linux-kernel - Re: race condition in crypto larval handling

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Date:	Sat, 7 Sep 2013 23:01:18 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"David S. Miller" <davem@...emloft.net>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-crypto <linux-crypto@...r.kernel.org>,
	Tyler Hicks <tyhicks@...onical.com>
Subject: Re: race condition in crypto larval handling

On Sat, Sep 7, 2013 at 9:54 PM, Herbert Xu <herbert@...dor.apana.org.au> wrote:
> On Sun, Sep 08, 2013 at 02:37:03PM +1000, Herbert Xu wrote:
>> On Sat, Sep 07, 2013 at 08:34:15PM -0700, Kees Cook wrote:
>> >
>> > However, I noticed on the "good" path (even without the above patch),
>> > I sometimes see a double-kfree triggered by the modprobe process. I
>> > can't, however, see how that's happening, since larval_destroy should
>> > only be called when refcnt == 0.
>>
>> Do you still see this double free with this patch? Without the
>> patch it is completely expected as killing the same lavral twice
>> will cause memory corruption leading to all sorts of weirdness,
>> even if you stop it from deleting the list entry twice.

I noticed while testing the larval_kill fix, and then tried it again
after reverting the fix -- both showed the behavior.

> Actually I know what it is.  sha512 registers two algorithms.
> Therefore, it will create two larvals in sequence and then destroy
> them in turn.  So it's not a double free at all.  If you put a
> printk in crypto_larval_alloc that should confirm this.

Ah! That would make sense; it just happens to re-allocate to the exact
same location, yes. Whew, that's certainly what's happening. I can
retest to confirm in my morning.

Thanks again for the larval_kill fix! I'll get it rolled out for wider
testing to confirm that it make our crash numbers go down.

-Kees

-- 
Kees Cook
Chrome OS Security
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