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

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Date:   Fri, 24 Mar 2017 09:37:36 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Bhupesh Sharma <bhsharma@...hat.com>
Subject: Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into
 KASLR VA space for randomization

On 24 March 2017 at 09:24, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Dave Young <dyoung@...hat.com> wrote:
>
>> > > So I applied this kexec fix and extended the changelog to clearly show why
>> > > this fix matters in practice.
>> >
>> > I thought it only impacts kexec, but Dave thought it will impact 1st kenrel
>> > either.
>>
>> Yes, I think no need to mention kexec, it is a general issue.
>>
>> First, the space is reserved for EFI, so kernel should not use it for kaslr.
>
> It's the kernel's EFI code, and we map whatever address we want (and then pass
> that to the EFI runtime), so wether it's randomized or not is the Linux kernel's
> policy decision...
>

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.

> So that's my question: can these memory regions include security sensitive data,
> and if yes, how can we best randomize it while kexec and other kernel and EFI
> features still work?
>
> Preserving virtual addresses for kexec is a red herring: the randomized offset
> could be passed to the kexec-ed kernel just fine.
>
> Thanks,
>
>         Ingo
> --
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