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

[Ard Biesheuvel]

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@xxxxxxxxxx>
---
 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

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