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Subject: Re: KCSAN + KVM = host reset
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <CANpmjNPqQHKUjqAzcFym5G8kHX0mjProOpGu8e4rBmuGRykAUg@mail.gmail.com>
Date:   Fri, 10 Apr 2020 15:57:12 -0400
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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To:     Marco Elver <elver@google.com>
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> On Apr 10, 2020, at 7:35 AM, Marco Elver <elver@google.com> wrote:
>=20
> On Fri, 10 Apr 2020 at 13:25, Qian Cai <cai@lca.pw> wrote:
>>=20
>>=20
>>=20
>>> On Apr 10, 2020, at 5:47 AM, Marco Elver <elver@google.com> wrote:
>>>=20
>>> That would contradict what you said about it working if KCSAN is
>>> "off". What kernel are you attempting to use in the VM?
>=20
> Ah, sorry this was a typo,
>  s/working if KCSAN/not working if KCSAN/
>=20
>> 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.
>>=20
>> github.com/cailca/linux-mm/blob/master/kvm.sh
>=20
> 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 =
misleading
information. I should be checking if svm.o was really recompiled in the =
first place.

Anyway, I=E2=80=99ll send a patch to add __no_kcsan for svm_vcpu_run() =
because 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 =
more ideas?

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];
=20
+       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);
=20
        mark_all_clean(svm->vmcb);
+       kcsan_enable_current();
 }
 STACK_FRAME_NON_STANDARD(svm_vcpu_run);
=20

=20