[PATCH 3/3] random: add random.rng_seed= bootconfig option

[Masami Hiramatsu <mhiramat@kernel.org>]

From: Mark Salyzyn <salyzyn@android.com>

A followup to commit 428826f5358c922dc378830a1717b682c0823160
("fdt: add support for rng-seed") to extend what was started
with Open Firmware (OF or Device Tree) parsing, but also add
it to the bootconfig.

If CONFIG_RANDOM_TRUST_BOOTLOADER is set, then feed the
random.rng_seed bootconfig data length as added trusted
entropy.

Always erase view of the random.rng_seed option from
/proc/bootconfig to prevent leakage to applications or modules,
to eliminate any attack vector.  Note that initcall embedded
code still have a chance to see it, but that will be unsafe
at different level.

It is preferred to add rng-seed to the Device Tree, but some
platforms do not have this option, so this adds the ability to
provide some bootconfig-limited data to the entropy through this
alternate mechanism.  Expect on average 6 bits of useful entropy
per character.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Hsin-Yi Wang <hsinyi@chromium.org>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexander Potapenko <glider@google.com>
---
v4
- Use bootconfig instead of command line
- Move the documentation under Documentation/admin-guide/bootconfig/.
v3
- Add Documentation (all other new v2 patches unchanged)

v2
- Split into four bite sized patches.
- Correct spelling in commit message.
- rng-seed is assumed to be utf-8, so correct both to 6 bits/character
  of collected entropy.
- Move entropy collection to a static __always_inline helper function.
---
 Documentation/admin-guide/bootconfig/random.rst |   21 ++++++++++++
 drivers/char/Kconfig                            |    1 +
 drivers/char/random.c                           |    8 ++++
 fs/proc/bootconfig.c                            |    4 ++
 include/linux/random.h                          |    7 ++++
 init/main.c                                     |   41 ++++++++++++++++-------
 6 files changed, 70 insertions(+), 12 deletions(-)
 create mode 100644 Documentation/admin-guide/bootconfig/random.rst

diff --git a/Documentation/admin-guide/bootconfig/random.rst b/Documentation/admin-guide/bootconfig/random.rst
new file mode 100644
index 000000000000..d4ee513c5136
--- /dev/null
+++ b/Documentation/admin-guide/bootconfig/random.rst
@@ -0,0 +1,21 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===============================
+The Random Subsystem Bootconfig
+===============================
+
+The keys start with "random." configures random number generator subsystem.
+
+Options
+=======
+
+random.rng_seed
+  Provide a trusted seed for the kernel's CRNG. Seed only trusted if
+  CONFIG_RANDOM_TRUST_BOOTLOADER=y.  After collection, this option is not
+  shown in /proc/bootconfig.
+  The seed is given a weight of 6 bits per character with the assumption that
+  it is a printable utf8 string.  It is expected that the supplier of the
+  seed, typically a bootloader or virtualization, will supply a new random
+  seed for each kernel instance.
+  A fixed serial number is typically not appropriate for security features
+  like ASLR.
diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 26956c006987..43fbbd307204 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -554,6 +554,7 @@ config RANDOM_TRUST_CPU
 
 config RANDOM_TRUST_BOOTLOADER
 	bool "Trust the bootloader to initialize Linux's CRNG"
+	select BOOT_CONFIG
 	help
 	Some bootloaders can provide entropy to increase the kernel's initial
 	device randomness. Say Y here to assume the entropy provided by the
diff --git a/drivers/char/random.c b/drivers/char/random.c
index ee21a6a584b1..83c77306e18e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2311,3 +2311,11 @@ void add_bootloader_randomness(const void *buf, unsigned int size)
 		add_device_randomness(buf, size);
 }
 EXPORT_SYMBOL_GPL(add_bootloader_randomness);
+
+#if defined(CONFIG_RANDOM_TRUST_BOOTLOADER)
+/* caller called add_device_randomness, but it is from a trusted source */
+void __init credit_trusted_entropy_bits(unsigned int nbits)
+{
+	credit_entropy_bits(&input_pool, nbits);
+}
+#endif
diff --git a/fs/proc/bootconfig.c b/fs/proc/bootconfig.c
index 9955d75c0585..6d1a819f2df4 100644
--- a/fs/proc/bootconfig.c
+++ b/fs/proc/bootconfig.c
@@ -8,6 +8,7 @@
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
 #include <linux/bootconfig.h>
+#include <linux/random.h>
 #include <linux/slab.h>
 
