linux-kernel - Re: [PATCH] wireless: fix 64K kernel heap content leak via ioctl

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Date:	Fri, 27 Aug 2010 14:43:57 -0700
From:	Kees Cook <kees.cook@...onical.com>
To:	Jean Tourrilhes <jt@....hp.com>
Cc:	linux-kernel@...r.kernel.org,
	"John W. Linville" <linville@...driver.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Johannes Berg <johannes@...solutions.net>,
	Joe Perches <joe@...ches.com>, Tejun Heo <tj@...nel.org>,
	linux-wireless@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] wireless: fix 64K kernel heap content leak via ioctl

Hi Jean,

On Fri, Aug 27, 2010 at 02:22:54PM -0700, Jean Tourrilhes wrote:
> On Fri, Aug 27, 2010 at 02:02:41PM -0700, Kees Cook wrote:
> > This problem was originally tracked down by Brad Spengler.
> > 
> > When calling wireless ioctls, if a driver does not correctly
> > validate/shrink iwp->length, the resulting copy_to_user can leak up to
> > 64K of kernel heap contents.
> > 
> > It seems that this is triggerable[1] in 2.6.32 at least on ath5k, but
> > I was not able to track down how. The twisty maze of ioctl handlers
> > stumped me. :)
> 
> 	You can always ask.
> 
> > Other drivers I checked did not appear to have any problems,
> > but the potential remains. I'm not sure if this patch is the right approach;
> > it was fixed differently[2] in grsecurity.
> 
> 	Did you tried your patch for real ? With large scan request ?
> 	I ask because at first glance, it looks incorrect, asI believe
> it kills large request. But someone would need to test, for sure.

I did not, no. Since I couldn't reproduce the original problem (I lacked
the hardware to test ath5k, and all the other drivers correctly managed
iwp->length).

> > [1] http://forums.grsecurity.net/viewtopic.php?f=3&t=2290&start=0
> > [2] http://grsecurity.net/~spender/wireless-infoleak-fix2.patch
> 
> 	I believe this patch would make the situation worse.
> 
> 	Would you mind validating the following patch ? I've just
> verified that it compiles and I believe it does what you are asking in
> a much more predictable way.

> -               if (copy_to_user(iwp->pointer, extra,
> -                                iwp->length *
> -                                descr->token_size)) {
> +               /* Verify how much we should return. Some driver
> +                * may abuse iwp->length... */
> +               if((iwp->length * descr->token_size) < extra_size)
> +                       extra_size = iwp->length * descr->token_size;
> +
> +               if (copy_to_user(iwp->pointer, extra, extra_size)) {

The comment should probably be clarified -- it's the caller's iwp->length
that may be causing problems (when combined with a driver that forgets to
adjusted iwp->length downward). Regardless, the above patch would appear to
limit the copy_to_user to only the kzalloced region.

Thanks!

-Kees

-- 
Kees Cook
Ubuntu Security Team
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