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From: Mathias Krause <minipli@googlemail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Kees Cook <keescook@chromium.org>,
	Andy Lutomirski <luto@kernel.org>,
	"kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Hoeun Ryu <hoeun.ryu@gmail.com>, PaX Team <pageexec@freemail.hu>,
	Emese Revfy <re.emese@gmail.com>,
	Russell King <linux@armlinux.org.uk>, X86 ML <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()
Date: Fri, 7 Apr 2017 12:51:15 +0200
Message-ID: <CA+rthh9-EJBBO6nYYY6noP0ybcvk+xJ+S5h6BCNPtidjMCv8VQ@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1704071048360.1716@nanos>

On 7 April 2017 at 11:46, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Fri, 7 Apr 2017, Mathias Krause wrote:
>> On 6 April 2017 at 17:59, Andy Lutomirski <luto@amacapital.net> wrote:
>> > On Wed, Apr 5, 2017 at 5:14 PM, Kees Cook <keescook@chromium.org> wrote:
>> >> static __always_inline rare_write_begin(void)
>> >> {
>> >>     preempt_disable();
>> >>     local_irq_disable();
>> >>     barrier();
>> >>     __arch_rare_write_begin();
>> >>     barrier();
>> >> }
>> >
>> > Looks good, except you don't need preempt_disable().
>> > local_irq_disable() also disables preemption.  You might need to use
>> > local_irq_save(), though, depending on whether any callers already
>> > have IRQs off.
>>
>> Well, doesn't look good to me. NMIs will still be able to interrupt
>> this code and will run with CR0.WP = 0.
>>
>> Shouldn't you instead question yourself why PaX can do it "just" with
>> preempt_disable() instead?!
>
> That's silly. Just because PaX does it, doesn't mean it's correct. To be
> honest, playing games with the CR0.WP bit is outright stupid to begin with.

Why that? It allows fast and CPU local modifications of r/o memory.
OTOH, an approach that needs to fiddle with page table entries
requires global synchronization to keep the individual TLB states in
sync. Hmm.. Not that fast, I'd say.

> Whether protected by preempt_disable or local_irq_disable, to make that
> work it needs CR0 handling in the exception entry/exit at the lowest
> level. And that's just a nightmare maintainence wise as it's prone to be
> broken over time.

It seems to be working fine for more than a decade now in PaX. So it
can't be such a big maintenance nightmare ;)

>                    Aside of that it's pointless overhead for the normal case.

>
> The proper solution is:
>
> write_rare(ptr, val)
> {
>         mp = map_shadow_rw(ptr);
>         *mp = val;
>         unmap_shadow_rw(mp);
> }
>
> map_shadow_rw() is essentially the same thing as we do in the highmem case
> where the kernel creates a shadow mapping of the user space pages via
> kmap_atomic().

The "proper solution" seems to be much slower compared to just
toggling CR0.WP (which is costly in itself, already) because of the
TLB invalidation / synchronisation involved.

> It's valid (at least on x86) to have a shadow map with the same page
> attributes but write enabled. That does not require any fixups of CR0 and
> just works.

"Just works", sure -- but it's not as tightly focused as the PaX
solution which is CPU local, while your proposed solution is globally
visible.


Cheers,
Mathias

  reply index

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-29 18:15 [RFC v2] Introduce rare_write() infrastructure Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 01/11] " Kees Cook
2017-03-29 18:23   ` Kees Cook
2017-03-30  7:44     ` Ho-Eun Ryu
2017-03-30 17:02       ` Kees Cook
2017-04-07  8:09   ` Ho-Eun Ryu
2017-04-07 20:38     ` Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 02/11] lkdtm: add test for " Kees Cook
2017-03-30  9:34   ` [kernel-hardening] " Ian Campbell
2017-03-30 16:16     ` Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 03/11] net: switch sock_diag handlers to rare_write() Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap() Kees Cook
2017-03-29 22:38   ` Andy Lutomirski
2017-03-30  1:41     ` Kees Cook
2017-04-05 23:57       ` Andy Lutomirski
2017-04-06  0:14         ` Kees Cook
2017-04-06 15:59           ` Andy Lutomirski
2017-04-07  8:34             ` [kernel-hardening] " Mathias Krause
2017-04-07  9:46               ` Thomas Gleixner
2017-04-07 10:51                 ` Mathias Krause [this message]
2017-04-07 13:14                   ` Thomas Gleixner
2017-04-07 13:30                     ` Mathias Krause
2017-04-07 16:14                       ` Andy Lutomirski
2017-04-07 16:22                         ` Mark Rutland
2017-04-07 19:58                         ` PaX Team
2017-04-08  4:58                           ` Andy Lutomirski
2017-04-09 12:47                             ` PaX Team
2017-04-10  0:10                               ` Andy Lutomirski
2017-04-10 10:42                                 ` PaX Team
2017-04-10 16:01                                   ` Andy Lutomirski
2017-04-07 20:44                         ` Thomas Gleixner
2017-04-07 21:20                           ` Kees Cook
2017-04-08  4:12                             ` Daniel Micay
2017-04-08  4:13                               ` Daniel Micay
2017-04-08  4:21                         ` Daniel Micay
2017-04-08  5:07                           ` Andy Lutomirski
2017-04-08  7:33                             ` Daniel Micay
2017-04-08 15:20                               ` Andy Lutomirski
2017-04-09 10:53                                 ` Ingo Molnar
2017-04-10 10:22                                 ` Mark Rutland
2017-04-09 20:24                             ` PaX Team
2017-04-10  0:31                               ` Andy Lutomirski
2017-04-10 19:47                                 ` PaX Team
2017-04-10 20:27                                   ` Andy Lutomirski
2017-04-10 20:13                               ` Kees Cook
2017-04-10 20:17                                 ` Andy Lutomirski
2017-04-07 19:25                       ` Thomas Gleixner
2017-04-07 14:45                   ` Peter Zijlstra
2017-04-10 10:29                     ` Mark Rutland
2017-04-07 19:52                 ` PaX Team
2017-04-10  8:26                   ` Thomas Gleixner
2017-04-10 19:55                     ` PaX Team
2017-04-07  9:37   ` Peter Zijlstra
2017-03-29 18:15 ` [RFC v2][PATCH 05/11] ARM: mm: dump: Add domain to output Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 06/11] ARM: domains: Extract common USER domain init Kees Cook
2017-03-29 18:15 ` [RFC v2][PATCH 07/11] ARM: mm: set DOMAIN_WR_RARE for rodata Kees Cook
2017-03-29 18:16 ` [RFC v2][PATCH 08/11] ARM: Implement __arch_rare_write_begin/end() Kees Cook
2017-04-07  9:36   ` Peter Zijlstra
2017-03-29 18:16 ` [RFC v2][PATCH 09/11] list: add rare_write() list helpers Kees Cook
2017-03-29 18:16 ` [RFC v2][PATCH 10/11] gcc-plugins: Add constify plugin Kees Cook
2017-03-29 18:16 ` [RFC v2][PATCH 11/11] cgroups: force all struct cftype const Kees Cook
2017-03-29 19:00 ` [RFC v2] Introduce rare_write() infrastructure Russell King - ARM Linux
2017-03-29 19:14   ` Kees Cook

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