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

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