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Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map kernel in user mode

From Thomas Garnier <thgarnie@google.com>
Newsgroups linux.kernel
Subject Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map kernel in user mode
Date 2017-05-05 17:50 +0200
Message-ID <tDNEt-3DR-7@gated-at.bofh.it> (permalink)
References <tDm1z-24V-7@gated-at.bofh.it> <tDqRA-5hr-17@gated-at.bofh.it> <tDGMF-7Ep-9@gated-at.bofh.it>
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On Fri, May 5, 2017 at 1:23 AM, Daniel Gruss
<daniel.gruss@iaik.tugraz.at> wrote:
>
> On 04.05.2017 17:28, Thomas Garnier wrote:
>>
>> Please read the documentation on submitting patches [1] and coding style [2].
>
>
> I will have a closer look at that.
>
>>  - How this approach prevent the hardware attacks you mentioned? You
>> still have to keep a part of _text in the pagetable and an attacker
>> could discover it no? (and deduce the kernel base address).
>
>
> These parts are moved to a different section (.user_mapped) which is at a possibly predictable location - the location of the randomized parts of the kernel is independent of the location of .user_mapped.
> The code/data footprint for .user_mapped is quite small, helping to reduce or eliminate the attack surface...
>

If I get it right, it means you can leak the per-cpu address instead
of the kernel. Correct? That would be a problem because you can
elevate privilege by overwriting per-cpu variables. Leaking this
address means also defeating KASLR memory randomization [3] (cf paper
in the commit).

In theory you could put the code in the fixmap but you still have the
per-cpu variables and changing that is hard.

[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=021182e52fe01c1f7b126f97fd6ba048dc4234fd

>> You also need to make it clear that btb attacks are still possible.
>
>
> By just increasing the KASLR randomization range, btb attacks can be mitigated (for free).

Correct, I hope we can do that.

>
>>  - What is the perf impact?
>
>
> It will vary for different machines. We have promising results (<1%) for an i7-6700K with representative benchmarks. However, for older systems or for workloads with a lot of pressure on some TLB levels, the performance may be much worse.

I think including performance data in both cases would be useful.


-- 
Thomas

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[RFC, PATCH] x86_64: KAISER - do not map kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-04 12:20 +0200
  Re: [RFC, PATCH] x86_64: KAISER - do not map kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-04 14:30 +0200
  Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Thomas Garnier <thgarnie@google.com> - 2017-05-04 17:30 +0200
    Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-05 10:30 +0200
      Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Thomas Garnier <thgarnie@google.com> - 2017-05-05 17:50 +0200
        Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode David Gens <david.gens@cs.tu-darmstadt.de> - 2017-05-06 06:40 +0200
          Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-06 10:40 +0200
            Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Mark Rutland <mark.rutland@arm.com> - 2017-05-08 12:30 +0200
              Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-08 13:00 +0200
                Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Mark Rutland <mark.rutland@arm.com> - 2017-05-08 15:30 +0200
                Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-08 15:50 +0200
            Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-08 16:00 +0200
              Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Thomas Garnier <thgarnie@google.com> - 2017-05-08 16:20 +0200
                Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-08 16:20 +0200
      Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-08 15:30 +0200
  Re: [RFC, PATCH] x86_64: KAISER - do not map kernel in user mode Christoph Hellwig <hch@infradead.org> - 2017-05-04 17:50 +0200
    Re: [RFC, PATCH] x86_64: KAISER - do not map kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-05 09:50 +0200
      Re: [kernel-hardening] Re: [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-07 23:50 +0200
        Re: [kernel-hardening] Re: [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Richard Weinberger <richard@nod.at> - 2017-05-08 00:10 +0200
          Re: [kernel-hardening] Re: [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-08 00:20 +0200
            Re: [kernel-hardening] Re: [RFC, PATCH] x86_64: KAISER - do not mapkernel in user mode "Fogh, Anders" <anders.fogh@gdata-adan.de> - 2017-05-09 17:00 +0200
              Re: [kernel-hardening] Re: [RFC, PATCH] x86_64: KAISER - do not  mapkernel in user mode Richard Weinberger <richard@nod.at> - 2017-05-09 17:00 +0200
                Re: [kernel-hardening] Re: [RFC, PATCH] x86_64: KAISER - do not  mapkernel in user mode Rik van Riel <riel@redhat.com> - 2017-05-09 17:40 +0200
      Re: [kernel-hardening] Re: [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Richard Weinberger <richard.weinberger@gmail.com> - 2017-05-08 00:10 +0200
  Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Jann Horn <jannh@google.com> - 2017-05-05 18:00 +0200
    Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Daniel Gruss <daniel.gruss@iaik.tugraz.at> - 2017-05-06 10:30 +0200
  Re: [kernel-hardening] [RFC, PATCH] x86_64: KAISER - do not map  kernel in user mode Jann Horn <jannh@google.com> - 2017-05-05 18:00 +0200

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