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

Naoya Horiguchi
2017-07-25T06:22:31Z

On Mon, Jul 24, 2017 at 02:17:07PM +0100, Matt Fleming wrote:
> 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.

OK, I'll update this.