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

[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 command line.

If CONFIG_RANDOM_TRUST_BOOTLOADER is set, then feed the rng-seed
command line option length as added trusted entropy.

Always rrase all views of the rng-seed option, except early command
line parsing, to prevent leakage to applications or modules, to
eliminate any attack vector.

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 command-line-limited data to the entropy through this
alternate mechanism.  Expect all 8 bits to be used, but must exclude
space to be accounted in the command line.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Cc: linux-kernel@vger.kernel.org
Cc: kernel-team@android.com
---
 drivers/char/random.c  |  8 +++++
 include/linux/random.h |  5 +++
 init/main.c            | 73 +++++++++++++++++++++++++++++++++++-------
 3 files changed, 74 insertions(+), 12 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index c7f9584de2c8b..2f386e411fb7b 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(unsigned int size)
+{
+	credit_entropy_bits(&input_pool, size * 8);
+}
+#endif
diff --git a/include/linux/random.h b/include/linux/random.h
index d319f9a1e4290..1e09eeadc613c 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -20,6 +20,11 @@ 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(unsigned int b);
+#else
+static inline void credit_trusted_entropy(unsigned int b) {}
+#endif
 
 #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 cc0ee4873419c..ae976b2dea5dc 100644
--- a/init/main.c
+++ b/init/main.c
@@ -524,24 +524,53 @@ static inline void smp_prepare_cpus(unsigned int maxcpus) { }
  * parsing is performed in place, and we should allow a component to
  * store reference of name/value for future reference.
  */
+static const char rng_seed_str[] __initconst = "rng-seed=";
+/* try to clear rng-seed so it won't be found by user applications. */
+static void __init copy_command_line(char *dest, char *src, size_t r)
+{
+	char *rng_seed = strnstr(src, rng_seed_str, r);
+
+	if (rng_seed) {
+		size_t l = rng_seed - src;
+		char *end;
+
+		memcpy(dest, src, l);
+		dest += l;
+		src = rng_seed + strlen(rng_seed_str);
+		r -= l + strlen(rng_seed_str);
+		end = strnchr(src, r, ' ');
+		if (end) {
+			if (l && rng_seed[-1] == ' ')
+				++end;
+			r -= end - src;
+			src = end;
+		}
+	}
+	memcpy(dest, src, r);
+	dest[r] = '\0';
+}
+
 static void __init setup_command_line(char *command_line)
 {
 	size_t len, xlen = 0, ilen = 0;
+	static const char argsep_str[] __initconst = " -- ";
+	static const char alloc_fail_msg[] __initconst =
+		"%s: Failed to allocate %zu bytes\n";
 
 	if (extra_command_line)
 		xlen = strlen(extra_command_line);
 	if (extra_init_args)
-		ilen = strlen(extra_init_args) + 4; /* for " -- " */
+		ilen = strlen(extra_init_args) + strlen(argsep_str);
 
-	len = xlen + strlen(boot_command_line) + 1;
+	len = xlen + strnlen(boot_command_line, sizeof(boot_command_line)) + 1;
 
 	saved_command_line = memblock_alloc(len + ilen, SMP_CACHE_BYTES);
 	if (!saved_command_line)
-		panic("%s: Failed to allocate %zu bytes\n", __func__, len + ilen);
+		panic(alloc_fail_msg, __func__, len + ilen);
 
 	static_command_line = memblock_alloc(len, SMP_CACHE_BYTES);
 	if (!static_command_line)
-		panic("%s: Failed to allocate %zu bytes\n", __func__, len);
+		panic(alloc_fail_msg, __func__, len);
 
 	if (xlen) {
 		/*
@@ -549,11 +578,14 @@ static void __init setup_command_line(char *command_line)
 		 * lines because there could be dashes (separator of init
 		 * command line) in the command lines.
 		 */
-		strcpy(saved_command_line, extra_command_line);
-		strcpy(static_command_line, extra_command_line);
+		copy_command_line(saved_command_line, extra_command_line, xlen);
+		copy_command_line(static_command_line, extra_command_line,
+				  xlen);
 	}
-	strcpy(saved_command_line + xlen, boot_command_line);
-	strcpy(static_command_line + xlen, command_line);
+	copy_command_line(saved_command_line + xlen, boot_command_line,
+			  len - xlen - 1);
+	copy_command_line(static_command_line + xlen, command_line,
+			  len - xlen - 1);
 
 	if (ilen) {
 		/*
@@ -562,13 +594,15 @@ static void __init setup_command_line(char *command_line)
 		 * to init.
 		 */
 		len = strlen(saved_command_line);
-		if (!strstr(boot_command_line, " -- ")) {
-			strcpy(saved_command_line + len, " -- ");
-			len += 4;
+		if (!strnstr(boot_command_line, argsep_str,
+			     sizeof(boot_command_line))) {
+			strcpy(saved_command_line + len, argsep_str);
+			len += strlen(argsep_str);
 		} else
 			saved_command_line[len++] = ' ';
 
-		strcpy(saved_command_line + len, extra_init_args);
+		copy_command_line(saved_command_line + len, extra_init_args,
+				  ilen - strlen(argsep_str));
 	}
 }
 
@@ -875,6 +909,21 @@ asmlinkage __visible void __init start_kernel(void)
 	rand_initialize();
 	add_latent_entropy();
 	add_device_randomness(command_line, strlen(command_line));
+	if (IS_BUILTIN(CONFIG_RANDOM_TRUST_BOOTLOADER)) {
+		size_t l = strlen(command_line);
+		char *rng_seed = strnstr(command_line, rng_seed_str, l);
+
+		if (rng_seed) {
+			char *end;
+
+			rng_seed += strlen(rng_seed_str);
+			l -= rng_seed - command_line;
+			end = strnchr(rng_seed, l, ' ');
+			if (end)
+				l = end - rng_seed;
+			credit_trusted_entropy(l);
+		}
+	}
 	boot_init_stack_canary();
 
 	time_init();
-- 
2.25.0.341.g760bfbb309-goog