Re: [PATCH v3 1/2] x86/boot/KASLR: exclude EFI_BOOT_SERVICES_{CODE|DATA} from KASLR's choice

Matt Fleming
2017-07-24T13:17:18Z

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@ah.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.