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 16:34:51 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
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

On 03/24/17 at 09:08am, Ingo Molnar wrote:
> > 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.

I thought it only impacts kexec, but Dave thought it will impact 1st
kenrel either.
> 
> 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?

The EFI region is reserved for EFI runtime services virtual mapping, the
original purpose is to preserve this region so that they can be reused
by kexec-ed kernel. This was introduced by Boris in commit d2f7cbe7b26a7
("x86/efi: Runtime services virtual mapping").

So it will be establishing and stay there. According to Dave's telling,
efi will still fetch efi variables or time/date by runtime service by
switching the efi pgd and entering into efi mode. And then switch back
to normal OS. Not sure if I am right for efi part in 1st kernel. For 2nd
kernel, if want to reuse the them, the efi region has to be kept.

Thanks
Baoquan