Re: [PATCH] random: add rng-seed= command line option

[Masami Hiramatsu <mhiramat@kernel.org>]

On Thu, 13 Feb 2020 20:24:54 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> > > My preference would be to pass in the random seed *not* on the
> > > command-line at all, but as a separate parameter which is passed to
> > > the bootloader, just as we pass in the device-tree, the initrd and the
> > > command-line as separate things.  The problem is that how we pass in
> > > extra boot parameters is architecture specific, and how we might do it
> > > for x86 is different than for arm64.  So yeah, it's a bit more
> > > inconvenient to do things that way; but I think it's also much
> > > cleaner.
> > 
> > Hmm, if the boot loader could add on to the bootconfig that Masami just
> > added, then it could add some "random" seed for each boot! The
> > bootconfig is just an appended file at the end of the initrd.
> 
> Yeah, it is easy to add bootconfig support to a bootloader. It can add
> a entropy number as "rng.seed=XXX" text after initrd image with size
> and checksum. That is architecutre independent way to pass such hidden
> parameter.
> (hidden key must be filtered out when printing out the /proc/bootconfig,
> but that is very easy too, just need a strncmp)
> 

And here is the patch to support "random.rng_seed = XXX" option by
bootconfig. Now you can focus on what you want to do. No need to
modify command line strings.

BTW, if you think you need to pass UTF-8 code as a data, I'm happy to
update the bootconfig to support it. Just for the safeness, I have
limited it by isprint() || isspace().

Thank you,

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 c7f9584de2c8..0ae33bbbd338 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..aace466c56ed 100644
--- a/fs/proc/bootconfig.c
+++ b/fs/proc/bootconfig.c
@@ -36,6 +36,9 @@ 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..6c3f51bc76d5 100644
--- a/init/main.c
+++ b/init/main.c
@@ -776,6 +776,32 @@ 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)
+			credit_trusted_entropy_bits(strlen(rng_seed) * 6);
+	}
+	boot_init_stack_canary();
+}
+
 asmlinkage __visible void __init start_kernel(void)
 {
 	char *command_line;
@@ -887,18 +913,7 @@ 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();

-- 
Masami Hiramatsu <mhiramat@kernel.org>