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 erase view of the rng-seed option, except our 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 on average 6 bits of useful entropy
per character.
Signed-off-by: Mark Salyzyn <salyzyn@xxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: kernel-team@xxxxxxxxxxx
Cc: "Theodore Ts'o" <tytso@xxxxxxx>
Cc: Arnd Bergmann <arnd@xxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Richard Henderson <richard.henderson@xxxxxxxxxx>
Cc: Mark Brown <broonie@xxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
Cc: Vasily Gorbik <gor@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
---
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.
---
.../admin-guide/kernel-parameters.txt | 11 +++
drivers/char/random.c | 8 ++
include/linux/random.h | 5 ++
init/main.c | 88 +++++++++++++++----
4 files changed, 95 insertions(+), 17 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index dbc22d6846275..f3c373cc40f9a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4334,6 +4334,17 @@
[KNL] Disable ring 3 MONITOR/MWAIT feature on supported
CPUs.
+ rng-seed= [KNL] Provide a trusted seed for the kernel's CRNG.
+ Seed only trusted if CONFIG_RANDOM_TRUST_BOOTLOADER.
+ After collection, this option is wiped from the command
+ line views. 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.
+
ro [KNL] Mount root device read-only on boot
rodata= [KNL]
diff --git a/drivers/char/random.c b/drivers/char/random.c
index ee21a6a584b15..83c77306e18e7 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/include/linux/random.h b/include/linux/random.h
index d319f9a1e4290..efe8cbe2255ab 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_bits(unsigned int nbits);
+#else
+static inline void credit_trusted_entropy_bits(unsigned int nbits) {}
+#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 9f4ce0356057e..ad52f03fb8de4 100644
--- a/init/main.c
+++ b/init/main.c
@@ -524,6 +524,31 @@ 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;
+ }
+ }
+ strlcpy(dest, src, r);
+}
+
static const char alloc_fail_msg[] __initconst =
"%s: Failed to allocate %zu bytes\n";
static void __init setup_command_line(char *command_line)
@@ -552,11 +577,15 @@ 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 + 1);
+ copy_command_line(static_command_line, extra_command_line,
+ xlen + 1);
}
- strlcpy(saved_command_line + xlen, boot_command_line, len - xlen);
- strcpy(static_command_line + xlen, command_line);
+ copy_command_line(saved_command_line + xlen, boot_command_line,
+ len - xlen);
+ copy_command_line(static_command_line + xlen, command_line,
+ len - xlen);
if (ilen) {
/*
@@ -572,7 +601,8 @@ static void __init setup_command_line(char *command_line)
} 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) + 1);
}
}
@@ -757,6 +787,41 @@ 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 command line device randomness above,
+ * now add entropy credit for just rng-seed=<data>
+ */
+ 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_bits(l * 6);
+ }
+ }
+ boot_init_stack_canary();
+}
+
asmlinkage __visible void __init start_kernel(void)
{
char *command_line;
@@ -868,18 +933,7 @@ 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();
--
2.25.0.341.g760bfbb309-goog
