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

Ard Biesheuvel
2015-12-30T15:26:57Z

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.

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