On Fri, Oct 07, 2016 at 03:05:43PM +0000, Roberts, William C wrote: > > > > -----Original Message----- > > From: Jann Horn [mailto:jann@thejh.net] > > Sent: Friday, October 7, 2016 10:30 AM > > To: Roberts, William C > > Cc: Kees Cook ; kernel- > > hardening@lists.openwall.com > > Subject: Re: [kernel-hardening] RE: [PATCH] printk: introduce kptr_restrict level 3 > > > > 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 > > > > 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 > > > > > > > > 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 > > > > >> Cc: kernel-hardening@lists.openwall.com; Jonathan Corbet > > > > >> ; linux-doc@vger.kernel.org; LKML > > > > >> ; Nick Desaulniers > > > > >> ; Dave Weinstein > > > > >> Subject: Re: [PATCH] printk: introduce kptr_restrict level 3 > > > > >> > > > > >> On Wed, Oct 5, 2016 at 11:04 AM, wrote: > > > > >> > From: William Roberts > > > > >> > > > > > >> > 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'm running Ubuntu 14.04, and perhaps they just got it wrong, but I can do dmesg as > an unprivileged user. I usually only work on Android, and its restricted to root. I'm > not sure how other distro's set this up. Ideally, on non-debug systems, I'd like to > see %p's filtered no matter what, I'd think opt in (whitelist) is ok. At least than, > you did it intentionally. I verified that %p filtering gets us coredumps and kernel > oops's have addresses. On a system with panic_on_oops=0 and dmesg_restrict=0, if you want to leak where the kernel and data are to be able to exploit a memory corruption properly, you can probably often just trigger the bug, get the addresses from the resulting oops, set up the addresses in your exploit properly and retry the attack. > Ideally on user images debug interfaces are closed, but I would like to make rogue %p's > without opt in go away. > I did a little research, and /proc/sys/kernel/dmesg_restrict seems to enable or disable this > ability. > > After setting that to 1, it appears I got the restriction I wanted: > $ dmesg > dmesg: klogctl failed: Operation not permitted Yup, exactly. > One of my friends who does hypervisor development and is familiar with a few systems > told me that Apple products just kill %p on production builds from logs. > > I'm kind of torn between the various approaches discussed on this thread.