Seed Linux RNG from Hardware RNG at boot during init.

The Linux RNG may have little entropy during boot. As more and more
devices have a Hardware RNG, we mix in 512 bytes from Hardware RNG
(if present) into Linux RNG early during boot (after
wait_for_coldboot_done and before property_service_init actions in
init).

To avoid having to trust the output of Hardware RNG, we do not mix it
into the Linux RNG's primary pool or increase the Linux RNG's entropy
estimates.

Bug: 10362513
Change-Id: I80617f21710400747f5e7533e518d90ea74e2f11

1 files changed, 85 insertions, 0 deletions

diff --git a/init/init.c b/init/init.c
index b7e34d0..94a2011 100644
--- a/init/init.c
+++ b/init/init.c
@@ -42,6 +42,7 @@
 #include <cutils/android_reboot.h>
 #include <cutils/sockets.h>
 #include <cutils/iosched_policy.h>
+#include <cutils/fs.h>
 #include <private/android_filesystem_config.h>
 #include <termios.h>
 
@@ -559,6 +560,84 @@ static int wait_for_coldboot_done_action(int nargs, char **args)
     return ret;
 }
 
+/*
+ * Writes 512 bytes of output from Hardware RNG (/dev/hw_random, backed
+ * by Linux kernel's hw_random framework) into Linux RNG's via /dev/urandom.
+ * Does nothing if Hardware RNG is not present.
+ *
+ * Since we don't yet trust the quality of Hardware RNG, these bytes are not
+ * mixed into the primary pool of Linux RNG and the entropy estimate is left
+ * unmodified.
+ *
+ * If the HW RNG device /dev/hw_random is present, we require that at least
+ * 512 bytes read from it are written into Linux RNG. QA is expected to catch
+ * devices/configurations where these I/O operations are blocking for a long
+ * time. We do not reboot or halt on failures, as this is a best-effort
+ * attempt.
+ */
+static int mix_hwrng_into_linux_rng_action(int nargs, char **args)
+{
+    int result = -1;
+    int hwrandom_fd = -1;
+    int urandom_fd = -1;
+    char buf[512];
+    ssize_t chunk_size;
+    size_t total_bytes_written = 0;
+
+    hwrandom_fd = TEMP_FAILURE_RETRY(
+            open("/dev/hw_random", O_RDONLY | O_NOFOLLOW));
+    if (hwrandom_fd == -1) {
+        if (errno == ENOENT) {
+          ERROR("/dev/hw_random not found\n");
+          /* It's not an error to not have a Hardware RNG. */
+          result = 0;
+        } else {
+          ERROR("Failed to open /dev/hw_random: %s\n", strerror(errno));
+        }
+        goto ret;
+    }
+
+    urandom_fd = TEMP_FAILURE_RETRY(
+            open("/dev/urandom", O_WRONLY | O_NOFOLLOW));
+    if (urandom_fd == -1) {
+        ERROR("Failed to open /dev/urandom: %s\n", strerror(errno));
+        goto ret;
+    }
+
+    while (total_bytes_written < sizeof(buf)) {
+        chunk_size = TEMP_FAILURE_RETRY(
+                read(hwrandom_fd, buf, sizeof(buf) - total_bytes_written));
+        if (chunk_size == -1) {
+            ERROR("Failed to read from /dev/hw_random: %s\n", strerror(errno));
+            goto ret;
+        } else if (chunk_size == 0) {
+            ERROR("Failed to read from /dev/hw_random: EOF\n");
+            goto ret;
+        }
+
+        chunk_size = TEMP_FAILURE_RETRY(write(urandom_fd, buf, chunk_size));
+        if (chunk_size == -1) {
+            ERROR("Failed to write to /dev/urandom: %s\n", strerror(errno));
+            goto ret;
+        }
+        total_bytes_written += chunk_size;
+    }
+
+    INFO("Mixed %d bytes from /dev/hw_random into /dev/urandom",
+                total_bytes_written);
+    result = 0;
+
+ret:
+    if (hwrandom_fd != -1) {
+        close(hwrandom_fd);
+    }
+    if (urandom_fd != -1) {
+        close(urandom_fd);
+    }
+    memset(buf, 0, sizeof(buf));
+    return result;
+}
+
 static int keychord_init_action(int nargs, char **args)
 {
     keychord_init();
@@ -962,6 +1041,7 @@ int main(int argc, char **argv)
     action_for_each_trigger("early-init", action_add_queue_tail);
 
     queue_builtin_action(wait_for_coldboot_done_action, "wait_for_coldboot_done");
+    queue_builtin_action(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");
     queue_builtin_action(keychord_init_action, "keychord_init");
     queue_builtin_action(console_init_action, "console_init");
 
@@ -976,6 +1056,11 @@ int main(int argc, char **argv)
         action_for_each_trigger("post-fs-data", action_add_queue_tail);
     }
 
+    /* Repeat mix_hwrng_into_linux_rng in case /dev/hw_random or /dev/random
+     * wasn't ready immediately after wait_for_coldboot_done
+     */
+    queue_builtin_action(mix_hwrng_into_linux_rng_action, "mix_hwrng_into_linux_rng");
+
     queue_builtin_action(property_service_init_action, "property_service_init");
     queue_builtin_action(signal_init_action, "signal_init");
     queue_builtin_action(check_startup_action, "check_startup");