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

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Message-ID: <20170324080833.GA15200@gmail.com>
Date:   Fri, 24 Mar 2017 09:08:33 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...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,
        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>,
        Dave Young <dyoung@...hat.com>,
        Bhupesh Sharma <bhsharma@...hat.com>
Subject: Re: [PATCH v2] x86/mm/KASLR: EFI region is mistakenly included into
 KASLR VA space for randomization


* Baoquan He <bhe@...hat.com> wrote:

> Currently KASLR is enabled on three regions: the direct mapping of physical
> memory, vamlloc and vmemmap. However EFI region is also mistakenly included
> for VA space randomization because of misusing EFI_VA_START macro and
> assuming EFI_VA_START < EFI_VA_END.
> 
> The EFI region is reserved for EFI runtime services virtual mapping which
> should not be included in kaslr ranges. In Documentation/x86/x86_64/mm.txt,
> we can see:
>   ffffffef00000000 - fffffffeffffffff (=64 GB) EFI region mapping space
> EFI use the space from -4G to -64G thus EFI_VA_START > EFI_VA_END,
> Here EFI_VA_START = -4G, and EFI_VA_END = -64G.
> 
> Changing EFI_VA_START to EFI_VA_END in mm/kaslr.c fixes this problem.
> 
> Cc: <stable@...r.kernel.org> #4.8+
> Signed-off-by: Baoquan He <bhe@...hat.com>
> Acked-by: Dave Young <dyoung@...hat.com>
> Reviewed-by: Bhupesh Sharma <bhsharma@...hat.com>
> Acked-by: Thomas Garnier <thgarnie@...gle.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: linux-efi@...r.kernel.org
> Cc: Thomas Garnier <thgarnie@...gle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Cc: Dave Young <dyoung@...hat.com>
> Cc: Bhupesh Sharma <bhsharma@...hat.com>

So I applied this kexec fix and extended the changelog to clearly show why this 
fix matters in practice.

Also, to make sure I understood it correctly: these addresses are all dynamic on 
64-bit kernels, i.e. we are establishing and then tearing down these page tables 
around EFI calls, and they are 'normally' not present at all, right?

Thanks,

	Ingo