CVE-2022-49124 — x86/mce: Work around an erratum on fast string copy instructions

Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

x86/mce: Work around an erratum on fast string copy instructions

A rare kernel panic scenario can happen when the following conditions are met due to an erratum on fast string copy instructions:

1) An uncorrected error. 2) That error must be in first cache line of a page. 3) Kernel must execute page_copy from the page immediately before that page.

The fast string copy instructions ("REP; MOVS*") could consume an uncorrectable memory error in the cache line _right after_ the desired region to copy and raise an MCE.

Bit 0 of MSR_IA32_MISC_ENABLE can be cleared to disable fast string copy and will avoid such spurious machine checks. However, that is less preferable due to the permanent performance impact. Considering memory poison is rare, it's desirable to keep fast string copy enabled until an MCE is seen.

Intel has confirmed the following: 1. The CPU erratum of fast string copy only applies to Skylake, Cascade Lake and Cooper Lake generations.

Directly return from the MCE handler: 2. Will result in complete execution of the "REP; MOVS*" with no data loss or corruption. 3. Will not result in another MCE firing on the next poisoned cache line due to "REP; MOVS*". 4. Will resume execution from a correct point in code. 5. Will result in the same instruction that triggered the MCE firing a second MCE immediately for any other software recoverable data fetch errors. 6. Is not safe without disabling the fast…

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CVE-2022-49124 — x86/mce: Work around an erratum on fast string copy instructions