No RDRAND online health tests (NIST SP 800-90B repetition / adaptive-proportion)
Summary
rdrand_rng_harvest feeds RDRAND output directly into CSPRNG pool with no per-source health checks (no repetition-count test no adaptive-proportion test no startup/self-test). NIST SP 800-90B requires these noise-source health tests. Driver relies entirely on downstream L15/IBAA/csprng mixing for safety. rdrand_rng assembly performs no retry budget single CF=0 immediately terminates read so flaky RDRAND silently yields fewer bytes. In virtualized environment hostile hypervisor can intercept RDRAND return biased/attacker-chosen values with no health test driver has no way to detect. Design observation not demonstrable exploit. [VERIFIED pass2: source-confirmed absence of SP 800-90B health tests; RDRAND one mixed source so compliance hardening; recommendation only]
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-2406 · 7 files| File | Type | Description | Size | |
|---|---|---|---|---|
| VERDICT.md | — | 1.6 KB | ↓ raw | |
| README.md | — | 248 B | ↓ raw | |
| build.sh | — | 85 B | view raw | |
| run.sh | — | 49 B | view raw | |
| env.txt | — | 314 B | view raw | |
| manifest.json | — | 598 B | view raw | |
| verdict.json | — | 2.0 KB | view raw |
DF-2406 evidence pack
- finding: sys/dev/crypto/rdrand/rdrand.c (159-188)
- verdict: reproduced (likely) - see VERDICT.md
- source-only confirmation (no runtime PoC)
- reproduce: build.sh / run.sh
- fix.diff: documented recommendation (no diff)
DF-2406 - Verification Verdict
Status: reproduced Impact: none (hardening) Confidence: likely
Mechanism
rdrand_rng_harvest performs no NIST SP 800-90B health tests (repetition count / adaptive proportion) on the harvested RDRAND stream before mixing it into the kernel pool.
Verdict
source-confirmed (no runtime PoC; deterministic code trace of the cited path:line in the read-only sys/ tree). Code-certain absence. Honest assessment: RDRAND is a self-tested on-chip DRBG (validated per SP 800-90A), the harvest is one entropy SOURCE among several mixed by add_buffer_randomness (not a sole-source RBG constructor), so the compliance gap is hardening, not a demonstrated weakness. No code fix authored - a documented recommendation: if rdrand is ever promoted to a sole/primary source, implement startup + continuous health tests per SP 800-90B 4.4 at the harvest site.
Threat / reachability
Latent code defect; see VERDICT.md.
Privilege escalation assessment
n/a (hardening)
Fix
Documented recommendation (no fix.diff): add SP 800-90B health tests if RDRAND becomes a primary entropy source.
Kernel references
Fix status (batch validation)
Applied with the 36 sibling fix.diffs of this batch in findings/poc/_batch41/all_fixes.patch; single in-guest make -j6 nativekernel KERNCONF=X86_64_GENERIC completed RC=0 with zero errors under -Werror (full log: findings/poc/_batch41/fix_build8.log). Behavior A/B intentionally skipped for this Info-tier item.
(No fix.diff for this item: documented recommendation only - see Fix above.)
Fix verification
not_testableNo fix.diff (documented recommendation in VERDICT.md): Documented recommendation (no fix.diff): add SP 800-90B health tests if RDRAND becomes a primary entropy source.
n/a (recommendation item)
Confirmed kernel references
Detail
Exploit chain
no escalation path (Info-tier hardening; see VERDICT.md)
Evidence (decisive lines)
VERDICT.md mechanism/verdict sections; fix.diff
PoC changes
none (source-only confirmation for this Info-tier item)
Verified recommended fix
Documented recommendation (no fix.diff): add SP 800-90B health tests if RDRAND becomes a primary entropy source.
Verdict
source-confirmed (no runtime PoC; deterministic code trace of the cited path:line in the read-only sys/ tree). Code-certain absence. Honest assessment: RDRAND is a self-tested on-chip DRBG (validated per SP 800-90A), the harvest is one entropy SOURCE among several mixed by add_buffer_randomness (not a sole-source RBG constructor), so the compliance gap is hardening, not a demonstrated weakness. No code fix authored - a documented recommendation: if rdrand is ever promoted to a sole/primary source, implement startup + continuous health tests per SP 800-90B 4.4 at the harvest site.
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