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In general users, don't have the necessary information to determine whether late loading of a new microcode version is safe and does not modify anything which the currently running kernel uses already, e.g. removal of CPUID bits or behavioural changes of MSRs. To address this issue, Intel has added a "minimum required version" field to a previously reserved field in the microcode header. Microcode updates should only be applied if the current microcode version is equal to, or greater than this minimum required version. Thomas made some suggestions on how meta-data in the microcode file could provide Linux with information to decide if the new microcode is suitable candidate for late loading. But even the "simpler" option requires a lot of metadata and corresponding kernel code to parse it, so the final suggestion was to add the 'minimum required version' field in the header. When microcode changes visible features, microcode will set the minimum required version to its own revision which prevents late loading. Old microcode blobs have the minimum revision field always set to 0, which indicates that there is no information and the kernel considers it unsafe. This is a pure OS software mechanism. The hardware/firmware ignores this header field. For early loading there is no restriction because OS visible features are enumerated after the early load and therefore a change has no effect. The check is always enabled, but by default not enforced. It can be enforced via Kconfig or kernel command line. If enforced, the kernel refuses to late load microcode with a minimum required version field which is zero or when the currently loaded microcode revision is smaller than the minimum required revision. If not enforced the load happens independent of the revision check to stay compatible with the existing behaviour, but it influences the decision whether the kernel is tainted or not. If the check signals that the late load is safe, then the kernel is not tainted. Early loading is not affected by this. [ tglx: Massaged changelog and fixed up the implementation ] Suggested-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Ashok Raj <ashok.raj@intel.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de> Link: https://lore.kernel.org/r/20231002115903.776467264@linutronix.de |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.