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 11:36:24 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.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


* Ard Biesheuvel <ard.biesheuvel@...aro.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! 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.

Turning KASLR off actively degrades that randomization of the kernel virtual 
addresses.

Am I missing anything?

Thanks,

	Ingo