linux-kernel - Re: [PATCH v3 1/2] x86/boot/KASLR: exclude EFI_BOOT_SERVICES

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Date:   Mon, 24 Jul 2017 14:17:07 +0100
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:     Baoquan He <bhe@...hat.com>, Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        izumi.taku@...fujitsu.com, Thomas Garnier <thgarnie@...gle.com>,
        fanc.fnst@...fujitsu.com, Jun'ichi Nomura <j-nomura@...jp.nec.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH v3 1/2] x86/boot/KASLR: exclude
 EFI_BOOT_SERVICES_{CODE|DATA} from KASLR's choice

On Mon, 10 Jul, at 02:51:35PM, Naoya Horiguchi wrote:
> KASLR chooses kernel location from E820_TYPE_RAM regions by walking over
> e820 entries now. E820_TYPE_RAM includes EFI_BOOT_SERVICES_CODE and
> EFI_BOOT_SERVICES_DATA, so those regions can be the target. According to
> UEFI spec, all memory regions marked as EfiBootServicesCode and
> EfiBootServicesData are available for free memory after the first call
> of ExitBootServices(). So such regions should be usable for kernel on
> spec basis.
> 
> In x86, however, we have some workaround for broken firmware, where we
> keep such regions reserved until SetVirtualAddressMap() is done.
> See the following code in should_map_region():
> 
> 	static bool should_map_region(efi_memory_desc_t *md)
> 	{
> 		...
> 		/*
> 		 * Map boot services regions as a workaround for buggy
> 		 * firmware that accesses them even when they shouldn't.
> 		 *
> 		 * See efi_{reserve,free}_boot_services().
> 		 */
> 		if (md->type == EFI_BOOT_SERVICES_CODE ||
> 			md->type == EFI_BOOT_SERVICES_DATA)
> 				return false;
> 
> This workaround suppressed a boot crash, but potential issues still
> remain because no one prevents the regions from overlapping with kernel
> image by KASLR.
> 
> So let's make sure that EFI_BOOT_SERVICES_{CODE|DATA} regions are never
> chosen as kernel memory for the workaround to work fine.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> ---
> v2 -> v3:
> - skip EFI_LOADER_CODE and EFI_LOADER_DATA in region scan
> 
> v1 -> v2:
> - switch efi_mirror_found to local variable
> - insert break when EFI_MEMORY_MORE_RELIABLE found
> ---
>  arch/x86/boot/compressed/kaslr.c | 44 +++++++++++++++++++++++++++++-----------
>  1 file changed, 32 insertions(+), 12 deletions(-)

I think the patch content is fine, but please update the comments and
the changelog to say why EFI_CONVENTIONAL_MEMORY was chosen, i.e. that
it's the only memory type that we know to be free.