 static char *saved_boot_config;
@@ -36,6 +37,9 @@ static int __init copy_xbc_key_value_list(char *dst, size_t size)
 		ret = xbc_node_compose_key(leaf, key, XBC_KEYLEN_MAX);
 		if (ret < 0)
 			break;
+		/* For keeping security reason, remove randomness key */
+		if (!strcmp(key, RANDOM_SEED_XBC_KEY))
+			continue;
 		ret = snprintf(dst, rest(dst, end), "%s = ", key);
 		if (ret < 0)
 			break;
diff --git a/include/linux/random.h b/include/linux/random.h
index d319f9a1e429..c8f41ab4f342 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -20,6 +20,13 @@ struct random_ready_callback {
 
 extern void add_device_randomness(const void *, unsigned int);
 extern void add_bootloader_randomness(const void *, unsigned int);
+#if defined(CONFIG_RANDOM_TRUST_BOOTLOADER)
+extern void __init credit_trusted_entropy_bits(unsigned int nbits);
+#else
+static inline void credit_trusted_entropy_bits(unsigned int nbits) {}
+#endif
+
+#define RANDOM_SEED_XBC_KEY "random.rng_seed"
 
 #if defined(LATENT_ENTROPY_PLUGIN) && !defined(__CHECKER__)
 static inline void add_latent_entropy(void)
diff --git a/init/main.c b/init/main.c
index f95b014a5479..d0e5a95b4182 100644
--- a/init/main.c
+++ b/init/main.c
@@ -776,6 +776,34 @@ void __init __weak arch_call_rest_init(void)
 	rest_init();
 }
 
+static __always_inline void __init collect_entropy(const char *command_line)
+{
+	/*
+	 * For best initial stack canary entropy, prepare it after:
+	 * - setup_arch() for any UEFI RNG entropy and boot cmdline access
+	 * - timekeeping_init() for ktime entropy used in rand_initialize()
+	 * - rand_initialize() to get any arch-specific entropy like RDRAND
+	 * - add_latent_entropy() to get any latent entropy
+	 * - adding command line entropy
+	 */
+	rand_initialize();
+	add_latent_entropy();
+	add_device_randomness(command_line, strlen(command_line));
+	if (IS_BUILTIN(CONFIG_RANDOM_TRUST_BOOTLOADER)) {
+		/*
+		 * Added bootconfig device randomness above,
+		 * now add entropy credit for just random.rng_seed=<data>
+		 */
+		const char *rng_seed = xbc_find_value(RANDOM_SEED_XBC_KEY, NULL);
+
+		if (rng_seed) {
+			add_device_randomness(rng_seed, strlen(rng_seed));
+			credit_trusted_entropy_bits(strlen(rng_seed) * 6);
+		}
+	}
+	boot_init_stack_canary();
+}
+
 asmlinkage __visible void __init start_kernel(void)
 {
 	char *command_line;
@@ -887,18 +915,7 @@ asmlinkage __visible void __init start_kernel(void)
 	softirq_init();
 	timekeeping_init();
 
-	/*
-	 * For best initial stack canary entropy, prepare it after:
-	 * - setup_arch() for any UEFI RNG entropy and boot cmdline access
-	 * - timekeeping_init() for ktime entropy used in rand_initialize()
-	 * - rand_initialize() to get any arch-specific entropy like RDRAND
-	 * - add_latent_entropy() to get any latent entropy
-	 * - adding command line entropy
-	 */
-	rand_initialize();
-	add_latent_entropy();
-	add_device_randomness(command_line, strlen(command_line));
-	boot_init_stack_canary();
+	collect_entropy(command_line);
 
 	time_init();
 	printk_safe_init();