| #
f8e8a06d |
|
10-Oct-2020 |
Conrad Meyer <cem@FreeBSD.org> |
random(4) FenestrasX: Push root seed version to arc4random(3)
Push the root seed version to userspace through the VDSO page, if
the RANDOM_FENESTRASX algorithm is enabled. Otherwise, there is no
functional change. The mechanism can be disabled with
debug.fxrng_vdso_enable=0.
arc4random(3) obtains a pointer to the root seed version published by
the kernel in the shared page at allocation time. Like arc4random(9),
it maintains its own per-process copy of the seed version corresponding
to the root seed version at the time it last rekeyed. On read requests,
the process seed version is compared with the version published in the
shared page; if they do not match, arc4random(3) reseeds from the
kernel before providing generated output.
This change does not implement the FenestrasX concept of PCPU userspace
generators seeded from a per-process base generator. That change is
left for future discussion/work.
Reviewed by: kib (previous version)
Approved by: csprng (me -- only touching FXRNG here)
Differential Revision: https://reviews.freebsd.org/D22839
|
| #
10b1a175 |
|
10-Oct-2020 |
Conrad Meyer <cem@FreeBSD.org> |
arc4random(9): Integrate with RANDOM_FENESTRASX push-reseed
There is no functional change for the existing Fortuna random(4)
implementation, which remains the default in GENERIC.
In the FenestrasX model, when the root CSPRNG is reseeded from pools due to
an (infrequent) timer, child CSPRNGs can cheaply detect this condition and
reseed. To do so, they just need to track an additional 64-bit value in the
associated state, and compare it against the root seed version (generation)
on random reads.
This revision integrates arc4random(9) into that model without substantially
changing the design or implementation of arc4random(9). The motivation is
that arc4random(9) is immediately reseeded when the backing random(4)
implementation has additional entropy. This is arguably most important
during boot, when fenestrasX is reseeding at 1, 3, 9, 27, etc., second
intervals. Today, arc4random(9) has a hardcoded 300 second reseed window.
Without this mechanism, if arc4random(9) gets weak entropy during initial
seed (and arc4random(9) is used early in boot, so this is quite possible),
it may continue to emit poorly seeded output for 5 minutes. The FenestrasX
push-reseed scheme corrects consumers, like arc4random(9), as soon as
possible.
Reviewed by: markm
Approved by: csprng (markm)
Differential Revision: https://reviews.freebsd.org/D22838
|
| #
a3c41f8b |
|
10-Oct-2020 |
Conrad Meyer <cem@FreeBSD.org> |
Add "Fenestras X" alternative /dev/random implementation
Fortuna remains the default; no functional change to GENERIC.
Big picture:
- Scalable entropy generation with per-CPU, buffered local generators.
- "Push" system for reseeding child generators when root PRNG is
reseeded. (Design can be extended to arc4random(9) and userspace
generators.)
- Similar entropy pooling system to Fortuna, but starts with a single
pool to quickly bootstrap as much entropy as possible early on.
- Reseeding from pooled entropy based on time schedule. The time
interval starts small and grows exponentially until reaching a cap.
Again, the goal is to have the RNG state depend on as much entropy as
possible quickly, but still periodically incorporate new entropy for
the same reasons as Fortuna.
Notable design choices in this implementation that differ from those
specified in the whitepaper:
- Blake2B instead of SHA-2 512 for entropy pooling
- Chacha20 instead of AES-CTR DRBG
- Initial seeding. We support more platforms and not all of them use
loader(8). So we have to grab the initial entropy sources in kernel
mode instead, as much as possible. Fortuna didn't have any mechanism
for this aside from the special case of loader-provided previous-boot
entropy, so most of these sources remain TODO after this commit.
Reviewed by: markm
Approved by: csprng (markm)
Differential Revision: https://reviews.freebsd.org/D22837
|