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

Ard Biesheuvel
2016-01-06T07:51:55Z

On 5 January 2016 at 20:53, Kees Cook <keescook@chromium.org> wrote:
> 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.)
>

Indeed.

>>
>> 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