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Date: Fri, 7 Oct 2016 14:19:43 +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: Thursday, October 6, 2016 5:05 PM
> To: Roberts, William C <william.c.roberts@...el.com>
> Cc: kernel-hardening@...ts.openwall.com
> Subject: Re: [PATCH] printk: introduce kptr_restrict level 3
>
> On Thu, Oct 6, 2016 at 8:18 AM, Roberts, William C <william.c.roberts@...el.com>
> wrote:
> >
> >
> >> -----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?
>
> It looks like what Grsecurity is doing is saying "if the address is outside of user-
> space" (" > TASK_SIZE") and it's not whitelisted ('P',
> 'X') and it's going to land in a user-space buffer ("is_usercopy_object()", censor it.
> ("K" is already censored -- they're just optimizing to avoid re-checking it
> needlessly.)
>
> So, in this case, all kernel memory, bss and stack included, would be outside the
> user-space address range. (I am curious, however, how to apply this to an
> architecture like s390 which has overlapping address ranges... probably the
> TASK_SIZE test needs to use some other "is in kernel memory" check that
> compiles down to TASK_SIZE on non-s390, and DTRT on s390, etc.)
>
Before I go off and attempt this, I just have another dumb question to ask:
If the printk copies it into the kernel ring buffer, at some point, someone comes
And asks for a copy into a userspace buffer either via dmesg or proc/kmsg interfaces.
Heck, it may even be on an open uart serial. How would the check actually work,
My understanding, at the time %p is resolved into a string, it might not be heading
to a userspace buffer. Perhaps you can help fill in what I am missing?
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