Messages in this thread | | | From | Nadav Amit <> | Subject | Re: [GIT PULL] x86/topology changes for v5.3 | Date | Thu, 11 Jul 2019 17:09:09 +0000 |
| |
> On Jul 11, 2019, at 8:08 AM, Kees Cook <keescook@chromium.org> wrote: > > On Thu, Jul 11, 2019 at 10:01:34AM +0200, Peter Zijlstra wrote: >> On Thu, Jul 11, 2019 at 07:11:19AM +0000, Nadav Amit wrote: >>>> On Jul 10, 2019, at 7:22 AM, Jiri Kosina <jikos@kernel.org> wrote: >>>> >>>> On Wed, 10 Jul 2019, Peter Zijlstra wrote: >>>> >>>>> If we mark the key as RO after init, and then try and modify the key to >>>>> link module usage sites, things might go bang as described. >>>>> >>>>> Thanks! >>>>> >>>>> >>>>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c >>>>> index 27d7864e7252..5bf7a8354da2 100644 >>>>> --- a/arch/x86/kernel/cpu/common.c >>>>> +++ b/arch/x86/kernel/cpu/common.c >>>>> @@ -366,7 +366,7 @@ static __always_inline void setup_umip(struct cpuinfo_x86 *c) >>>>> cr4_clear_bits(X86_CR4_UMIP); >>>>> } >>>>> >>>>> -DEFINE_STATIC_KEY_FALSE_RO(cr_pinning); >>>>> +DEFINE_STATIC_KEY_FALSE(cr_pinning); >>>> >>>> Good catch, I guess that is going to fix it. >>>> >>>> At the same time though, it sort of destroys the original intent of Kees' >>>> patch, right? The exploits will just have to call static_key_disable() >>>> prior to calling native_write_cr4() again, and the protection is gone. >>> >>> Even with DEFINE_STATIC_KEY_FALSE_RO(), I presume you can just call >>> set_memory_rw(), make the page that holds the key writable, and then call >>> static_key_disable(), followed by a call to native_write_cr4(). >> >> Or call text_poke_bp() with the right set of arguments. > > Right -- the point is to make it defended against an arbitrary write, > not arbitrary execution. Nothing is safe from arbitrary exec, but we can > do our due diligence on making things read-only.
I don’t understand.
In the PoC that motivated this this patch [1], the attacker gained the ability to call a function, control its first argument and used it to disable SMEP/SMAP by calling native_write_cr4(), which he did before doing an arbitrary write (another ability he gain).
After this patch, the attacker can instead call three functions, and by controlling their first arguments (*) disable SMEP. I didn’t see something in the motivating PoC that prevented calling 3 functions one at a time.
So it seems to me that it raised the bar for an attack by very little.
--
(*) set_memory_rw() has a second argument - the number of pages - but many non-zero values may be fine (if not a warning from __cpa_process_fault() might appear).
[1] https://googleprojectzero.blogspot.com/2017/05/exploiting-linux-kernel-via-packet.html
| |