[PATCH v9 2/3] fdt: add support for rng-seed

[Hsin-Yi Wang]

Introducing a chosen node, rng-seed, which is an entropy that can be
passed to kernel called very early to increase initial device
randomness. Bootloader should provide this entropy and the value is
read from /chosen/rng-seed in DT.

Obtain of_fdt_crc32 for CRC check after early_init_dt_scan_nodes(),
since early_init_dt_scan_chosen() would modify fdt to erase rng-seed.

Add a new interface add_bootloader_randomness() for rng-seed use case.
Depends on whether the seed is trustworthy, rng seed would be passed to
add_hwgenerator_randomness(). Otherwise it would be passed to
add_device_randomness(). Decision is controlled by kernel config
RANDOM_TRUST_BOOTLOADER.

Signed-off-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
Reviewed-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
Reviewed-by: Rob Herring <robh@xxxxxxxxxx>
---
Change from v8:
* Add a new interface add_bootloader_randomness
* Add a new kernel config
---
 drivers/char/Kconfig   | 10 ++++++++++
 drivers/char/random.c  | 15 +++++++++++++++
 drivers/of/fdt.c       | 14 ++++++++++++--
 include/linux/random.h |  1 +
 4 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
index 96156c729a31..5974a5906fd0 100644
--- a/drivers/char/Kconfig
+++ b/drivers/char/Kconfig
@@ -551,3 +551,13 @@ config RANDOM_TRUST_CPU
 	has not installed a hidden back door to compromise the CPU's
 	random number generation facilities. This can also be configured
 	at boot with "random.trust_cpu=on/off".
+
+config RANDOM_TRUST_BOOTLOADER
+	bool "Trust the bootloader to initialize Linux's CRNG"
+	default n
+	help
+	Bootloader could provide rng-seed set in /chosen/rng-seed in DT to help
+	increase initial device randomness. Assume the entropy provided is
+	trustworthy, it would be regarded as true hardware RNGs and update the
+	entropy estimate. Otherwise it would be regarded as device input that
+	could help mix the entropy pool, but won't be added to actual entropy.
\ No newline at end of file
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 5d5ea4ce1442..29d3ff3de1e1 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2445,3 +2445,18 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 	credit_entropy_bits(poolp, entropy);
 }
 EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);
+
+/* Handle random seed passed by bootloader.
+ * If the seed is trustworthy, it would be regarded as hardware RNGs. Otherwise
+ * it would be regarded as device data.
+ * The decision is controlled by CONFIG_RANDOM_TRUST_BOOTLOADER.
+ */
+void add_bootloader_randomness(const void *buf, unsigned int size)
+{
+#ifdef CONFIG_RANDOM_TRUST_BOOTLOADER
+	add_hwgenerator_randomness(buf, size, size * 8);
+#else
+	add_device_randomness(buf, size);
+#endif
+}
+EXPORT_SYMBOL_GPL(add_bootloader_randomness);
\ No newline at end of file
diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index 9cdf14b9aaab..7d97ab6d0e31 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -24,6 +24,7 @@
 #include <linux/debugfs.h>
 #include <linux/serial_core.h>
 #include <linux/sysfs.h>
+#include <linux/random.h>
 
 #include <asm/setup.h>  /* for COMMAND_LINE_SIZE */
 #include <asm/page.h>
@@ -1044,6 +1045,7 @@ int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
 {
 	int l;
 	const char *p;
+	const void *rng_seed;
 
 	pr_debug("search \"chosen\", depth: %d, uname: %s\n", depth, uname);
 
@@ -1078,6 +1080,14 @@ int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
 
 	pr_debug("Command line is: %s\n", (char*)data);
 
+	rng_seed = of_get_flat_dt_prop(node, "rng-seed", &l);
+	if (rng_seed && l > 0) {
+		add_bootloader_randomness(rng_seed, l);
+
+		/* try to clear seed so it won't be found. */
+		fdt_nop_property(initial_boot_params, node, "rng-seed");
+	}
+
 	/* break now */
 	return 1;
 }
@@ -1166,8 +1176,6 @@ bool __init early_init_dt_verify(void *params)
 
 	/* Setup flat device-tree pointer */
 	initial_boot_params = params;
-	of_fdt_crc32 = crc32_be(~0, initial_boot_params,
-				fdt_totalsize(initial_boot_params));
 	return true;
 }
 
@@ -1197,6 +1205,8 @@ bool __init early_init_dt_scan(void *params)
 		return false;
 
 	early_init_dt_scan_nodes();
+	of_fdt_crc32 = crc32_be(~0, initial_boot_params,
+				fdt_totalsize(initial_boot_params));
 	return true;
 }
 
diff --git a/include/linux/random.h b/include/linux/random.h
index 1f7dced2bba6..f189c927fdea 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -19,6 +19,7 @@ struct random_ready_callback {
 };
 
 extern void add_device_randomness(const void *, unsigned int);
+extern void add_bootloader_randomness(const void *, unsigned int);
 
 #if defined(LATENT_ENTROPY_PLUGIN) && !defined(__CHECKER__)
 static inline void add_latent_entropy(void)
-- 
2.20.1