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Date: Mon, 1 Aug 2016 14:34:35 +0200 From: Pablo Neira Ayuso <pablo@...filter.org> To: "Eric W. Biederman" <ebiederm@...ssion.com> Cc: Michal Kubecek <mkubecek@...e.cz>, Patrick McHardy <kaber@...sh.net>, Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>, netfilter-devel@...r.kernel.org, coreteam@...filter.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH RESEND nf] netfilter: avoid a race between nf_register_hook() and cleanup_net() On Sat, Jul 30, 2016 at 08:24:37AM -0500, Eric W. Biederman wrote: > Michal Kubecek <mkubecek@...e.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@...ssion.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. Please, propagate up to the caller to register and unregister the hooks from init_net and exit_net instead as I suggested time ago. I understand that this is not as small as this patch, and that this will require a bit more boiler plate code in iptable_*.c and nftables itself, but we'll avoid the dependencies with both rtnl_lock and net_lock. Thanks.
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