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(®ion, 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(®ion, 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
>
