Re: [PATCH v5 4/4] x86/boot/KASLR: Restrict kernel to be randomized in mirror regions

[Kees Cook]

On Thu, Jul 13, 2017 at 7:19 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> Kernel text may be located in non-mirror regions (movable zone) when both
> address range mirroring feature and KASLR are enabled.
>
> The address range mirroring feature arranges such mirror region into
> normal zone and other region into movable zone in order to locate
> kernel code and data in mirror region. The physical memory region
> whose descriptors in EFI memory map has EFI_MEMORY_MORE_RELIABLE
> attribute (bit: 16) are mirrored.
>
> If efi is detected, iterate efi memory map and pick the mirror region to
> process for adding candidate of randomization slot. If efi is disabled
> or no mirror region found, still process e820 memory map.
>
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> ---
>  arch/x86/boot/compressed/kaslr.c | 68 ++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 66 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 99c7194f7ea6..9059b571eca1 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -37,7 +37,9 @@
>  #include <linux/uts.h>
>  #include <linux/utsname.h>
>  #include <linux/ctype.h>
> +#include <linux/efi.h>
>  #include <generated/utsrelease.h>
> +#include <asm/efi.h>
>
>  /* Macros used by the included decompressor code below. */
>  #define STATIC
> @@ -558,6 +560,65 @@ static void process_mem_region(struct mem_vector *entry,
>         }
>  }
>
> +#ifdef CONFIG_EFI
> +/*
> + * Returns true if mirror region found (and must have been processed
> + * for slots adding)
> + */
> +static bool process_efi_entries(unsigned long minimum,
> +                               unsigned long image_size)
> +{
> +       struct efi_info *e = &boot_params->efi_info;
> +       bool efi_mirror_found = false;
> +       struct mem_vector region;
> +       efi_memory_desc_t *md;
> +       unsigned long pmap;
> +       char *signature;
> +       u32 nr_desc;
> +       int i;
> +
> +       signature = (char *)&boot_params->efi_info.efi_loader_signature;
> +       if (strncmp(signature, EFI32_LOADER_SIGNATURE, 4) &&
> +           strncmp(signature, EFI64_LOADER_SIGNATURE, 4))
> +               return false;
> +
> +#ifdef CONFIG_X86_32
> +       /* Can't handle data above 4GB at this time */
> +       if (e->efi_memmap_hi) {
> +               warn("Memory map is above 4GB, EFI should be disabled.\n");
> +               return false;
> +       }
> +       pmap =  e->efi_memmap;
> +#else
> +       pmap = (e->efi_memmap | ((__u64)e->efi_memmap_hi << 32));
> +#endif
> +
> +       nr_desc = e->efi_memmap_size / e->efi_memdesc_size;
> +       for (i = 0; i < nr_desc; i++) {
> +               md = (efi_memory_desc_t *)(pmap + (i * e->efi_memdesc_size));
> +               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");

Should this text be capitalized to "EFI" or expanded to "EFI mirror"?

> +                               break;
> +                       }
> +               }
> +       }
> +
> +       return efi_mirror_found;
> +}
> +#else
> +static inline bool process_efi_entries(unsigned long minimum,
> +                                      unsigned long image_size)
> +{
> +       return false;
> +}
> +#endif
> +
>  static void process_e820_entries(unsigned long minimum,
>                                  unsigned long image_size)
>  {
> @@ -586,13 +647,16 @@ static unsigned long find_random_phys_addr(unsigned long minimum,
>  {
>         /* Check if we had too many memmaps. */
>         if (memmap_too_large) {
> -               debug_putstr("Aborted e820 scan (more than 4 memmap= args)!\n");
> +               debug_putstr("Aborted memory entries scan (more than 4 memmap= args)!\n");
>                 return 0;
>         }
>
>         /* Make sure minimum is aligned. */
>         minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
>
> +       if(process_efi_entries(minimum, image_size))

Style nit: space between "if" and "(".

> +               return slots_fetch_random();
> +
>         process_e820_entries(minimum, image_size);
>         return slots_fetch_random();
>  }
> @@ -652,7 +716,7 @@ void choose_random_location(unsigned long input,
>          */
>         min_addr = min(*output, 512UL << 20);
>
> -       /* Walk e820 and find a random address. */
> +       /* Walk available memory entries to find a random address. */
>         random_addr = find_random_phys_addr(min_addr, output_size);
>         if (!random_addr) {
>                 warn("Physical KASLR disabled: no suitable memory region!");
> --
> 2.5.5
>

Otherwise, looks fine to me for the KASLR piece; I can't speak
specifically to the EFI logic. :)

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

-- 
Kees Cook
Pixel Security