Scheduler-Contention Side Channel on AMD “Zen” Processors

Summary

A researcher reported a potential side-channel condition on certain AMD Zen-family processors involving REP-prefixed string instructions. Under certain conditions, differences in loop count may cause variations in internal resource usage, leading to measurable timing differences. In transient execution scenarios, this behavior could serve as a side channel when combined with code patterns that involve secret-dependent loop counts

Based on AMD’s assessment, this behavior reflects a variant of known side-channel and speculative-execution techniques rather than a new vulnerability. Existing mitigation recommendations remain applicable. Please see the Mitigation section below.

Mitigation

AMD recommends that software developers employ existing best practices, including constant-time algorithms and, where appropriate, avoiding secret-dependent control flows, to help mitigate this potential vulnerability.

Please refer to the white papers in the References section for additional guidance.

Affected Products

All “Zen” Processors

Acknowledgement 

AMD thanks Trevor E. Carlson of the National University of Singapore (NUS), Xiaoyu Cheng, and Fei Tong of Southeast University (SEU) for submitting the report and engaging in coordinated vulnerability disclosure. 

References

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