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

Hsin-Yi Wang
2019-08-22T07:16:48Z

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@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
---
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