efi: Add support for seeding the RNG from a UEFI config table

Specify a Linux specific UEFI configuration table that carries some
random bits, and use the contents during early boot to seed the kernel's
random number generator. This allows much strong random numbers to be
generated early on.

The entropy is fed to the kernel using add_device_randomness(), which is
documented as being appropriate for being called very early.

Since UEFI configuration tables may also be consumed by kexec'd kernels,
register a reboot notifier that updates the seed in the table.

Note that the config table could be generated by the EFI stub or by any
other UEFI driver or application (e.g., GRUB), but the random seed table
GUID and the associated functionality should be considered an internal
kernel interface (unless it is promoted to ABI later on)
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20161112213237.8804-4-matt@codeblueprint.co.ukSigned-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/efi/efi.c

@@ -23,7 +23,10 @@
 #include <linux/of.h>
 #include <linux/of_fdt.h>
 #include <linux/io.h>
+#include <linux/kexec.h>
 #include <linux/platform_device.h>
+#include <linux/random.h>
+#include <linux/reboot.h>
 #include <linux/slab.h>
 #include <linux/acpi.h>
 #include <linux/ucs2_string.h>
@@ -48,6 +51,7 @@ struct efi __read_mostly efi = {
 	.esrt			= EFI_INVALID_TABLE_ADDR,
 	.properties_table	= EFI_INVALID_TABLE_ADDR,
 	.mem_attr_table		= EFI_INVALID_TABLE_ADDR,
+	.rng_seed		= EFI_INVALID_TABLE_ADDR,
 };
 EXPORT_SYMBOL(efi);
 
@@ -440,6 +444,7 @@ static __initdata efi_config_table_type_t common_tables[] = {
 	{EFI_SYSTEM_RESOURCE_TABLE_GUID, "ESRT", &efi.esrt},
 	{EFI_PROPERTIES_TABLE_GUID, "PROP", &efi.properties_table},
 	{EFI_MEMORY_ATTRIBUTES_TABLE_GUID, "MEMATTR", &efi.mem_attr_table},
+	{LINUX_EFI_RANDOM_SEED_TABLE_GUID, "RNG", &efi.rng_seed},
 	{NULL_GUID, NULL, NULL},
 };
 
@@ -501,6 +506,29 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,
 	pr_cont("\n");
 	set_bit(EFI_CONFIG_TABLES, &efi.flags);
 
+	if (efi.rng_seed != EFI_INVALID_TABLE_ADDR) {
+		struct linux_efi_random_seed *seed;
+		u32 size = 0;
+
+		seed = early_memremap(efi.rng_seed, sizeof(*seed));
+		if (seed != NULL) {
+			size = seed->size;
+			early_memunmap(seed, sizeof(*seed));
+		} else {
+			pr_err("Could not map UEFI random seed!\n");
+		}
+		if (size > 0) {
+			seed = early_memremap(efi.rng_seed,
+					      sizeof(*seed) + size);
+			if (seed != NULL) {
+				add_device_randomness(seed->bits, seed->size);
+				early_memunmap(seed, sizeof(*seed) + size);
+			} else {
+				pr_err("Could not map UEFI random seed!\n");
+			}
+		}
+	}
+
 	/* Parse the EFI Properties table if it exists */
 	if (efi.properties_table != EFI_INVALID_TABLE_ADDR) {
 		efi_properties_table_t *tbl;
@@ -824,3 +852,47 @@ int efi_status_to_err(efi_status_t status)
 
 	return err;
 }
+
+#ifdef CONFIG_KEXEC
+static int update_efi_random_seed(struct notifier_block *nb,
+				  unsigned long code, void *unused)
+{
+	struct linux_efi_random_seed *seed;
+	u32 size = 0;
+
+	if (!kexec_in_progress)
+		return NOTIFY_DONE;
+
+	seed = memremap(efi.rng_seed, sizeof(*seed), MEMREMAP_WB);
+	if (seed != NULL) {
+		size = min(seed->size, 32U);
+		memunmap(seed);
+	} else {
+		pr_err("Could not map UEFI random seed!\n");
+	}
+	if (size > 0) {
+		seed = memremap(efi.rng_seed, sizeof(*seed) + size,
+				MEMREMAP_WB);
+		if (seed != NULL) {
+			seed->size = size;
+			get_random_bytes(seed->bits, seed->size);
+			memunmap(seed);
+		} else {
+			pr_err("Could not map UEFI random seed!\n");
+		}
+	}
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block efi_random_seed_nb = {
+	.notifier_call = update_efi_random_seed,
+};
+
+static int register_update_efi_random_seed(void)
+{
+	if (efi.rng_seed == EFI_INVALID_TABLE_ADDR)
+		return 0;
+	return register_reboot_notifier(&efi_random_seed_nb);
+}
+late_initcall(register_update_efi_random_seed);
+#endif

include/linux/efi.h

@@ -599,6 +599,7 @@ void efi_native_runtime_setup(void);
  */
 #define LINUX_EFI_ARM_SCREEN_INFO_TABLE_GUID	EFI_GUID(0xe03fc20a, 0x85dc, 0x406e,  0xb9, 0x0e, 0x4a, 0xb5, 0x02, 0x37, 0x1d, 0x95)
 #define LINUX_EFI_LOADER_ENTRY_GUID		EFI_GUID(0x4a67b082, 0x0a4c, 0x41cf,  0xb6, 0xc7, 0x44, 0x0b, 0x29, 0xbb, 0x8c, 0x4f)
+#define LINUX_EFI_RANDOM_SEED_TABLE_GUID	EFI_GUID(0x1ce1e5bc, 0x7ceb, 0x42f2,  0x81, 0xe5, 0x8a, 0xad, 0xf1, 0x80, 0xf5, 0x7b)
 
 typedef struct {
 	efi_guid_t guid;
@@ -872,6 +873,7 @@ extern struct efi {
 	unsigned long esrt;		/* ESRT table */
 	unsigned long properties_table;	/* properties table */
 	unsigned long mem_attr_table;	/* memory attributes table */
+	unsigned long rng_seed;		/* UEFI firmware random seed */
 	efi_get_time_t *get_time;
 	efi_set_time_t *set_time;
 	efi_get_wakeup_time_t *get_wakeup_time;
@@ -1493,4 +1495,10 @@ efi_status_t efi_exit_boot_services(efi_system_table_t *sys_table,
 				    struct efi_boot_memmap *map,
 				    void *priv,
 				    efi_exit_boot_map_processing priv_func);
+
+struct linux_efi_random_seed {
+	u32	size;
+	u8	bits[];
+};
+
 #endif /* _LINUX_EFI_H */