Re: [PATCH nf] netfilter: sync with packet rx also after removing queue entries

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Thu, Oct 08, 2015 at 11:38:07PM +0200, Florian Westphal wrote:
> We need to sync packet rx again after flushing the queue entries.
> Otherwise, the following race could happen:
> 
> cpu1: nf_unregister_hook(H) called, H unliked from lists, calls
> synchronize_net() to wait for packet rx completion.
> 
> Problem is that while no new nf_queue_entry structs that use H can be
> allocated, another CPU might receive a verdict from userspace just before
> cpu1 calls nf_queue_nf_hook_drop to remove this entry:
> 
> cpu2: receive verdict from userspace, lock queue
> cpu2: unlink nf_queue_entry struct E, which references H, from queue list
> cpu1: calls nf_queue_nf_hook_drop, blocks on queue spinlock
> cpu2: unlock queue
> cpu1: nf_queue_nf_hook_drop drops affected queue entries
> cpu2: call nf_reinject for E
> cpu1: kfree(H)
> cpu2: potential use-after-free for H
> 
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Fixes: 085db2c04557 ("netfilter: Per network namespace netfilter hooks.")
> Signed-off-by: Florian Westphal <fw@strlen.de>
> ---
> Found during code review, no actual panic was observed.
> 
> diff --git a/net/netfilter/core.c b/net/netfilter/core.c
> index 8e47f81..21a0856 100644
> --- a/net/netfilter/core.c
> +++ b/net/netfilter/core.c
> @@ -152,6 +152,8 @@ void nf_unregister_net_hook(struct net *net, const struct nf_hook_ops *reg)
>  #endif
>  	synchronize_net();
>  	nf_queue_nf_hook_drop(net, &entry->ops);
> +	/* other cpu might still process nfqueue verdict that used reg */
> +	synchronize_net();
>  	kfree(entry);
>  }
>  EXPORT_SYMBOL(nf_unregister_net_hook);

I cannot find any better way to resolve this.

Applied, thanks Florian!