The attached patch should apply cleanly to the Debian urandom. My apologies for attaching the wrong patch previously. I picked up the Ubuntu patch instead of the Debian patch.
>From ef7dda4ef592758a845ad28d0795743ac9677371 Mon Sep 17 00:00:00 2001 From: root <r...@localhost.localdomain> Date: Sat, 11 Sep 2010 15:27:05 -0700 Subject: [PATCH] Explain why we /write/ the seed at boot time. Also document assumptions about random.seed. --- urandom | 20 +++++++++++++++++++- 1 files changed, 19 insertions(+), 1 deletions(-) diff --git a/urandom b/urandom index ffca615..79344cb 100755 --- a/urandom +++ b/urandom @@ -10,10 +10,19 @@ # It is called from the boot, halt and reboot scripts. ### END INIT INFO +## Assumption 1: We assume $SAVEDFILE is a file (or a symlink +## to a file) that resides on a non-volatile medium that persists +## across reboots. +## Case 1a: Ideally, it is readable and writeable. Its is unshared, +## i.e. its contents are unique to this machine. It is protected so +## that its contents are not known to attackers. +## Case 1b: Less than ideally, it is read-only. Its contents are +## unique to this machine and not known to attackers. +SAVEDFILE=/var/lib/urandom/random-seed + [ -c /dev/urandom ] || exit 0 PATH=/sbin:/usr/sbin:/bin:/usr/bin -SAVEDFILE=/var/lib/urandom/random-seed if ! POOLBYTES=$(( ($(cat /proc/sys/kernel/random/poolsize 2>/dev/null) + 7) / 8 )) ; then @@ -33,6 +42,13 @@ case "$1" in cat "$SAVEDFILE" >/dev/urandom fi rm -f $SAVEDFILE + # Write a new seed into $SAVEDFILE because re-using a seed + # compromises security. Each time we re-seed, we want the + # seed to be as different as possible. + # Write it now, in case the machine crashes without doing + # an orderly shutdown. + # The write will fail if $SAVEDFILE is read-only, but it + # doesn't hurt to try. umask 077 dd if=/dev/urandom of=$SAVEDFILE bs=$POOLBYTES count=1 >/dev/null 2>&1 ES=$? @@ -41,6 +57,8 @@ case "$1" in ;; stop) # Carry a random seed from shut-down to start-up; + # Write it on shutdown, in case the one written at startup + # has been lost, snooped, or otherwise compromised. # see documentation in linux/drivers/char/random.c [ "$VERBOSE" = no ] || log_action_begin_msg "Saving random seed" umask 077 -- 1.5.6.5
