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

Kees Cook
2010-08-27T21:45:10Z

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