[LEDE-DEV] [PATCH v3] base-files: seed /dev/urandom

[Etienne CHAMPETIER]

This commit:
1) seed /dev/urandom with a saved seed as early as possible
   (using /lib/preinit/81_urandom_seed)
2) save a new seed using getrandom() so we are sure /dev/urandom
   pool is initialized (using /etc/init.d/urandom_seed)

seed size is 512 bytes (ie /proc/sys/kernel/random/poolsize / 8)
it's the same size as in ubuntu 14.04 and all systemd systems

seed file is /etc/urandom.seed (need a writable path)

seeding /dev/urandom doesn't change entropy estimation, so we still have
"random: ubus urandom read with 4 bits of entropy available"
messages in the logs, but we can now ignore them if
after "urandom-seed: Seeding with ..." message

We could also add an urandom.seed at build time to improve first boot

v2: log preinit messages to /dev/kmsg
v3: use non generic function name for logging, as /lib/preinit/ files
    are all sourced together in /etc/preinit

Signed-off-by: Etienne CHAMPETIER <champetier.etienne at gmail.com>
---
 package/base-files/files/etc/init.d/urandom_seed   | 22 ++++++++++++++++++++++
 .../base-files/files/lib/preinit/81_urandom_seed   | 19 +++++++++++++++++++
 2 files changed, 41 insertions(+)
 create mode 100755 package/base-files/files/etc/init.d/urandom_seed
 create mode 100644 package/base-files/files/lib/preinit/81_urandom_seed

diff --git a/package/base-files/files/etc/init.d/urandom_seed b/package/base-files/files/etc/init.d/urandom_seed
new file mode 100755
index 0000000..d7c1f71
--- /dev/null
+++ b/package/base-files/files/etc/init.d/urandom_seed
@@ -0,0 +1,22 @@
+#!/bin/sh /etc/rc.common
+
+START=99
+
+SEED=/etc/urandom.seed
+
+error_exit() {
+    logger -t urandom_seed "$1"
+    exit 1
+}
+
+save_seed() {
+    touch $SEED.tmp || error_exit "touch failed"
+    chown root:root $SEED.tmp || error_exit "chown failed"
+    chmod 600 $SEED.tmp || error_exit "chmod failed"
+    getrandom 512 > $SEED.tmp || error_exit "getrandom failed"
+    mv $SEED.tmp $SEED || error_exit "mv failed"
+}
+
+boot() {
+    save_seed
+}
diff --git a/package/base-files/files/lib/preinit/81_urandom_seed b/package/base-files/files/lib/preinit/81_urandom_seed
new file mode 100644
index 0000000..76c849a
--- /dev/null
+++ b/package/base-files/files/lib/preinit/81_urandom_seed
@@ -0,0 +1,19 @@
+#!/bin/sh
+
+log_urandom_seed() {
+    echo "urandom-seed: $1" > /dev/kmsg
+}
+
+do_urandom_seed() {
+    S=/etc/urandom.seed
+    U=/dev/urandom
+
+    [ -c $U ] || { log_urandom_seed "Something is wrong with $U"; return; }
+    [ -f $S ] || { log_urandom_seed "Seed file not found: $S"; return; }
+    [ -O $S -a -G $S -a ! -x $S ] || { log_urandom_seed "Wrong owner / permissions for $S"; return; }
+
+    log_urandom_seed "Seeding with $S"
+    cat $S > $U
+}
+
+boot_hook_add preinit_main do_urandom_seed
-- 
1.9.1