Re: Sporadic PV guest malloc.c assertion failures and segfaults unless pv-l1tf=false is set

[Andy Smith <andy@strugglers.net>]

Hello,

On Sun, Nov 25, 2018 at 06:18:49AM +0000, Andy Smith wrote:
> In the text for XSA-273 it says:
> 
>     "Shadowing comes with a workload-dependent performance hit to
>     the guest.  Once the guest kernel software updates have been
>     applied, a well behaved guest will not write vulnerable PTEs,
>     and will therefore avoid the performance penalty (or crash)
>     entirely."
> 
> Does anyone have a reference to what is needed in the Linux kernel
> for that?

Perhaps stupidly, I have only just now thought to check whether the
one guest I have an easy reproducer on (predictable failure of
php-fpm) was actually running an up to date kernel. It was not.

It is Debian stretch and was running kernel package
linux-image-4.9.0-7-amd64 version 4.9.110-3+deb9u2. The guest's
administrator obviously had not done any upgrades since install time
because updated kernel linux-image-4.9.0-8-amd64 version 4.9.130-2
was available.

After installing and booting with that, the guest no longer causes
"L1TF-vulnerable L1e 000000006a6ff960 - Shadowing" to be emitted in
the hypervisor dmesg, and the problems I described disappear.

I assume this is because of:

https://metadata.ftp-master.debian.org/changelogs//main/l/linux/linux_4.9.130-2_changelog

    linux (4.9.110-3+deb9u3) stretch-security; urgency=high

      [ Salvatore Bonaccorso ]
        * Add L1 Terminal Fault fixes (CVE-2018-3620, CVE-2018-3646)

So, I can tell affected guest administrators to upgrade their
kernels to include Linux's L1TF protections and their problems
should go away.

Do you care that without those fixes there appear to be memory
corruption issues? If so, I can keep this reproducer guest around
and debug it further.

Cheers,
Andy

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