OpenBSD Journal

Meltdown fix committed by guenther@

Contributed by Paul 'WEiRD' de Weerd on from the so-hot-of-the-press-it-melts-your-cpu dept.

Meltdown mitigation is coming to OpenBSD. Philip Guenther (guenther@) has just committed a diff that implements a new mitigation technique to OpenBSD: Separation of page tables for kernel and userland. This fixes the Meltdown problems that affect most CPUs from Intel. Both Philip and Mike Larkin (mlarkin@) spent a lot of time implementing this solution, talking to various people from other projects on best approaches.

In the commit message, Philip briefly describes the implementation:

Meltdown: implement user/kernel page table separation.

On Intel CPUs which speculate past user/supervisor page permission checks,
use a separate page table for userspace with only the minimum of kernel code
and data required for the transitions to/from the kernel (still marked as
supervisor-only, of course):
- the IDT (RO)
- three pages of kernel text in the .kutext section for interrupt, trap,
and syscall trampoline code (RX)
- one page of kernel data in the .kudata section for TLB flush IPIs (RW)
- the lapic page (RW, uncachable)
- per CPU: one page for the TSS+GDT (RO) and one page for trampoline
stacks (RW)

When a syscall, trap, or interrupt takes a CPU from userspace to kernel the
trampoline code switches page tables, switches stacks to the thread's real
kernel stack, then copies over the necessary bits from the trampoline stack.
On return to userspace the opposite occurs: recreate the iretq frame on the
trampoline stack, switch stack, switch page tables, and return to userspace.

mlarkin@ implemented the pmap bits and did 90% of the debugging, diagnosing
issues on MP in particular, and drove the final push to completion.
Many rounds of testing by naddy@, sthen@, and others
Thanks to Alex Wilson from Joyent for early discussions about trampolines
and their data requirements.
Per-CPU page layout mostly inspired by DragonFlyBSD.

Even with extensive testing by developers, editors@ are sure more testing would be welcome. So grab a snapshot (if they're from February 22nd or later, they should contain the diff) and see how this behaves in your environment!

(Comments are closed)


  1. By ilyes aiouaz (ilyes_aiouaz) i.aiouaz@ulysse-lab.com on https://www.ulysse-lab.com

    Good news,
    Thanks Guenther.

    1. By grab mov (grabmov) moviejl21@gmail.com on http://sakubola.com

      wow, cool bro congratulations,

  2. By Renaud Allard (renaud) renaud@allard.it on

    Congratulations on the good job, that was fast.

  3. By Noryungi (noryungi) noryungi@yahoo.com on

    That is excellent news! Thanks to everyone involved!

  4. By brynet (Brynet) on https://brynet.biz.tm/

    Congrats! Many thanks to mlarkin@ & guenther@ for their tireless efforts to get this work in.

  5. By Troy (rits) register@redinsight.com on

    Thank you!

  6. By Peter J. Philipp (pjp) nospam@solarscale.de on http://centroid.eu

    Thank you! This is great. I installed the snapshot from Feb 22nd. So far there seems no panics, no inhibitions, and it's fast AND zzz worked too over night. My workstation is a Xeon E3-1275v3 where I tested this on. Thanks again!

    1. By Amit Kulkarni (amitkulz) on

      There's no snapshot for Feb 22 released yet! You probably installed from Feb 21.

  7. By Amit Kulkarni (amitkulz) on

    Wow, superb work as always!

  8. By Just another grumpy fart (grumpy_fart) grumpy_fart@gmail.com on http://www.openbsd.org/

    Isn't it amazing how a couple of volunteers, in their spare time, manage to roll out a working solution to this dumpster truck fire for our favourite OS, where several vendors were fumbling around for months.

    Responsible disclosure my ass. Highly irresponsible, if you ask me. No wonder people are taking Intel to court over this shit storm.

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