linux-kernel - Re: [PATCH v3 2/2] x86/efi: clean up dead code around efi_reserve_boot

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Message-ID: <20170726011331.GA24304@x1>
Date:   Wed, 26 Jul 2017 09:13:31 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:     Matt Fleming <matt@...eblueprint.co.uk>,
        Kees Cook <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        "izumi.taku@...fujitsu.com" <izumi.taku@...fujitsu.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        "fanc.fnst@...fujitsu.com" <fanc.fnst@...fujitsu.com>,
        Junichi Nomura <j-nomura@...jp.nec.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>, dyoung@...hat.com
Subject: Re: [PATCH v3 2/2] x86/efi: clean up dead code around
 efi_reserve_boot_services()

On 07/26/17 at 12:12am, Naoya Horiguchi wrote:
> On Mon, Jul 24, 2017 at 02:20:44PM +0100, Matt Fleming wrote:
> > On Mon, 10 Jul, at 02:51:36PM, Naoya Horiguchi wrote:
> > > EFI_BOOT_SERVICES_{CODE|DATA} regions never overlap the kernel now,
> > > so we can clean up the check in efi_reserve_boot_services().
> > > 
> > > Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> > > ---
> > >  arch/x86/platform/efi/quirks.c | 23 +----------------------
> > >  1 file changed, 1 insertion(+), 22 deletions(-)
> > 
> > Is this true for kernels not using KASLR? 
> 
> Thank you for pointing out this. It's not true depending on memmap layout.
> If a firmware does not define the memory around the kernel address
> (0x1000000 or CONFIG_PHYSICAL_START) as EFI_BOOT_SERVICES_*, no overlap
> happens.  That's true in my testing server, but I don't think that we can
> expect it generally.
> 
> So I think of adding some assertion in the patch 1/2 to detect this overlap
> in extract_kernel() even for no KASLR case.

EFI_BOOT_SERVICES_* memory are collected as e820 region of
E820_TYPE_RAM, how can we guarantee kernel won't use them after jumping
into the running kernel whether KASLR enabled or not? We can only wish
that EFI firmware engineer don't put EFI_BOOT_SERVICES_* far from
0x1000000 where normal kernel is loaded.