Re: [PATCH v4] x86/boot/KASLR: exclude EFI_BOOT_SERVICES_{CODE|DATA} from KASLR's choice

[Baoquan He <bhe@redhat.com>]

Hi Naoya,

Thanks for this fix. I saw NEC had reported a bug to rhel previously,
and the bug truly will corrupt OS, it can be fixed by this patch.

This patch looks good to me, just a small concern, please see below
inline comment.

On 08/24/17 at 07:33pm, 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. Furthermore,
> we choose kernel address only from EFI_CONVENTIONAL_MEMORY because it's
> the only memory type we know to be free.

Here, I think it's better to present why EFI_CONVENTIONAL_MEMORY is the
only memory type we should choose. EFI_BOOT_SERVICES_xxx has been clear
to us why it's not good. It might be worth to saying something about
EFI_LOADER_xxx why it's not ok to choose. Maybe one sentence to mention
it and take pgd as exampel as Matt ever said.

Thanks
Baoquan

> v3 -> v4:
> - update comment and patch description to mention why only
>   EFI_CONVENTIONAL_MEMORY is chosen.
> - use efi_early_memdesc_ptr()
> - I decided not to post cleanup patches (patch 2/2 in previous series)
>   because it's not necessary to fix the issue.
> 
> 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 | 35 ++++++++++++++++++++++++++---------
>  1 file changed, 26 insertions(+), 9 deletions(-)
> 
> diff --git tip/x86/boot/arch/x86/boot/compressed/kaslr.c tip/x86/boot_patched/arch/x86/boot/compressed/kaslr.c
> index 7de23bb..ba5e9e5 100644
> --- tip/x86/boot/arch/x86/boot/compressed/kaslr.c
> +++ tip/x86/boot_patched/arch/x86/boot/compressed/kaslr.c
> @@ -597,19 +597,36 @@ process_efi_entries(unsigned long minimum, unsigned long image_size)
>  	for (i = 0; i < nr_desc; i++) {
>  		md = efi_early_memdesc_ptr(pmap, e->efi_memdesc_size, i);
>  		if (md->attribute & EFI_MEMORY_MORE_RELIABLE) {
> -			region.start = md->phys_addr;
> -			region.size = md->num_pages << EFI_PAGE_SHIFT;
> -			process_mem_region(&region, minimum, image_size);
>  			efi_mirror_found = true;
> -
> -			if (slot_area_index == MAX_SLOT_AREA) {
> -				debug_putstr("Aborted EFI scan (slot_areas full)!\n");
> -				break;
> -			}
> +			break;
>  		}
>  	}
>  
> -	return efi_mirror_found;
> +	for (i = 0; i < nr_desc; i++) {
> +		md = efi_early_memdesc_ptr(pmap, e->efi_memdesc_size, i);
> +
> +		/*
> +		 * According to spec, EFI_BOOT_SERVICES_{CODE|DATA} are also
> +		 * available for kernel image, but we don't include them for
> +		 * the workaround for buggy firmware.
> +		 * Only EFI_CONVENTIONAL_MEMORY is guaranteed to be free.
> +		 */
> +		if (md->type != EFI_CONVENTIONAL_MEMORY)
> +			continue;
> +
> +		if (efi_mirror_found &&
> +		    !(md->attribute & EFI_MEMORY_MORE_RELIABLE))
> +			continue;
> +
> +		region.start = md->phys_addr;
> +		region.size = md->num_pages << EFI_PAGE_SHIFT;
> +		process_mem_region(&region, minimum, image_size);
> +		if (slot_area_index == MAX_SLOT_AREA) {
> +			debug_putstr("Aborted EFI scan (slot_areas full)!\n");
> +			break;
> +		}
> +	}
> +	return true;
>  }
>  #else
>  static inline bool
> -- 
> 2.7.0
>