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

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Message-ID: <CANDhNCoFQXAGT9D=oRefjmAEU=LFVa4Y8-0+peJwiSMf1DuyeA@mail.gmail.com>
Date: Wed, 22 Jan 2025 10:24:33 -0800
From: John Stultz <jstultz@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.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>, 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 8:56 AM Jim Mattson <jmattson@...gle.com> wrote:
>
> 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.
> :)

Thanks Jim,

I've just reproduced this as well, after running the debug test in a
loop.  The one odd bit is that it's not always the same subtest that
fails.

I've seen:
FAIL: Single-step #DB w/ STI blocking
FAIL: Usermode Single-step #DB w/ STI blocking
FAIL: Single-step #DB on emulated instructions
FAIL: Single-step #DB w/ MOVSS blocking
FAIL: Usermode Single-step #DB basic test
FAIL: Single-step #DB w/ MOVSS blocking and DR7.GD=1
FAIL: hw breakpoint (test that dr6.BS is not cleared)

So yeah, hopefully all the same bug? :)

thanks
-john