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Subject[PATCH v3 3/3] fdt: add support for rng-seed
Date
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.

Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
---
change v2->v3:
1. use arch hook for fdt pgprot change
2. handle CONFIG_KEXEC
---
Documentation/devicetree/bindings/chosen.txt | 14 +++++
drivers/of/fdt.c | 55 ++++++++++++++++++++
2 files changed, 69 insertions(+)

diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
index 45e79172a646..fef5c82672dc 100644
--- a/Documentation/devicetree/bindings/chosen.txt
+++ b/Documentation/devicetree/bindings/chosen.txt
@@ -28,6 +28,20 @@ mode) when EFI_RNG_PROTOCOL is supported, it will be overwritten by
the Linux EFI stub (which will populate the property itself, using
EFI_RNG_PROTOCOL).

+rng-seed
+-----------
+
+This property served as an entropy to add device randomness. It is parsed
+as a byte array, e.g.
+
+/ {
+ chosen {
+ rng-seed = <0x31 0x95 0x1b 0x3c 0xc9 0xfa 0xb3 ...>;
+ };
+};
+
+This random value should be provided by bootloader.
+
stdout-path
-----------

diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
index e84971d1e9ea..e9862657268d 100644
--- a/drivers/of/fdt.c
+++ b/drivers/of/fdt.c
@@ -10,6 +10,7 @@

#include <linux/crc32.h>
#include <linux/kernel.h>
+#include <linux/kexec.h>
#include <linux/initrd.h>
#include <linux/memblock.h>
#include <linux/mutex.h>
@@ -24,6 +25,8 @@
#include <linux/debugfs.h>
#include <linux/serial_core.h>
#include <linux/sysfs.h>
+#include <linux/random.h>
+#include <linux/reboot.h>

#include <asm/setup.h> /* for COMMAND_LINE_SIZE */
#include <asm/page.h>
@@ -1087,11 +1090,14 @@ int __init early_init_dt_scan_memory(unsigned long node, const char *uname,
return 0;
}

+int rng_seed_size;
+
int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
int depth, void *data)
{
int l;
const char *p;
+ const void *rng_seed;

pr_debug("search \"chosen\", depth: %d, uname: %s\n", depth, uname);

@@ -1126,6 +1132,16 @@ 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", &rng_seed_size);
+ if (rng_seed && rng_seed_size > 0) {
+ add_device_randomness(rng_seed, rng_seed_size);
+
+ /* try to clear seed so it won't be found. */
+ update_fdt_pgprot(PAGE_KERNEL);
+ fdt_delprop(initial_boot_params, node, "rng-seed");
+ update_fdt_pgprot(PAGE_KERNEL_RO);
+ }
+
/* break now */
return 1;
}
@@ -1327,4 +1343,43 @@ static int __init of_fdt_raw_init(void)
late_initcall(of_fdt_raw_init);
#endif

+#ifdef CONFIG_KEXEC
+static int update_fdt_random_seed(struct notifier_block *nb,
+ unsigned long code, void *unused)
+{
+ int node;
+ void *rng_seed;
+
+ if (!kexec_in_progress || !rng_seed_size)
+ return NOTIFY_DONE;
+
+ node = fdt_path_offset(initial_boot_params, "/chosen");
+ if (node < 0)
+ node = fdt_path_offset(initial_boot_params, "/chosen@0");
+ if (node < 0)
+ return NOTIFY_DONE;
+
+ rng_seed = kmalloc(rng_seed_size, GFP_ATOMIC);
+ get_random_bytes(rng_seed, rng_seed_size);
+
+ update_fdt_pgprot(PAGE_KERNEL);
+ fdt_setprop(initial_boot_params, node, "rng-seed", rng_seed,
+ rng_seed_size);
+
+ kfree(rng_seed);
+
+ return NOTIFY_DONE;
+}
+
+static struct notifier_block fdt_random_seed_nb = {
+ .notifier_call = update_fdt_random_seed,
+};
+
+static int register_update_fdt_random_seed(void)
+{
+ return register_reboot_notifier(&fdt_random_seed_nb);
+}
+late_initcall(register_update_fdt_random_seed);
+#endif
+
#endif /* CONFIG_OF_EARLY_FLATTREE */
--
2.20.1
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