arm64: efi: invoke EFI_RNG_PROTOCOL to supply KASLR randomness

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 the /chosen/kaslr-seed DT property 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.
Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -810,6 +810,11 @@ config RANDOMIZE_BASE
 	  It is the bootloader's job to provide entropy, by passing a
 	  random u64 value in /chosen/kaslr-seed at kernel entry.
 
+	  When booting via the UEFI stub, it will invoke the firmware's
+	  EFI_RNG_PROTOCOL implementation (if available) to supply entropy
+	  to the kernel proper. In addition, it will randomise the physical
+	  location of the kernel Image as well.
+
 	  If unsure, say N.
 
 config RANDOMIZE_MODULE_REGION_FULL

drivers/firmware/efi/libstub/arm-stub.c

@@ -18,6 +18,8 @@
 
 #include "efistub.h"
 
+bool __nokaslr;
+
 static int efi_secureboot_enabled(efi_system_table_t *sys_table_arg)
 {
 	static efi_guid_t const var_guid = EFI_GLOBAL_VARIABLE_GUID;
@@ -207,14 +209,6 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
 		pr_efi_err(sys_table, "Failed to find DRAM base\n");
 		goto fail;
 	}
-	status = handle_kernel_image(sys_table, image_addr, &image_size,
-				     &reserve_addr,
-				     &reserve_size,
-				     dram_base, image);
-	if (status != EFI_SUCCESS) {
-		pr_efi_err(sys_table, "Failed to relocate kernel\n");
-		goto fail;
-	}
 
 	/*
 	 * Get the command line from EFI, using the LOADED_IMAGE
@@ -224,7 +218,28 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
 	cmdline_ptr = efi_convert_cmdline(sys_table, image, &cmdline_size);
 	if (!cmdline_ptr) {
 		pr_efi_err(sys_table, "getting command line via LOADED_IMAGE_PROTOCOL\n");
-		goto fail_free_image;
+		goto fail;
+	}
+
+	/* check whether 'nokaslr' was passed on the command line */
+	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
+		static const u8 default_cmdline[] = CONFIG_CMDLINE;
+		const u8 *str, *cmdline = cmdline_ptr;
+
+		if (IS_ENABLED(CONFIG_CMDLINE_FORCE))
+			cmdline = default_cmdline;
+		str = strstr(cmdline, "nokaslr");
+		if (str == cmdline || (str > cmdline && *(str - 1) == ' '))
+			__nokaslr = true;
+	}
+
+	status = handle_kernel_image(sys_table, image_addr, &image_size,
+				     &reserve_addr,
+				     &reserve_size,
+				     dram_base, image);
+	if (status != EFI_SUCCESS) {
+		pr_efi_err(sys_table, "Failed to relocate kernel\n");
+		goto fail_free_cmdline;
 	}
 
 	status = efi_parse_options(cmdline_ptr);
@@ -244,7 +259,7 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
 
 		if (status != EFI_SUCCESS) {
 			pr_efi_err(sys_table, "Failed to load device tree!\n");
-			goto fail_free_cmdline;
+			goto fail_free_image;
 		}
 	}
 
@@ -286,12 +301,11 @@ unsigned long efi_entry(void *handle, efi_system_table_t *sys_table,
 	efi_free(sys_table, initrd_size, initrd_addr);
 	efi_free(sys_table, fdt_size, fdt_addr);
 
-fail_free_cmdline:
-	efi_free(sys_table, cmdline_size, (unsigned long)cmdline_ptr);
-
 fail_free_image:
 	efi_free(sys_table, image_size, *image_addr);
 	efi_free(sys_table, reserve_size, reserve_addr);
+fail_free_cmdline:
+	efi_free(sys_table, cmdline_size, (unsigned long)cmdline_ptr);
 fail:
 	return EFI_ERROR;
 }

