News Tech: A Linux kernel “fix” that allowed STIBP while using the IBPB mode for Retbleed mitigations on AMD processors was released this morning. In other words, this improved manner of Retbleed mitigation requires more safeguards.
Retbleed, a novel speculative execution attack that takes use of return instructions, was made public last month. Retbleed impacts AMD Zen 1, Zen 1+, and Zen 2 processors as well as Intel Core 8th Gen and earlier CPUs. Retbleed Linux kernel mitigations, which were added last month, hurt AMD’s performance as well as Intel’s, especially on older Zen 1 CPUs.
The IBPB-based Retbleed protection is not the default because it is thought to be the most secure but also has the greatest performance impact. With the latest Linux kernel “patch” released today, if choosing IBPB-based protection, Simultaneous Multi-Threading (SMT) will be disabled if STIBP support isn’t present on the target system.
This weekend, Linux 6.0-rc1 mainline should adopt the security fix, and during the course of the next week, it will probably start to show up in backported stable kernel releases.
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