linux-kernel - Re: next-20170510 refcount_inc() on zero / use-after-free in key

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Date:   Fri, 12 May 2017 16:29:39 +0100
From:   David Howells <dhowells@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     dhowells@...hat.com, linux-kernel@...r.kernel.org,
        Elena Reshetova <elena.reshetova@...el.com>,
        keyrings@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        James Morris <james.l.morris@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: next-20170510 refcount_inc() on zero / use-after-free in key_lookup()

Mark Rutland <mark.rutland@....com> wrote:

> From a quick look at key_lookup(), the following looks very suspicious:
> 
> found:
>         /* pretend it doesn't exist if it is awaiting deletion */
>         if (refcount_read(&key->usage) == 0)
>                 goto not_found;
> 
>         /* this races with key_put(), but that doesn't matter since key_put()
>          * doesn't actually change the key
>          */
>         __key_get(key);
> 
> ... as if we can race with key_put(), we can see a zero refcount here,
> and the race *does* matter.

No, it doesn't.

If key_put() reduces a refcount to 0, it doesn't do anything other than poke
the gc thread:

	void key_put(struct key *key)
	{
		if (key) {
			key_check(key);

			if (refcount_dec_and_test(&key->usage))
				schedule_work(&key_gc_work);
		}
	}

in particular, no indication of the reduced key is passed.

The gc thread scans the entire key serial tree under the key_serial_lock
looking for keys that are no longer ref'd.  No one else is allowed to remove
keys from the tree.  This means that the gc thread can safely leave a cursor
pointing into the midst of the tree with no locks held whilst it yields to the
scheduler.

The code you quoted above in key_lookup() is inside the key_serial_lock, so it
prevents the gc thread from culling a key when it resurrects it.

So the problem isn't the key code, it's the refcount code.

As I've said before, the refcount code needs an increment op that permits
inc-from-0.  In this case, it's perfectly okay.

David