Re: [PATCH v2 13/13] arm64: efi: invoke EFI_RNG_PROTOCOL to supply KASLR randomness

Kees Cook
2016-01-05T19:53:57Z

On Wed, Dec 30, 2015 at 7:26 AM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> Since arm64 does not use a decompressor that supplies an execution
> environment where it is feasible to some extent to provide a source of
> randomness, the arm64 KASLR kernel depends on the bootloader to supply
> some random bits in register x1 upon kernel entry.
>
> On UEFI systems, we can use the EFI_RNG_PROTOCOL, if supplied, to obtain
> some random bits. At the same time, use it to randomize the offset of the
> kernel Image in physical memory.

This logic seems like it should be under the name
CONFIG_RANDOMIZE_BASE and depend on UEFI? (Again, I'm just trying to
keep naming conventions the same across architectures to avoid
confusion.)

-Kees

>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
> arch/arm64/kernel/efi-entry.S | 7 +-
> drivers/firmware/efi/libstub/arm-stub.c | 1 -
> drivers/firmware/efi/libstub/arm64-stub.c | 134 +++++++++++++++++---
> include/linux/efi.h | 5 +-
> 4 files changed, 127 insertions(+), 20 deletions(-)
>
> diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
> index f82036e02485..f41073dde7e0 100644
> --- a/arch/arm64/kernel/efi-entry.S
> +++ b/arch/arm64/kernel/efi-entry.S
> @@ -110,7 +110,7 @@ ENTRY(entry)
> 2:
> /* Jump to kernel entry point */
> mov x0, x20
> - mov x1, xzr
> + ldr x1, efi_rnd
> mov x2, xzr
> mov x3, xzr
> br x21
> @@ -119,6 +119,9 @@ efi_load_fail:
> mov x0, #EFI_LOAD_ERROR
> ldp x29, x30, [sp], #32
> ret
> +ENDPROC(entry)
> +
> +ENTRY(efi_rnd)
> + .quad 0, 0
>
> entry_end:
> -ENDPROC(entry)
> diff --git a/drivers/firmware/efi/libstub/arm-stub.c b/drivers/firmware/efi/libstub/arm-stub.c
> index 950c87f5d279..f580bcdfae4f 100644
> --- a/drivers/firmware/efi/libstub/arm-stub.c
> +++ b/drivers/firmware/efi/libstub/arm-stub.c
> @@ -145,7 +145,6 @@ void efi_char16_printk(efi_system_table_t *sys_table_arg,
> out->output_string(out, str);
> }
>
> -
> /*
> * This function handles the architcture specific differences between arm and
> * arm64 regarding where the kernel image must be loaded and any memory that
> diff --git a/drivers/firmware/efi/libstub/arm64-stub.c b/drivers/firmware/efi/libstub/arm64-stub.c
> index 78dfbd34b6bf..4e5c306346b4 100644
> --- a/drivers/firmware/efi/libstub/arm64-stub.c
> +++ b/drivers/firmware/efi/libstub/arm64-stub.c
> @@ -13,6 +13,68 @@
> #include <asm/efi.h>
> #include <asm/sections.h>
>
> +struct efi_rng_protocol_t {
> + efi_status_t (*get_info)(struct efi_rng_protocol_t *,
> + unsigned long *,
> + efi_guid_t *);
> + efi_status_t (*get_rng)(struct efi_rng_protocol_t *,
> + efi_guid_t *,
> + unsigned long,
> + u8 *out);
> +};
> +
> +extern struct {
> + u64 virt_seed;
> + u64 phys_seed;
> +} efi_rnd;
> +
> +static int efi_get_random_bytes(efi_system_table_t *sys_table)
> +{
> + efi_guid_t rng_proto = EFI_RNG_PROTOCOL_GUID;
> + efi_status_t status;
> + struct efi_rng_protocol_t *rng;
> +
> + status = sys_table->boottime->locate_protocol(&rng_proto, NULL,
> + (void **)&rng);
> + if (status == EFI_NOT_FOUND) {
> + pr_efi(sys_table, "EFI_RNG_PROTOCOL unavailable, no randomness supplied\n");
> + return EFI_SUCCESS;
> + }
> +
> + if (status != EFI_SUCCESS)
> + return status;
> +
> + return rng->get_rng(rng, NULL, sizeof(efi_rnd), (u8 *)&efi_rnd);
> +}
> +
> +static efi_status_t get_dram_top(efi_system_table_t *sys_table_arg, u64 *top)
> +{
> + unsigned long map_size, desc_size;
> + efi_memory_desc_t *memory_map;
> + efi_status_t status;
> + int l;
> +
> + status = efi_get_memory_map(sys_table_arg, &memory_map, &map_size,
> + &desc_size, NULL, NULL);
> + if (status != EFI_SUCCESS)
> + return status;
> +
> + for (l = 0; l < map_size; l += desc_size) {
> + efi_memory_desc_t *md = (void *)memory_map + l;
> +
> + if (md->attribute & EFI_MEMORY_WB) {
> + u64 phys_end = md->phys_addr +
> + md->num_pages * EFI_PAGE_SIZE;
> + if (phys_end > *top)
> + *top = phys_end;
> + }
> + }
> +
> + efi_call_early(free_pool, memory_map);
> +
> + return EFI_SUCCESS;
> +}
> +
> efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
> unsigned long *image_addr,
> unsigned long *image_size,
> @@ -27,6 +89,14 @@ efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
> void *old_image_addr = (void *)*image_addr;
> unsigned long preferred_offset;
>
