fdt: add support for rng-seed

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>
Reviewed-by: Theodore Ts'o <tytso@mit.edu> # drivers/char/random.c
Signed-off-by: Will Deacon <will@kernel.org>

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;
 }