RustCrypto Utilities cmov: `thumbv6m-none-eabi` compiler emits non-constant time assembly when using `cmovnz`
Published Jan 15, 2026 · Updated Nov 3, 2026
Original summary
Normalized public record · Updated Nov 3, 2026
RustCrypto Utilities cmov: `thumbv6m-none-eabi` compiler emits non-constant time assembly when using `cmovnz`
Publisher and source text is retained separately from QFinch interpretation.
QFinch assessment
thumbv6m-none-eabi (Cortex M0, M0+ and M1) compiler emits non-constant time assembly when using cmovnz (portable version). I did not found any other target with the same behaviour but I did not go through all targets supported by Rust.
Potential impact
The exact impact is unclear, especially since cmov clearly warns users that the portable version is best-effort.
Technical context
It seems that, during mask computation, an LLVM optimisation pass is detecting that bitnz is returning 0 or 1, that can be interpreted as a boolean. This intermediate value is not masked by a call to blackbox and thus the subsequent .wrappingsub(1) can be interpreted as a conditional bitwise conditional not.
Evidence: public source recordsAffected products
| Affected package, product or range | Fix status |
|---|---|
| Consult the referenced vendor advisory for affected versions. | Consult source advisory |
Matching a version is only one part of assessing exposure. QFinch has not assessed your environment on this public page.
Remediation
Apply the vendor-supported fixed release or mitigation, then verify the affected component is no longer reachable.
- Confirm the affected product and reachable code path exist in your environment.
- Apply the vendor-supported patch or mitigation from the primary advisory.
- Rescan the exact deployed revision and retain verification evidence.
Evidence coverage
- SSVC priority
- MEDIUM
- Confidence
- Not available
- Aliases
- None reported
Scores appear only when corresponding evidence exists. Missing data is never interpreted as zero risk.
Sources and record history
Record history
Published Jan 15, 2026 · Last refreshed Nov 3, 2026