linux-kernel - Re: BUG: Occasional unexpected DR6 value seen with nested virtualization on x86

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Message-ID: <CALMp9eTcsDcnm2cqWW-tjLJEXb5PeoLUr-Pk=0oOH=_3oowREg@mail.gmail.com>
Date: Wed, 22 Jan 2025 08:55:55 -0800
From: Jim Mattson <jmattson@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: John Stultz <jstultz@...gle.com>, Sean Christopherson <seanjc@...gle.com>, kvm <kvm@...r.kernel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Frederic Weisbecker <fweisbec@...il.com>, 
	Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>, Alex Bennée <alex.bennee@...aro.org>, 
	Will Deacon <will@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, LKML <linux-kernel@...r.kernel.org>, 
	"Team, Android" <kernel-team@...roid.com>
Subject: Re: BUG: Occasional unexpected DR6 value seen with nested
 virtualization on x86

On Wed, Jan 22, 2025 at 1:01 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> Il mer 22 gen 2025, 07:07 John Stultz <jstultz@...gle.com> ha scritto:
> >
> > I then cut down and ported the bionic test out so it could build under
> > a standard debian environment:
> > https://github.com/johnstultz-work/bionic-ptrace-reproducer
> >
> > Where I was able to reproduce the same problem in a debian VM (after
> > running the test in a loop for a short while).
>
>
> Thanks, that's nice to have.
>
> > Now, here's where it is odd. I could *not* reproduce the problem on
> > bare metal hardware, *nor* could I reproduce the problem in a virtual
> > environment.  I can *only* reproduce the problem with nested
> > virtualization (running the VM inside a VM).
>
> Typically in that case the best thing to do is turn it into a
> kvm-unit-test or selftest (though that's often an endeavor of its own,
> as it requires distilling the Linux kernel and userspace code into a
> guest that runs without an OS). But what you've done is already a good
> step.

Just run the kvm-unit-tests 'x86/debug' test in a loop inside an L1
VM. It will eventually fail. Maybe not the same bug, but we can hope.
:)

> > I have reproduced this on my intel i12 NUC using the same v6.12 kernel
> > on metal + virt + nested environments.  It also reproduced on the NUC
> > with v5.15 (metal) + v6.1 (virt) + v6.1(nested).
>
> Good that you can use a new kernel. Older kernels are less reliable
> with nested virt (especially since the one that matters the most is
> the metal one).
>
> Paolo
>
> > I've tried to do some tracing in the arch/x86/kvm/x86.c logic, but
> > I've not yet directly correlated anything on the hosts to the point
> > where we read the zero DR6 value in the nested guest.
> >
> > But I'm not very savvy around virtualization or ptrace watchpoints or
> > low level details around intel DB6 register, so I wanted to bring this
> > up on the list to see if folks had suggestions or ideas to further
> > narrow this down?  Happy to test things as it's pretty simple to
> > reproduce here.
> >
> > Many thanks to Alex Bennee and Jim Mattson for their testing
> > suggestions to help narrow this down so far.
> >
> > thanks
> > -john
> >
>