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I just learned about Meltdown and Spectre bugs. I read that:
There are patches against Meltdown for Linux (KPTI (formerly KAISER)), Windows, and OS X.
Following the link in the quote I get to an article which is too obscure for me to understand. Still, it says:
The resulting patch set (still called "KAISER") is in its third revision and seems likely to find its way upstream in a relatively short period of time.
Following again the link in the above quote I get into a page, updated the 10th of Novermber of 2017, where I read the following:
KAISER makes it harder to defeat KASLR, but makes syscalls and interrupts slower. These patches are based on work from a team at Graz University of Technology posted here. The major addition is support for Intel PCIDs which builds on top of Andy Lutomorski's PCID work merged for 4.14. PCIDs make KAISER's overhead very reasonable for a wide variety of use cases.
The above page also links to the code of the fix (?), here, where I can also see kernel 4.14.
From this I conclude that the fix is available only for kernels 4.14 (and above?). However, all currently supported versions of Ubuntu use a lower kernel.
The latest Ubuntu (17.10) uses kernel 4.13. The latest LTS Ubuntu (16.04) uses 4.4.
Does this mean that the fix for such bug is not available for Ubuntu? It seems that Ubuntu 18.04 will be based on kernel 4.15, but this is still not released.
Notice also that the fix seems to refer only to Meltdown and not to Spectre. This would mean that there is currently no fix for such bug anywhere.