Re: [PATCH] ubi: avoid workqueue format string leak

[Ezequiel Garcia <ezequiel.garcia@free-electrons.com>]

On Apr 08, Kees Cook wrote:
> On Tue, Apr 8, 2014 at 7:43 AM, Artem Bityutskiy
> <artem.bityutskiy@linux.intel.com> wrote:
> > On Tue, 2014-04-08 at 10:57 -0300, Ezequiel Garcia wrote:
> >> Hello Kees,
> >>
> >> Thanks for the patch.
> >>
> >> On Apr 07, Kees Cook wrote:
> >> > When building the name for the workqueue thread, make sure a format
> >> > string cannot leak in from the disk name.
> >> >
> >>
> >> Could you enlighten me and explain why you want to avoid the name leak?
> >> Is it a security concern?
> >>
> >> I'd like to understad this better, so I can avoid making such mistakes
> >> in the future.
> >
> > Well, the basics seem to be simple, attacker makes sure gd->disk_name
> > contains a bunch of "%s" and other placeholders, and this leads
> > "workqueue_alloc()" to read kernel memory and form the workqueue name.
> 
> Right. I don't think there is an actual exploitable vulnerability
> here, but it's a best-practice to not pass variable strings in as a
> potential format string.
> 

I see, thanks for explanation. I'll certainly try to keep this in mind!

> > I did not think it through further, though, but that was enough for me
> > to apply the patch right away. But yeah, curios parts are:
> >
> > 1. How attacker could end up with a crafted "gd->disk_name"
> 
> At present, the only way I know how to set that is via some special
> controls in md, but I assume that would not work via ubi.
> 

I guess it's not possible in our case, given we are hard-setting the name
to ubiblock%d_%d:

  sprintf(gd->disk_name, "ubiblock%d_%d", dev->ubi_num, dev->vol_id);

Nevertheless the fix is valid, so thanks a lot and keep them coming!
-- 
Ezequiel García, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com