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From: Andy Lutomirski <luto@kernel.org>
To: PaX Team <pageexec@freemail.hu>
Cc: Andy Lutomirski <luto@kernel.org>,
	Mathias Krause <minipli@googlemail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Kees Cook <keescook@chromium.org>,
	"kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Hoeun Ryu <hoeun.ryu@gmail.com>, 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: Sun, 9 Apr 2017 17:10:16 -0700
Message-ID: <CALCETrWvGLtObdk3V0k_Esa_L=EOf+Whe=6DpcJ7H_W8Z-5Btw@mail.gmail.com> (raw)
In-Reply-To: <58EA2D58.17782.6ADE22BD@pageexec.freemail.hu>

On Sun, Apr 9, 2017 at 5:47 AM, PaX Team <pageexec@freemail.hu> wrote:
> On 7 Apr 2017 at 21:58, Andy Lutomirski wrote:
>
>> On Fri, Apr 7, 2017 at 12:58 PM, PaX Team <pageexec@freemail.hu> wrote:
>> > On 7 Apr 2017 at 9:14, Andy Lutomirski wrote:
>> >> Then someone who cares about performance can benchmark the CR0.WP
>> >> approach against it and try to argue that it's a good idea.  This
>> >> benchmark should wait until I'm done with my PCID work, because PCID
>> >> is going to make use_mm() a whole heck of a lot faster.
>> >
>> > in my measurements switching PCID is hovers around 230 cycles for snb-ivb
>> > and 200-220 for hsw-skl whereas cr0 writes are around 230-240 cycles. there's
>> > of course a whole lot more impact for switching address spaces so it'll never
>> > be fast enough to beat cr0.wp.
>> >
>>
>> If I'm reading this right, you're saying that a non-flushing CR3 write
>> is about the same cost as a CR0.WP write.  If so, then why should CR0
>> be preferred over the (arch-neutral) CR3 approach?
>
> cr3 (page table switching) isn't arch neutral at all ;). you probably meant
> the higher level primitives except they're not enough to implement the scheme
> as discussed before since the enter/exit paths are very much arch dependent.

Yes.

>
> on x86 the cost of the pax_open/close_kernel primitives comes from the cr0
> writes and nothing else, use_mm suffers not only from the cr3 writes but
> also locking/atomic ops and cr4 writes on its path and the inevitable TLB
> entry costs. and if cpu vendors cared enough, they could make toggling cr0.wp
> a fast path in the microcode and reduce its overhead by an order of magnitude.
>

If the CR4 writes happen in for this use case, that's a bug.

>>  And why would switching address spaces obviously be much slower?
>> There'll be a very small number of TLB fills needed for the actual
>> protected access.
>
> you'll be duplicating TLB entries in the alternative PCID for both code
> and data, where they will accumulate (=take room away from the normal PCID
> and expose unwanted memory for access) unless you also flush them when
> switching back (which then will cost even more cycles). also i'm not sure
> that processors implement all the 12 PCID bits so depending on how many PCIDs
> you plan to use, you could be causing even more unnecessary TLB replacements.
>

Unless the CPU is rather dumber than I expect, the only duplicated
entries should be for the writable aliases of pages that are written.
The rest of the pages are global and should be shared for all PCIDs.

--Andy

  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
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 [this message]
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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