Re: [kernel-hardening] RE: [PATCH] printk: introduce kptr_restrict level 3

[Kees Cook <keescook@chromium.org>]

On Fri, Oct 7, 2016 at 7:29 AM, Jann Horn <jann@thejh.net> wrote:
> On Fri, Oct 07, 2016 at 02:19:43PM +0000, Roberts, William C wrote:
>>
>>
>> > -----Original Message-----
>> > From: keescook@google.com [mailto:keescook@google.com] On Behalf Of Kees
>> > Cook
>> > Sent: Thursday, October 6, 2016 5:05 PM
>> > To: Roberts, William C <william.c.roberts@intel.com>
>> > Cc: kernel-hardening@lists.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@intel.com>
>> > wrote:
>> > >
>> > >
>> > >> -----Original Message-----
>> > >> From: keescook@google.com [mailto:keescook@google.com] On Behalf Of
>> > >> Kees Cook
>> > >> Sent: Wednesday, October 5, 2016 3:34 PM
>> > >> To: Roberts, William C <william.c.roberts@intel.com>
>> > >> Cc: kernel-hardening@lists.openwall.com; Jonathan Corbet
>> > >> <corbet@lwn.net>; linux-doc@vger.kernel.org; LKML
>> > >> <linux-kernel@vger.kernel.org>; Nick Desaulniers
>> > >> <ndesaulniers@google.com>; Dave Weinstein <olorin@google.com>
>> > >> Subject: Re: [PATCH] printk: introduce kptr_restrict level 3
>> > >>
>> > >> On Wed, Oct 5, 2016 at 11:04 AM,  <william.c.roberts@intel.com> wrote:
>> > >> > From: William Roberts <william.c.roberts@intel.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@grsecurity.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.
>
> IMO that's fine - I don't think pointers in the kernel ring buffer should be restricted.
> Instead, access to dmesg / proc/kmsg should be restricted appropriately.
>
> I guess it depends on what the goal here is. Do we really want to stop root from
> ever seeing a kernel pointer (in which case OOPS messages wouldn't really work
> anymore)? My view is that restricting these interfaces so far that only root can
> access them and it's unlikely that root accidentally does so is sufficient.

I don't think it's worth worrying about dmesg right now. Maybe later
on, but I don't think it's worth it right now. Assuming it'll work for
things landing in /proc and /sys, focusing on the user-buffer-destined
stuff seems the best use of time to me.

-Kees

-- 
Kees Cook
Nexus Security