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Subject[GIT PULL] random number generator updates for 5.19-rc1
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Hi Linus,

Please pull the following random number generator updates for 5.19-rc1. These
updates continue to refine the work began in 5.17 and 5.18 of modernizing the
RNG's crypto and streamlining and documenting its code. New for 5.19, the
updates aim to improve entropy collection methods and make some initial
decisions regarding the "premature next" problem and our threat model. The
cloc utility now reports that random.c is 931 lines of code and 466 lines of
comments, not that basic metrics like that mean all that much, but at the very
least it tells you that this is very much a manageable driver now.

Here's a summary of the various patches in this pull:

- The random_get_entropy() function now always returns something at least
minimally useful. This is the primary entropy source in most collectors,
which in the best case expands to something like RDTSC, but prior to this
change, in the worst case it would just return 0, contributing nothing. For
5.19, additional architectures are wired up, and architectures that are
entirely missing a cycle counter now have a generic fallback path, which
uses the highest resolution clock available from the timekeeping subsystem.
Some of those clocks can actually be quite good, despite the CPU not having
a cycle counter of its own, and going off-core for a stamp is generally
thought to increase jitter, something positive from the perspective of
entropy gathering. Done very early on in the development cycle, this has
been sitting in next getting some testing for a while now and has relevant
acks from the archs, so it should be pretty well tested and fine, but is
nonetheless the thing I'll be keeping my eye on most closely.

- Of particular note with the random_get_entropy() improvements is MIPS,
which, on CPUs that lack the c0 count register, will now combine the
high-speed but short-cycle c0 random register with the lower-speed but
long-cycle generic fallback path.

- With random_get_entropy() now always returning something useful, the
interrupt handler now collects entropy in a consistent construction.

- Rather than comparing two samples of random_get_entropy() for the jitter
dance, the algorithm now tests many samples, and uses the amount of
differing ones to determine whether or not jitter entropy is usable and how
laborious it must be. The problem with comparing only two samples was that
if the cycle counter was extremely slow, but just so happened to be on the
cusp of a change, the slowness wouldn't be detected. Taking many samples
fixes that to some degree. This, combined with the other improvements to
random_get_entropy(), should make future unification of /dev/random and
/dev/urandom maybe more possible. At the very least, were we to attempt it
again today (we're not), it wouldn't break any of Guenter's test rigs that
broke when we tried it with 5.18. So, not today, but perhaps down the road,
that's something we can revisit.

- We attempt to reseed the RNG immediately upon waking up from system suspend
or hibernation, making use of the various timestamps about suspend time and
such available, as well as the usual inputs such as RDRAND when available.

- Batched randomness now falls back to ordinary randomness before the RNG is
initialized. This provides more consistent guarantees to the types of random
numbers being returned by the various accessors.

- The "pre-init injection" code is now gone for good. I suspect you in
particular will be happy to read that, as I recall you expressing your
distaste for it a few months ago. Instead, to avoid a "premature first"
issue, while still allowing for maximal amount of entropy availability
during system boot, the first 128 bits of estimated entropy are used
immediately as it arrives, with the next 128 bits being buffered. And, as
before, after the RNG has been fully initialized, it winds up reseeding
anyway a few seconds later in most cases. This resulted in a pretty big
simplification of the initialization code and let us remove various ad-hoc
mechanisms like the ugly crng_pre_init_inject().

- The RNG no longer pretends to handle the "premature next" security model,
something that various academics and other RNG designs have tried to care
about in the past. After an interesting mailing list thread, these issues
are thought to be a) mainly academic and not practical at all, and b)
actively harming the real security of the RNG by delaying new entropy
additions after a potential compromise, making a potentially bad situation
even worse. As well, in the first place, our RNG never even properly handled
the premature next issue, so removing an incomplete solution to a fake
problem was particularly nice. This allowed for numerous other
simplifications in the code, which is a lot cleaner as a consequence. If you
didn't see it before, https://lore.kernel.org/lkml/YmlMGx6+uigkGiZ0@zx2c4.com/
may be a thread worth skimming through.

- While the interrupt handler received a separate code path years ago that
avoids locks by using per-cpu data structures and a faster mixing algorithm,
in order to reduce interrupt latency, input and disk events that are
triggered in hardirq handlers were still hitting locks and more expensive
algorithms. Those are now redirected to use the faster per-cpu data
structures.

- Rather than having the fake-crypto almost-siphash-based random32
implementation be used right and left, and in many places where
cryptographically secure randomness is desirable, the batched entropy code
is now fast enough to replace that.

- As usual, numerous code quality and documentation cleanups. For example, the
initialization state machine now uses enum symbolic constants instead of
just hard coding numbers everywhere.

- Since the RNG initializes once, and then is always initialized thereafter, a
pretty heavy amount of code used during that initialization is never used
again. It is now completely cordoned off using static branches and it winds
up in the .text.unlikely section so that it doesn't reduce cache compactness
after the RNG is ready.

- A variety of functions meant for waiting on the RNG to be initialized were
only used by vsprintf, and in not a particularly optimal way. Replacing that
usage with a more ordinary setup made it possible to remove those functions.

- A cleanup of how we warn userspace about the use of uninitialized
/dev/urandom and uninitialized get_random_bytes() usage. Interestingly, with
the change you merged for 5.18 that attempts to use jitter (but does not
block if it can't), the majority of users should never see those warnings
for /dev/urandom at all now, and the one for in-kernel usage is mainly a
debug thing.

