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

On 8/9/21 11:39 PM, Florian Westphal wrote:

Pavel Skripkin [off-list ref] wrote:

quoted

I think, there a missing lock in this function:

	for_each_possible_cpu(cpu) {
		ct = per_cpu(nft_ct_pcpu_template, cpu);

(*)

quoted

		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 *

Hi, Florian!

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

Dumb question: why per_cpu() will return 2 different pointers for CPU 1 and CPU 0? As I understand for_each_possible_cpu() will iterate over all
CPUs which could ever be enabled. So, we can hit situation when 2 concurrent processes call per_cpu() with same cpu value (*).

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?

Anyway, I think, moving locking a bit higher is good here, let's test it. I will prepare a patch, if it will pass syzbot testing, thanks!


#syz test
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master



With regards,
Pavel Skripkin