[PATCH v5 0/1] random: getrandom(2): warn on large CRNG waits, introduce new flags

Ahmed S. Darwish
2019-09-26T20:42:33Z

Summary / Changelog-v5:

- Add the new flags GRND_INSECURE and GRND_SECURE_UNBOUNDED_INITIAL_WAIT
to getrandom(2), instead of introducing a new getrandom2(2) system
call, which nobody liked.

- Fix a bug discovered through testing where "int ret =
wait_event_interruptible_timeout(waitq, true, MAX_SCHEDULE_TIMEOUT)"
returns failure (-1) due to implicit LONG_MAX => int truncation

- WARN if a process is stuck on getrandom(,,flags=0) for more than 30
seconds ... defconfig and bootparam configurable

- Add documentation for "random.getrandom_wait_threshold" kernel param

- Extra comments @ include/uapi/linux/random.h and random.c::getrandom.
Explicit recommendations to *exclusively* use the new flags.

- GRND_INSECURE never issue any warning, even if CRNG is not inited.
Similarly for GRND_SECURE_UNBOUNDED_INITIAL_WAIT, no matter how
big the unbounded wait is.

In a reply to the V4 patch, Linus posted a related patch [*] with the
following additions:

- Drop the original random.c behavior of having each /dev/urandom
"CRNG not inited" warning also _reset_ the crng_init_cnt entropy.

This is not included in this patch, as IMHO this can be done as a
separate patch on top.

- Limit GRND_RANDOM max count/buflen to 32MB instead of 2GB. This
is very sane obviously, and can be done in a separate patch on
top.

This V5 patch just tries to be as conservative as possible.

- GRND_WAIT_ENTROPY and GRND_EXCPLICIT: AFAIK these were primarily
added so that getrandom(,,flags=0) can be changed to return
weaker non-blocking crypto from non-inited CRG in a possible
future.

I hope we don't have to resort to that extreme measure.. Hopefully
the WARN() on this patch will be enough in nudging distributions to
enable more hwrng sources (RDRAND, etc.) .. and also for the
user-space developres badly pointed at (hi GDM and Qt) to fix their
code.

[*] https://lkml.kernel.org/r/CAHk-=wiCqDiU7SE3FLn2W26MS_voUAuqj5XFa1V_tiGTrrW-zQ@mail.gmail.com

Ahmed S. Darwish (1):
random: getrandom(2): warn on large CRNG waits, introduce new flags

.../admin-guide/kernel-parameters.txt | 7 ++
drivers/char/Kconfig | 60 ++++++++++-
drivers/char/random.c | 102 +++++++++++++++---
include/uapi/linux/random.h | 27 ++++-
4 files changed, 177 insertions(+), 19 deletions(-)

--
2.23.0