32-bit PTI For Mitigating Meltdown Is Causing Problems With Early Linux 4.19 Builds

If you tend to routinely ride the latest Linux Git code and are still using 32-bit (x86) hardware, you may want to watch out for problems around the new Kernel Page Table Isolation (K/PTI) functionality.

One of the additions to the Linux 4.19 kernel that ended its merge window last weekend and proceeded to 4.19-rc1 is 32-bit KPTI support for page table isolation similar to x86_64 for mitigating the Meltdown vulnerability on Intel CPUs.

It really hasn't been a priority to get the PTI support in order for i686 hardware given the age of the hardware and most servers or other systems really having much exposure to potential exploit by Meltdown are more modern x86_64 systems (not aware of any corporations still running old Intel 32-bit servers let alone any clouds...). But with Linux 4.19 the page table isolation support is available.

Tests I did last month of the initial 32-bit PTI support on some old Intel laptops found there is indeed a performance hit similar to PTI on x86_64 CPUs.

But if the performance hit isn't bad enough, it looks like the 32-bit PTI support is causing some problems for early Linux 4.19 testers.

Recent reports include EFI boot failures when using PTI with 32-bit builds on Intel CPUs. Also there are reports of user-space corruption caused by the 32-bit PTI support. Those are just the reports I've been seeing on the kernel mailing list over the past day.

Long story short, you've been forewarned, you may also want to consider upgrading your hardware, or if you really do want to help in testing the Linux 4.19 kernel there is at least the nopti kernel parameter for disabling page table isolation at boot-time.

Fortunately, with these problems reported early, the 32-bit PTI support should be cleaned up by the time Linux 4.19.0 ships as stable around the middle of October. 32-bit PTI is on top of many other new features for Linux 4.19.