfilter hooks.")
> Signed-off-by: Michal Kubecek <mkubecek@xxxxxxx>
> ---
> net/netfilter/core.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/net/netfilter/core.c b/net/netfilter/core.c
> index f39276d1c2d7..860978c9f82e 100644
> --- a/net/netfilter/core.c
> +++ b/net/netfilter/core.c
> @@ -193,6 +193,8 @@ int nf_register_hook(struct nf_hook_ops *reg)
> struct net *net, *last;
> int ret;
>
> + /* prevent race with cleanup_net() */
> + mutex_lock(&net_mutex);
> rtnl_lock();
> for_each_net(net) {
> ret = nf_register_net_hook(net, reg);
> @@ -201,6 +203,7 @@ int nf_register_hook(struct nf_hook_ops *reg)
> }
> list_add_tail(®->list, &nf_hook_list);
> rtnl_unlock();
> + mutex_unlock(&net_mutex);
>
> return 0;
> rollback:
> @@ -211,6 +214,7 @@ rollback:
> nf_unregister_net_hook(net, reg);
> }
> rtnl_unlock();
> + mutex_unlock(&net_mutex);
> return ret;
> }
> EXPORT_SYMBOL(nf_register_hook);
> @@ -219,11 +223,14 @@ void nf_unregister_hook(struct nf_hook_ops *reg)
> {
> struct net *net;
>
> + /* prevent race with cleanup_net() */
> + mutex_lock(&net_mutex);
> rtnl_lock();
> list_del(®->list);
> for_each_net(net)
> nf_unregister_net_hook(net, reg);
> rtnl_unlock();
> + mutex_unlock(&net_mutex);
> }
> EXPORT_SYMBOL(nf_unregister_hook);
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