Re: [PATCH v3 2/2] x86/boot/KASLR: Restrict kernel to be randomized in mirror regions

[Kees Cook <keescook@chromium.org>]

On Wed, Jul 5, 2017 at 1:06 AM, Baoquan He <bhe@redhat.com> 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@redhat.com>
> ---
>  arch/x86/boot/compressed/kaslr.c | 52 +++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 51 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 85c360eec4a6..94f08fd375ae 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 +560,48 @@ static void process_mem_region(struct mem_vector *entry,
>         }
>  }
>
> +/* Marks if efi mirror regions have been found and handled. */
> +static bool efi_mirror_found;
> +
> +static void process_efi_entry(unsigned long minimum, unsigned long image_size)

Since this walks all entries (not a single entry), please name it
process_efi_entries() (or some other more clear name).

Thanks!

-Kees

> +{
> +       struct efi_info *e = &boot_params->efi_info;
> +       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;
> +
> +#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;
> +       }
> +       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;
> +               }
> +       }
> +}
> +
>  static void process_e820_entry(unsigned long minimum, unsigned long image_size)
>  {
>         int i;
> @@ -592,6 +636,12 @@ static unsigned long find_random_phys_addr(unsigned long minimum,
>         /* Make sure minimum is aligned. */
>         minimum = ALIGN(minimum, CONFIG_PHYSICAL_ALIGN);
>
> +#ifdef CONFIG_EFI
> +       process_efi_entry(minimum, image_size);
> +       if (efi_mirror_found)
> +               return slots_fetch_random();
> +#endif
> +
>         process_e820_entry(minimum, image_size);
>         return slots_fetch_random();
>  }
> @@ -651,7 +701,7 @@ 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
>



-- 
Kees Cook
Pixel Security