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

Pavel Skripkin [off-list ref] 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.