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

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Message-ID: <CAGXu5jLQ8oex=PMftRsFcoB78SXSRM3LTVoib61agA392vSvTA@mail.gmail.com>
Date:	Sun, 8 Sep 2013 08:52:11 -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 11:01 PM, Kees Cook <keescook@...omium.org> wrote:
> 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.
>>
>> 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.

Confirmed: 2 allocs happen, and then 2 kfrees. :)

-Kees

-- 
Kees Cook
Chrome OS Security
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