Re: [PATCH] printk: introduce kptr_restrict level 3

From: Kees Cook
Date: Wed Oct 05 2016 - 15:36:07 EST

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

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

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");
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:

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.


Kees Cook
Nexus Security