base-files: seed /dev/urandom

This commit:
1) seed /dev/urandom with the saved seeds as early as possible
   (see /lib/preinit/81_urandom_seed)
2) save a seed at /etc/urandom.seed if it doesn't exists
3) save a new seed each boot at "system.@system[0].urandom_seed"
   (see /etc/init.d/urandom_seed)

We use getrandom() so we are sure /dev/urandom pool is initialized

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

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

Saving a new seed on each boot is disabled by default to avoid too much
writes without user consent

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
v4: after a lot of discussion on the ML, use a uci config param
v5: config param is now the path of the seed

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>

package/base-files/files/bin/config_generate

@@ -230,6 +230,7 @@ generate_static_system() {
 		set system.@system[-1].timezone='UTC'
 		set system.@system[-1].ttylogin='0'
 		set system.@system[-1].log_size='64'
+		set system.@system[-1].urandom_seed='0'
 
 		delete system.ntp
 		set system.ntp='timeserver'

package/base-files/files/etc/init.d/urandom_seed

@@ -0,0 +1,29 @@
+#!/bin/sh /etc/rc.common
+
+START=99
+
+EXTRA_COMMANDS="save"
+
+_log() {
+    logger -t urandom_seed "$1"
+}
+
+_save() {
+    touch $1.tmp || { _log "touch $1 failed"; return; }
+    chown root:root $1.tmp || { _log "chown $1 failed"; return; }
+    chmod 600 $1.tmp || { _log "chmod $1 failed"; return; }
+    getrandom 512 > $1.tmp || { _log "getrandom failed"; return; }
+    mv $1.tmp $1 || { _log "mv $1 failed"; return; }
+}
+
+save() {
+    SEED="$(uci -q get system.@system[0].urandom_seed)"
+    [ "${SEED:0:1}" == "/" ] && _save "$SEED" && _log "Seed saved ($SEED)"
+
+    SEED=/etc/urandom.seed
+    [ ! -f $SEED ] && _save "$SEED" && _log "Seed saved ($SEED)"
+}
+
+boot() {
+    save
+}

package/base-files/files/lib/preinit/81_urandom_seed

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+log_urandom_seed() {
+    echo "urandom-seed: $1" > /dev/kmsg
+}
+
+_do_urandom_seed() {
+    [ -f "$1" ] || { log_urandom_seed "Seed file not found ($1)"; return; }
+    [ -O "$1" -a -G "$1" -a ! -x "$1" ] || { log_urandom_seed "Wrong owner / permissions for $1"; return; }
+
+    log_urandom_seed "Seeding with $1"
+    cat "$1" > /dev/urandom
+}
+
+do_urandom_seed() {
+    [ -c /dev/urandom ] || { log_urandom_seed "Something is wrong with /dev/urandom"; return; }
+
+    _do_urandom_seed "/etc/urandom.seed"
+
+    SEED="$(uci -q get system.@system[0].urandom_seed)"
+    [ "${SEED:0:1}" == "/" -a "$SEED" != "/etc/urandom.seed" ] && _do_urandom_seed "$SEED"
+}
+
+boot_hook_add preinit_main do_urandom_seed