RE: [PATCH] printk: introduce kptr_restrict level 3

[Roberts, William C]

> -----Original Message-----
> From: keescook@xxxxxxxxxx [mailto:keescook@xxxxxxxxxx] On Behalf Of Kees
> Cook
> Sent: Wednesday, October 5, 2016 3:34 PM
> To: Roberts, William C <william.c.roberts@xxxxxxxxx>
> Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx; Jonathan Corbet <corbet@xxxxxxx>;
> linux-doc@xxxxxxxxxxxxxxx; LKML <linux-kernel@xxxxxxxxxxxxxxx>; Nick
> Desaulniers <ndesaulniers@xxxxxxxxxx>; Dave Weinstein <olorin@xxxxxxxxxx>
> Subject: Re: [PATCH] printk: introduce kptr_restrict level 3
> 
> On Wed, Oct 5, 2016 at 11:04 AM,  <william.c.roberts@xxxxxxxxx> wrote:
> > From: William Roberts <william.c.roberts@xxxxxxxxx>
> >
> > Some out-of-tree modules do not use %pK and just use %p, as it's the
> > common C paradigm for printing pointers. Because of this,
> > kptr_restrict has no affect on the output and thus, no way to contain
> > the kernel address leak.
> 
> Solving this is certainly a good idea -- I'm all for finding a solid solution.
> 
> > Introduce kptr_restrict level 3 that causes the kernel to treat %p as
> > if it was %pK and thus always prints zeros.
> 
> I'm worried that this could break kernel internals where %p is being used and not
> exposed to userspace. Maybe those situations don't exist...

Not saying they don't I didn't find any.

> 
> Regardless, I would rather do what Grsecurity has done in this area, and whitelist
> known-safe values instead. For example, they have %pP for approved pointers,
> and %pX for approved
> dereference_function_descriptor() output. Everything else is censored if it is a
> value in kernel memory and destined for a user-space memory
> buffer:
> 
>         if ((unsigned long)ptr > TASK_SIZE && *fmt != 'P' && *fmt != 'X' && *fmt !=
> 'K' && is_usercopy_object(buf)) {
>                 printk(KERN_ALERT "grsec: kernel infoleak detected!
> Please report this log to spender@xxxxxxxxxxxxxxx\n");
>                 dump_stack();
>                 ptr = NULL;
>         }
> 
> The "is_usercopy_object()" test is something we can add, which is testing for a
> new SLAB flag that is used to mark slab caches as either used by user-space or
> not, which is done also through whitelisting.
> (For more details on this, see:
> http://www.openwall.com/lists/kernel-hardening/2016/06/08/10)
> 
> Would you have time/interest to add the slab flags and is_usercopy_object()?
> The hardened usercopy part of the slab whitelisting can be separate, since it likely
> needs a different usercopy interface to sanely integrate with upstream.

I could likely take this on. I would need to read up on the links and have a better concept
of what it is.

> 
> -Kees
> 
> --
> Kees Cook
> Nexus Security