> + if (IS_ENABLED(CONFIG_ARM64_RELOCATABLE_KERNEL)) {
> + status = efi_get_random_bytes(sys_table_arg);
> + if (status != EFI_SUCCESS) {
> + pr_efi_err(sys_table_arg, "efi_get_random_bytes() failed\n");
> + return status;
> + }
> + }
> +
> /*
> * The preferred offset of the kernel Image is TEXT_OFFSET bytes beyond
> * a 2 MB aligned base, which itself may be lower than dram_base, as
> @@ -36,13 +106,42 @@ efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
> if (preferred_offset < dram_base)
> preferred_offset += SZ_2M;
>
> - /* Relocate the image, if required. */
> kernel_size = _edata - _text;
> - if (*image_addr != preferred_offset) {
> - kernel_memsize = kernel_size + (_end - _edata);
> + kernel_memsize = kernel_size + (_end - _edata);
> +
> + if (IS_ENABLED(CONFIG_ARM64_RELOCATABLE_KERNEL) && efi_rnd.phys_seed) {
> + /*
> + * If KASLR is enabled, and we have some randomness available,
> + * locate the kernel at a randomized offset in physical memory.
> + */
> + u64 dram_top = dram_base;
> +
> + status = get_dram_top(sys_table_arg, &dram_top);
> + if (status != EFI_SUCCESS) {
> + pr_efi_err(sys_table_arg, "get_dram_size() failed\n");
> + return status;
> + }
> +
> + kernel_memsize += SZ_2M;
> + nr_pages = round_up(kernel_memsize, EFI_ALLOC_ALIGN) /
> + EFI_PAGE_SIZE;
>
> /*
> - * First, try a straight allocation at the preferred offset.
> + * Use the random seed to scale the size and add it to the DRAM
> + * base. Note that this may give suboptimal results on systems
> + * with discontiguous DRAM regions with large holes between them.
> + */
> + *reserve_addr = dram_base +
> + ((dram_top - dram_base) >> 16) * (u16)efi_rnd.phys_seed;
> +
> + status = efi_call_early(allocate_pages, EFI_ALLOCATE_MAX_ADDRESS,
> + EFI_LOADER_DATA, nr_pages,
> + (efi_physical_addr_t *)reserve_addr);
> +
> + *image_addr = round_up(*reserve_addr, SZ_2M) + TEXT_OFFSET;
> + } else {
> + /*
> + * Else, try a straight allocation at the preferred offset.
> * This will work around the issue where, if dram_base == 0x0,
> * efi_low_alloc() refuses to allocate at 0x0 (to prevent the
> * address of the allocation to be mistaken for a FAIL return
> @@ -52,27 +151,30 @@ efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
> * Mustang), we can still place the kernel at the address
> * 'dram_base + TEXT_OFFSET'.
> */
> + if (*image_addr == preferred_offset)
> + return EFI_SUCCESS;
> +
> *image_addr = *reserve_addr = preferred_offset;
> nr_pages = round_up(kernel_memsize, EFI_ALLOC_ALIGN) /
> EFI_PAGE_SIZE;
> status = efi_call_early(allocate_pages, EFI_ALLOCATE_ADDRESS,
> EFI_LOADER_DATA, nr_pages,
> (efi_physical_addr_t *)reserve_addr);
> + }
> +
> + if (status != EFI_SUCCESS) {
> + kernel_memsize += TEXT_OFFSET;
> + status = efi_low_alloc(sys_table_arg, kernel_memsize,
> + SZ_2M, reserve_addr);
> +
> if (status != EFI_SUCCESS) {
> - kernel_memsize += TEXT_OFFSET;
> - status = efi_low_alloc(sys_table_arg, kernel_memsize,
> - SZ_2M, reserve_addr);
> -
> - if (status != EFI_SUCCESS) {
> - pr_efi_err(sys_table_arg, "Failed to relocate kernel\n");
> - return status;
> - }
> - *image_addr = *reserve_addr + TEXT_OFFSET;
> + pr_efi_err(sys_table_arg, "Failed to relocate kernel\n");
> + return status;
> }
> - memcpy((void *)*image_addr, old_image_addr, kernel_size);
> - *reserve_size = kernel_memsize;
> + *image_addr = *reserve_addr + TEXT_OFFSET;
> }
> -
> + memcpy((void *)*image_addr, old_image_addr, kernel_size);
> + *reserve_size = kernel_memsize;
>
> return EFI_SUCCESS;
> }
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index 569b5a866bb1..13783fdc9bdd 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -299,7 +299,7 @@ typedef struct {
> void *open_protocol_information;
> void *protocols_per_handle;
> void *locate_handle_buffer;
> - void *locate_protocol;
> + efi_status_t (*locate_protocol)(efi_guid_t *, void *, void **);
> void *install_multiple_protocol_interfaces;
> void *uninstall_multiple_protocol_interfaces;
> void *calculate_crc32;
> @@ -599,6 +599,9 @@ void efi_native_runtime_setup(void);
> #define EFI_PROPERTIES_TABLE_GUID \
> EFI_GUID( 0x880aaca3, 0x4adc, 0x4a04, 0x90, 0x79, 0xb7, 0x47, 0x34, 0x08, 0x25, 0xe5 )
>
> +#define EFI_RNG_PROTOCOL_GUID \
> + EFI_GUID( 0x3152bca5, 0xeade, 0x433d, 0x86, 0x2e, 0xc0, 0x1c, 0xdc, 0x29, 0x1f, 0x44 )
> +
> typedef struct {
> efi_guid_t guid;
> u64 table;
> --
> 2.5.0
>

--
Kees Cook
Chrome OS & Brillo Security