[RESEND,nf] netfilter: avoid a race between nf_register_hook() and cleanup_net()

Message ID 20160729161904.4F00BA0BD9@unicorn.suse.cz
State Under Review
Delegated to: Pablo Neira
Headers show

Commit Message

Michal Kubecek July 29, 2016, 4:19 p.m.
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.

Fixes: 085db2c04557 ("netfilter: Per network namespace netfilter hooks.")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
---
 net/netfilter/core.c | 7 +++++++
 1 file changed, 7 insertions(+)

Comments

Eric W. Biederman July 30, 2016, 1:24 p.m. | #1
Michal Kubecek <mkubecek@suse.cz> 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@xmission.com>


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@suse.cz>
> ---
>  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(&reg->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(&reg->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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Pablo Neira Ayuso Aug. 26, 2016, 5:31 p.m. | #2
Hi Eric,

On Sat, Jul 30, 2016 at 08:24:37AM -0500, Eric W. Biederman wrote:
> Michal Kubecek <mkubecek@suse.cz> 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@xmission.com>
> 
> 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.

OK... I'm going to place this in the nf.git tree... but this is very ugly.

So Eric, I'd really appreciate if you can follow up once this has hit
nf-next.git and we get rid of the rtnl_lock and net_lock mutex by
propagating up to the the caller the hook registration from init_net()
and unregistering this from exit_net(). So we don't need to loop on
the existing netns but we use the existing netns init and exit
callbacks.

Let me know, thanks.
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Patch

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(&reg->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(&reg->list);
 	for_each_net(net)
 		nf_unregister_net_hook(net, reg);
 	rtnl_unlock();
+	mutex_unlock(&net_mutex);
 }
 EXPORT_SYMBOL(nf_unregister_hook);