| f8e8a06d | 10-Oct-2020 |
Conrad Meyer <[email protected]> |
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
fu
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
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| 10b1a175 | 10-Oct-2020 |
Conrad Meyer <[email protected]> |
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
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
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