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Date: Thu, 6 Oct 2016 15:18:03 +0000
From: "Roberts, William C" <william.c.roberts@...el.com>
To: Kees Cook <keescook@...omium.org>
CC: "kernel-hardening@...ts.openwall.com"
<kernel-hardening@...ts.openwall.com>
Subject: RE: [PATCH] printk: introduce kptr_restrict level 3
> -----Original Message-----
> From: keescook@...gle.com [mailto:keescook@...gle.com] On Behalf Of Kees
> Cook
> Sent: Wednesday, October 5, 2016 3:34 PM
> To: Roberts, William C <william.c.roberts@...el.com>
> Cc: kernel-hardening@...ts.openwall.com; Jonathan Corbet <corbet@....net>;
> linux-doc@...r.kernel.org; LKML <linux-kernel@...r.kernel.org>; Nick
> Desaulniers <ndesaulniers@...gle.com>; Dave Weinstein <olorin@...gle.com>
> Subject: Re: [PATCH] printk: introduce kptr_restrict level 3
>
> On Wed, Oct 5, 2016 at 11:04 AM, <william.c.roberts@...el.com> wrote:
> > From: William Roberts <william.c.roberts@...el.com>
> >
> > 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...
>
> 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@...ecurity.net.\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.
A couple of questions off hand:
1. What about bss statics? I am assuming that when the loader loads up a module
That it's dynamically allocating the .bss section or some equivalent. I would
Also assume the method you describe would catch that, is that correct?
2. What about stack variables?
>
> -Kees
>
> --
> Kees Cook
> Nexus Security
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