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From:   Marco Elver <elver@google.com>
Date:   Mon, 13 Apr 2020 10:35:38 +0200
Message-ID: <CANpmjNPSLkiEer3xQHHxJm_4o5Em0i3bvM7TMmNO46Vzv2cwWQ@mail.gmail.com>
Subject: Re: KCSAN + KVM = host reset
To:     Qian Cai <cai@lca.pw>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        "paul E. McKenney" <paulmck@kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org
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On Fri, 10 Apr 2020 at 21:57, Qian Cai <cai@lca.pw> wrote:
>
>
>
> > On Apr 10, 2020, at 7:35 AM, Marco Elver <elver@google.com> wrote:
> >
> > On Fri, 10 Apr 2020 at 13:25, Qian Cai <cai@lca.pw> wrote:
> >>
> >>
> >>
> >>> On Apr 10, 2020, at 5:47 AM, Marco Elver <elver@google.com> wrote:
> >>>
> >>> That would contradict what you said about it working if KCSAN is
> >>> "off". What kernel are you attempting to use in the VM?
> >
> > Ah, sorry this was a typo,
> >  s/working if KCSAN/not working if KCSAN/
> >
> >> Well, I said set KCSAN debugfs to =E2=80=9Coff=E2=80=9D did not help, =
i.e., it will reset the host running kvm.sh. It is the vanilla ubuntu 18.04=
 kernel in VM.
> >>
> >> github.com/cailca/linux-mm/blob/master/kvm.sh
> >
> > So, if you say that CONFIG_KCSAN_INTERRUPT_WATCHER=3Dn works, that
> > contradicts it not working when KCSAN is "off". Because if KCSAN is
> > off, it never sets up any watchpoints, and whether or not
> > KCSAN_INTERRUPT_WATCHER is selected or not shouldn't matter. Does that
> > make more sense?
>
> Yes, you are right. CONFIG_KCSAN_INTERRUPT_WATCHER=3Dn does not
> make it work. It was a mistake when I tested it because there was a stale=
 svm.o
> leftover from the previous run, and then it will not trigger a rebuild (a=
 bug?) when
> only modify the Makefile to remove KCSAN_SANITIZE :=3D n. Sorry for the m=
isleading
> information. I should be checking if svm.o was really recompiled in the f=
irst place.
>
> Anyway, I=E2=80=99ll send a patch to add __no_kcsan for svm_vcpu_run() be=
cause I tried
> to narrow down more with a kcsan_[disable|enable]_current() pair, but it =
does NOT
> work even by enclosing the almost whole function below until Marcro has m=
ore ideas?

This is expected. Instrumentation is not removed if you add
kcsan_{disable,enable}_current() (it has the same effect as a
localized "off"). Since it seems just the instrumentation and
associated calls before every memory access is enough, this won't
work. The attribute __no_kcsan removes instrumentation entirely from
the function. If the non-instrumented code should be reduced, it is
conceivable to take the problematic portion of code and factor it into
a function that has attribute '__no_kcsan_or_inline'.

Thanks,
-- Marco

> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 2be5bbae3a40..e58b2d5a575c 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3286,6 +3286,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>         svm->vmcb->save.rsp =3D vcpu->arch.regs[VCPU_REGS_RSP];
>         svm->vmcb->save.rip =3D vcpu->arch.regs[VCPU_REGS_RIP];
>
> +       kcsan_disable_current();
>         /*
>          * A vmexit emulation is required before the vcpu can be executed
>          * again.
> @@ -3410,6 +3411,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
>                 svm_handle_mce(svm);
>
>         mark_all_clean(svm->vmcb);
> +       kcsan_enable_current();
>  }
>  STACK_FRAME_NON_STANDARD(svm_vcpu_run);
>
>
>
>
>
>