Subject: Re: [syzbot] KASAN: use-after-free Write in nft_ct_tmpl_put_pcpu

From: Florian Westphal <[email-masked]>
Date: Mon, 9 Aug 2021 22:39:16 +0200

Pavel Skripkin <[email-masked]> wrote:
> I think, there a missing lock in this function:
> 
> 	for_each_possible_cpu(cpu) {
> 		ct = per_cpu(nft_ct_pcpu_template, cpu);
> 		if (!ct)
> 			break;
> 		nf_ct_put(ct);
> 		per_cpu(nft_ct_pcpu_template, cpu) = NULL;
> 		
> 	}
> 
> Syzbot hit a UAF in nft_ct_tmpl_put_pcpu() (*), but freed template should be
> NULL.
> 
> So I suspect following scenario:
> 
> 
> CPU0:			CPU1:
> = per_cpu()
> 			= per_cpu()
> 
> nf_ct_put
> per_cpu = NULL
> 			nf_ct_put()
> 			* UAF *

Yes and no.  The above is fine since pcpu will return different pointers
for cpu 0 and 1.

The race is between two different net namespaces that race when
changing nft_ct_pcpu_template_refcnt.
This happens since

commit f102d66b335a417d4848da9441f585695a838934
netfilter: nf_tables: use dedicated mutex to guard transactions

Before this, all transactions were serialized by a global mutex,
now we only serialize transactions in the same netns.

Its probably best to add
DEFINE_MUTEX(nft_ct_pcpu_mutex) and then acquire that when we need to
inc/dec the nft_ct_pcpu_template_refcnt so we can't have two distinct
cpus hitting a zero refcount.

Would you send a patch for this?

Thanks.