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Date: Fri, 7 Oct 2016 12:12:50 -0700 From: Kees Cook <keescook@...omium.org> To: Jann Horn <jann@...jh.net> Cc: "Roberts, William C" <william.c.roberts@...el.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: RE: [PATCH] printk: introduce kptr_restrict level 3 On Fri, Oct 7, 2016 at 7:29 AM, Jann Horn <jann@...jh.net> wrote: > On Fri, Oct 07, 2016 at 02:19:43PM +0000, Roberts, William C wrote: >> >> >> > -----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. > > 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
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