drivers/firmware/efi/libstub/arm64-stub.c

@@ -13,6 +13,10 @@
 #include <asm/efi.h>
 #include <asm/sections.h>
 
+#include "efistub.h"
+
+extern bool __nokaslr;
+
 efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
 					unsigned long *image_addr,
 					unsigned long *image_size,
@@ -23,26 +27,52 @@ efi_status_t __init handle_kernel_image(efi_system_table_t *sys_table_arg,
 {
 	efi_status_t status;
 	unsigned long kernel_size, kernel_memsize = 0;
-	unsigned long nr_pages;
 	void *old_image_addr = (void *)*image_addr;
 	unsigned long preferred_offset;
+	u64 phys_seed = 0;
+
+	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
+		if (!__nokaslr) {
+			status = efi_get_random_bytes(sys_table_arg,
+						      sizeof(phys_seed),
+						      (u8 *)&phys_seed);
+			if (status == EFI_NOT_FOUND) {
+				pr_efi(sys_table_arg, "EFI_RNG_PROTOCOL unavailable, no randomness supplied\n");
+			} else if (status != EFI_SUCCESS) {
+				pr_efi_err(sys_table_arg, "efi_get_random_bytes() failed\n");
+				return status;
+			}
+		} else {
+			pr_efi(sys_table_arg, "KASLR disabled on kernel command line\n");
+		}
+	}
 
 	/*
 	 * 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
 	 * long as the resulting offset equals or exceeds it.
 	 */
-	preferred_offset = round_down(dram_base, SZ_2M) + TEXT_OFFSET;
+	preferred_offset = round_down(dram_base, MIN_KIMG_ALIGN) + TEXT_OFFSET;
 	if (preferred_offset < dram_base)
-		preferred_offset += SZ_2M;
+		preferred_offset += MIN_KIMG_ALIGN;
 
-	/* 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_RANDOMIZE_BASE) && phys_seed != 0) {
+		/*
+		 * If KASLR is enabled, and we have some randomness available,
+		 * locate the kernel at a randomized offset in physical memory.
+		 */
+		*reserve_size = kernel_memsize + TEXT_OFFSET;
+		status = efi_random_alloc(sys_table_arg, *reserve_size,
+					  MIN_KIMG_ALIGN, reserve_addr,
+					  phys_seed);
 
+		*image_addr = *reserve_addr + TEXT_OFFSET;
+	} else {
 		/*
-		 * First, try a straight allocation at the preferred offset.
+		 * 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 +82,31 @@ 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;
+		*reserve_size = round_up(kernel_memsize, EFI_ALLOC_ALIGN);
+
 		status = efi_call_early(allocate_pages, EFI_ALLOCATE_ADDRESS,
-					EFI_LOADER_DATA, nr_pages,
+					EFI_LOADER_DATA,
+					*reserve_size / EFI_PAGE_SIZE,
 					(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) {
-				pr_efi_err(sys_table_arg, "Failed to relocate kernel\n");
-				return status;
-			}
-			*image_addr = *reserve_addr + TEXT_OFFSET;
+	if (status != EFI_SUCCESS) {
+		*reserve_size = kernel_memsize + TEXT_OFFSET;
+		status = efi_low_alloc(sys_table_arg, *reserve_size,
+				       MIN_KIMG_ALIGN, reserve_addr);
+
+		if (status != EFI_SUCCESS) {
+			pr_efi_err(sys_table_arg, "Failed to relocate kernel\n");
+			*reserve_size = 0;
+			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);
 
 	return EFI_SUCCESS;
 }

drivers/firmware/efi/libstub/fdt.c

@@ -147,6 +147,20 @@ efi_status_t update_fdt(efi_system_table_t *sys_table, void *orig_fdt,
 	if (status)
 		goto fdt_set_fail;
 
+	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
+		efi_status_t efi_status;
+
+		efi_status = efi_get_random_bytes(sys_table, sizeof(fdt_val64),
+						  (u8 *)&fdt_val64);
+		if (efi_status == EFI_SUCCESS) {
+			status = fdt_setprop(fdt, node, "kaslr-seed",
+					     &fdt_val64, sizeof(fdt_val64));
+			if (status)
+				goto fdt_set_fail;
+		} else if (efi_status != EFI_NOT_FOUND) {
+			return efi_status;
+		}
+	}
 	return EFI_SUCCESS;
 
 fdt_set_fail: