CVS commit: src

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Fri Jun 30 21:42:06 UTC 2023

Modified Files:
        src/share/man/man7: entropy.7
        src/sys/kern: kern_clock.c kern_entropy.c

Log Message:
entropy(9): Reintroduce netbsd<=9 time-delta estimator for unblocking.

The system will (in a subsequent change) by default block for this
condition before almost all of userland is running (including
/etc/rc.d/sshd key generation).  That way, a never-blocking
getentropy(3) API will never return any data without at least
best-effort entropy like netbsd<=9 did to applications except in
single-user mode (where you have to be careful about everything
anyway) or in the few processes that run before a seed can even be
loaded (where blocking indefinitely, e.g. when generating a stack
protector cookie in libc, could pose a severe availability problem
that can't be configured away, but where the security impact is low).

However, (in another subsequent change) we will continue to use
_only_ HWRNG driver estimates and seed estimates, and _not_
time-delta estimator, for _warning_ about security in motd, daily
security report, etc.  And if HWRNG/seed provides enough entropy
before time-delta estimator does, that will unblock /dev/random too.

The result is:

- Machines with HWRNG or seed won't warn about entropy and will
  essentially never block -- even on first boot without a seed, it
  will take only as long as the fastest HWRNG to unblock.

- Machines with neither HWRNG nor seed:
  . will warn about entropy, giving feedback about security;
    and
  . will avoid returning anything more predictable than netbsd<=9;
    but
  . won't block (much) longer than netbsd<=9 would (and won't block
    again after blocking once, except with kern.entropy.depletion=1 for
    testing).

  (The threshold for unblocking is now somewhat higher than before:
  512 samples that pass the time-delta estimator, rather than 80 as
  it used to be.)

  And, of course, adding a seed (or HWRNG) will prevent both warnings
  and blocking.

The mechanism is:

1. /dev/random will block until _either_

   (a) enough bits of entropy (256) from reliable sources have been
       added to the pool, _or_

   (b) enough samples have been added from any sources (512), passing
       the old time-delta entropy estimator, that the possible
       security benefit doesn't justify holding up availability any
       longer (`best effort'), except on systems with higher security
       requirements like securelevel=2 which can disable non-HWRNG,
       non-seed sources with rndctl_flags in rc.conf(5).

2. dmesg will report `entropy: ready' when 1(a) is satisfied, but if
   1(b) is satisfied first, it will report `entropy: best effort', so
   the concise log messages will reflect the timing and whether in
   any period of time any of the system might be relying on best
   effort entropy.

3. The sysctl knob kern.entropy.needed (and the ioctl RNDGETPOOLSTAT
   variable rndpoolstat_t::added) still reflects the number of bits
   of entropy from reliable sources, so we can still use this to
   suggest regenerating ssh keys.

   This matters on platforms that can only be reached, after flashing
   an installation image, by sshing in over a (private) network, like
   small network appliances or remote virtual machines without
   (interactive) serial consoles.  If we blocked indefinitely at boot
   when generating ssh keys, such platforms would be unusable.  This
   way, platforms are usable, but operators can still be advised at
   login time to regenerate keys as soon as they can actually load
   entropy onto the system, e.g. with rndctl(8) on a seed file copied
   from a local machine over the (private) network.

4. On machines without HWRNG, using a seed file still suppresses
   warnings for users who need more confident security.  But it is no
   longer necessary for availability.

This is a compromise between availability and security:

- The security mechanism of blocking indefinitely on machines without
  HWRNG hurts availability too much, as painful experience over the
  multiple years since I made the mistake of introducing it have
  shown.  (Sorry!)

- The other main alternative, not having a blocking path at all (as I
  pushed for, and as OpenBSD has done for a long time) could
  potentially reduce security vs netbsd<=9, and would run against the
  expectations set by many popular operating systems to the severe
  detriment of public perception of NetBSD security.

Even though we can't _confidently_ assess enough entropy from, e.g.,
sampling interrupt timings, this is the traditional behaviour that
most operating systems provide -- and the result here is a net
nondecrease in security over netbsd<=9, because all paths from the
entropy pool to userland now have at least as high a standard before
returning data as they did in netbsd<=9.

PR kern/55641
PR pkg/55847
PR kern/57185
https://mail-index.netbsd.org/current-users/2020/09/02/msg039470.html
https://mail-index.netbsd.org/current-users/2020/11/21/msg039931.html
https://mail-index.netbsd.org/current-users/2020/12/05/msg040019.html

XXX pullup-10


To generate a diff of this commit:
cvs rdiff -u -r1.8 -r1.9 src/share/man/man7/entropy.7
cvs rdiff -u -r1.148 -r1.149 src/sys/kern/kern_clock.c
cvs rdiff -u -r1.61 -r1.62 src/sys/kern/kern_entropy.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.