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Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into KASLR VA space for randomization

From Ard Biesheuvel <ard.biesheuvel@linaro.org>
Newsgroups linux.kernel
Subject Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into KASLR VA space for randomization
Date 2017-03-24 11:50 +0100
Message-ID <touX7-gm-3@gated-at.bofh.it> (permalink)
References (2 earlier) <tosVj-7gO-1@gated-at.bofh.it> <tot4Z-7ku-5@gated-at.bofh.it> <totHH-7Sh-13@gated-at.bofh.it> <totRo-7Xq-23@gated-at.bofh.it> <touNr-aN-9@gated-at.bofh.it>
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On 24 March 2017 at 10:36, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
>
>> No. It is the firmware's EFI code, and the virtual translation applied by the OS
>> is made known to the firmware by means of a call into the runtime service
>> SetVirtualAddressMap(). This service can only be called once after each boot,
>> and so kexec kernels are forced to use the same VA mapping for runtime services
>> as the first kernel. This is the whole point of having a VA region reserved for
>> this, so that kexec kernels are guaranteed to be able to use the same VA
>> mapping.
>
> Yes, but it's the kernel's EFI code that determines the area!

Indeed. It seems I misunderstood you there.

There are some known limitations, though, which prevent us from using
userland mappings on x86 like we do on ARM (Macs don't support it),
but I don't think randomizing the mappings inside this 64 GB window is
going to trigger any latent firmware bugs.

> So my suggestion:
>
>> > Preserving virtual addresses for kexec is a red herring: the randomized offset
>> > could be passed to the kexec-ed kernel just fine.
>
> Would solve the kexec problem, right?
>
> I.e. the first kernel that boots randomizes the address range - and passes that
> offset off to any subsequent kernels.
>

Yes, that sounds feasible to me.

> Turning KASLR off actively degrades that randomization of the kernel virtual
> addresses.
>
> Am I missing anything?
>

No, I think you are right. UEFI runtime services region are likely to
consist of R+W+X mappings for the foreseeable future on x86, and the
more we tighten down security in other places, the more appealing the
UEFI regions become for exploitation (even if they are only mapped
while runtime services calls are in progress).

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[PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into KASLR VA space for randomization Baoquan He <bhe@redhat.com> - 2017-03-24 06:10 +0100
  [tip:x86/urgent] x86/mm/KASLR: Exclude EFI region from KASLR VA  space randomization tip-bot for Baoquan He <tipbot@zytor.com> - 2017-03-24 09:20 +0100
  Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Ingo Molnar <mingo@kernel.org> - 2017-03-24 09:20 +0100
    Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Baoquan He <bhe@redhat.com> - 2017-03-24 09:40 +0100
      Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Dave Young <dyoung@redhat.com> - 2017-03-24 09:50 +0100
        Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Ingo Molnar <mingo@kernel.org> - 2017-03-24 10:30 +0100
          Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Borislav Petkov <bp@alien8.de> - 2017-03-24 10:40 +0100
          Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-03-24 10:40 +0100
            Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Borislav Petkov <bp@alien8.de> - 2017-03-24 10:50 +0100
              Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-03-24 11:00 +0100
            Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Borislav Petkov <bp@alien8.de> - 2017-03-24 10:50 +0100
              Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-03-24 10:50 +0100
            Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Ingo Molnar <mingo@kernel.org> - 2017-03-24 11:40 +0100
              Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-03-24 11:50 +0100
                Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Ingo Molnar <mingo@kernel.org> - 2017-03-24 12:00 +0100
          Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Baoquan He <bhe@redhat.com> - 2017-03-24 13:00 +0100
    Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into  KASLR VA space for randomization Dave Young <dyoung@redhat.com> - 2017-03-24 10:00 +0100

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