- The file_operations struct for /dev/[u]random now implements .read_iter and
.write_iter instead of .read and .write, allowing it to also implement
.splice_read and .splice_write, which makes splice(2) work again after it
was broken here (and in many other places in the tree) during the set_fs()
removal. This was a bit of a last minute arrival from Jens that hasn't had
as much time to bake, so I'll be keeping my eye on this as well, but it
seems fairly ordinary. Unfortunately, read_iter() is around 3% slower than
read() in my tests, which I'm not thrilled about. But Jens and Al, spurred
by this observation, seem to be making progress in removing the bottlenecks
on the iter paths in the VFS layer in general, which should remove the
performance gap for all drivers.

- Assorted other bug fixes, cleanups, and optimizations.

- A small SipHash cleanup. Note: Greg has an SPDX SipHash patch in his tree
that changes the same files, which git should handle automatically, but
heads up in case there's merge trouble.

Regards,
Jason

The following changes since commit c5eb0a61238dd6faf37f58c9ce61c9980aaffd7a:

Linux 5.18-rc6 (2022-05-08 13:54:17 -0700)

are available in the Git repository at:

https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git tags/random-5.19-rc1-for-linus

for you to fetch changes up to 1ce6c8d68f8ac587f54d0a271ac594d3d51f3efb:

random: check for signals after page of pool writes (2022-05-22 22:34:31 +0200)

----------------------------------------------------------------
Random number generator updates for Linux 5.19-rc1.
----------------------------------------------------------------

Jason A. Donenfeld (46):
random: fix sysctl documentation nits
init: call time_init() before rand_initialize()
ia64: define get_cycles macro for arch-override
s390: define get_cycles macro for arch-override
parisc: define get_cycles macro for arch-override
alpha: define get_cycles macro for arch-override
powerpc: define get_cycles macro for arch-override
openrisc: start CPU timer early in boot
timekeeping: Add raw clock fallback for random_get_entropy()
m68k: use fallback for random_get_entropy() instead of zero
riscv: use fallback for random_get_entropy() instead of zero
mips: use fallback for random_get_entropy() instead of just c0 random
arm: use fallback for random_get_entropy() instead of zero
nios2: use fallback for random_get_entropy() instead of zero
x86/tsc: Use fallback for random_get_entropy() instead of zero
um: use fallback for random_get_entropy() instead of zero
sparc: use fallback for random_get_entropy() instead of zero
xtensa: use fallback for random_get_entropy() instead of zero
random: insist on random_get_entropy() existing in order to simplify
random: vary jitter iterations based on cycle counter speed
random: mix in timestamps and reseed on system restore
random: do not use batches when !crng_ready()
random: use first 128 bits of input as fast init
random: do not pretend to handle premature next security model
random: order timer entropy functions below interrupt functions
random: do not use input pool from hard IRQs
random: help compiler out with fast_mix() by using simpler arguments
siphash: use one source of truth for siphash permutations
random32: use real rng for non-deterministic randomness
random: use symbolic constants for crng_init states
random: avoid initializing twice in credit race
random: move initialization out of reseeding hot path
random: remove ratelimiting for in-kernel unseeded randomness
random: use proper jiffies comparison macro
random: handle latent entropy and command line from random_init()
random: credit architectural init the exact amount
random: use static branch for crng_ready()
random: remove extern from functions in header
random: use proper return types on get_random_{int,long}_wait()
random: make consistent use of buf and len
random: move initialization functions out of hot pages
random: remove get_random_bytes_arch() and add rng_has_arch_random()
random: remove mostly unused async readiness notifier
random: move randomize_page() into mm where it belongs
random: unify batched entropy implementations
random: check for signals after page of pool writes

Jens Axboe (3):
random: convert to using fops->read_iter()
random: convert to using fops->write_iter()
random: wire up fops->splice_{read,write}_iter()

Documentation/admin-guide/sysctl/kernel.rst | 8 +-
arch/alpha/include/asm/timex.h | 1 +
arch/arm/include/asm/timex.h | 1 +
arch/ia64/include/asm/timex.h | 1 +
arch/m68k/include/asm/timex.h | 2 +-
arch/mips/include/asm/timex.h | 17 +-
arch/nios2/include/asm/timex.h | 3 +
arch/openrisc/include/asm/timex.h | 1 +
arch/openrisc/kernel/head.S | 9 +
arch/parisc/include/asm/timex.h | 3 +-
arch/powerpc/include/asm/timex.h | 1 +
arch/riscv/include/asm/timex.h | 2 +-
arch/s390/include/asm/timex.h | 1 +
arch/sparc/include/asm/timex_32.h | 4 +-
arch/um/include/asm/timex.h | 9 +-
arch/x86/include/asm/timex.h | 9 +
arch/x86/include/asm/tsc.h | 7 +-
arch/xtensa/include/asm/timex.h | 6 +-
drivers/char/random.c | 1346 +++++++++++----------------
include/linux/mm.h | 1 +
include/linux/prandom.h | 61 +-
include/linux/random.h | 90 +-
include/linux/siphash.h | 28 +
include/linux/timex.h | 8 +
init/main.c | 13 +-
kernel/time/timekeeping.c | 15 +
kernel/time/timer.c | 2 -
lib/Kconfig.debug | 3 +-
lib/random32.c | 347 +------
lib/siphash.c | 32 +-
lib/vsprintf.c | 67 +-
mm/util.c | 32 +
net/core/dev.c | 3 -
net/ipv4/devinet.c | 4 +-
net/ipv6/addrconf.c | 2 -
35 files changed, 784 insertions(+), 1355 deletions(-)

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