From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
Marc Zyngier <maz@kernel.org>, James Morse <james.morse@arm.com>,
Julien Thierry <julien.thierry.kdev@gmail.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
linux-arm-kernel@lists.infradead.org,
Huacai Chen <chenhc@lemote.com>,
Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
linux-mips@vger.kernel.org, Paul Mackerras <paulus@ozlabs.org>,
kvm-ppc@vger.kernel.org,
Christian Borntraeger <borntraeger@de.ibm.com>,
Janosch Frank <frankja@linux.ibm.com>,
David Hildenbrand <david@redhat.com>,
Cornelia Huck <cohuck@redhat.com>,
Claudio Imbrenda <imbrenda@linux.ibm.com>
Subject: Re: [RFC PATCH 3/3] KVM: x86: Use KVM_BUG/KVM_BUG_ON to handle bugs that are fatal to the VM
Date: Fri, 25 Sep 2020 10:12:33 -0700 [thread overview]
Message-ID: <20200925171233.GC31528@linux.intel.com> (raw)
In-Reply-To: <87k0wichht.fsf@vitty.brq.redhat.com>
On Fri, Sep 25, 2020 at 11:50:38AM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@intel.com> writes:
>
> > On Thu, Sep 24, 2020 at 02:34:14PM +0200, Vitaly Kuznetsov wrote:
> >> Sean Christopherson <sean.j.christopherson@intel.com> writes:
> >> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> >> > index 6f9a0c6d5dc5..810d46ab0a47 100644
> >> > --- a/arch/x86/kvm/vmx/vmx.c
> >> > +++ b/arch/x86/kvm/vmx/vmx.c
> >> > @@ -4985,14 +4986,13 @@ static int handle_cr(struct kvm_vcpu *vcpu)
> >> > }
> >> > break;
> >> > case 2: /* clts */
> >> > - WARN_ONCE(1, "Guest should always own CR0.TS");
> >> > - vmx_set_cr0(vcpu, kvm_read_cr0_bits(vcpu, ~X86_CR0_TS));
> >> > - trace_kvm_cr_write(0, kvm_read_cr0(vcpu));
> >> > - return kvm_skip_emulated_instruction(vcpu);
> >> > + KVM_BUG(1, vcpu->kvm, "Guest always owns CR0.TS");
> >> > + return -EIO;
> >> > case 1: /*mov from cr*/
> >> > switch (cr) {
> >> > case 3:
> >> > WARN_ON_ONCE(enable_unrestricted_guest);
> >> > +
> >>
> >> Here, were you intended to replace WARN_ON_ONCE() with KVM_BUG_ON() or
> >> this is just a stray newline added?
> >
> > I think it's just a stray newline. At one point I had converted this to a
> > KVM_BUG_ON(), but then reversed direction because it's not fatal to the guest,
> > i.e. KVM should continue to function even though it's spuriously intercepting
> > CR3 loads.
> >
> > Which, rereading this patch, completely contradicts the KVM_BUG() for CLTS.
> >
> > That's probably something we should sort out in this RFC: is KVM_BUG() only
> > to be used if the bug is fatal/dangerous, or should it be used any time the
> > error is definitely a KVM (or hardware) bug.
>
> Personally, I'm feeling adventurous so my vote goes to the later :-)
> Whenever a KVM bug was discovered by a VM it's much safer to stop
> executing it as who knows what the implications might be?
Not necessarily, e.g. terminating the VM may corrupt the VM's file system,
which is less safe, for lack of a better word, from the VM's perspective.
> In this particular case I can think of a nested scenario when L1 didn't
> ask for CR3 intercept but L0 is still injecting it. It is not fatal by
> itself but probably there is bug in calculating intercepts in L0 so
> if we're getting something extra maybe we're also missing some? And this
> doesn't sound good at all.
Hmm, but by that argument this scenario would fall into the "dangerous" part
of "bug is fatal/dangerous". I guess my opinion is that we should set a
fairly high bar for using KVM_BUG() so that KVM can be aggressive in shutting
down.
> > In theory, it should be impossible to reach this again as "r = -EIO" will
> > bounce this out to userspace, the common checks to deny all ioctls() will
> > prevent reinvoking KVM_RUN.
>
> Do we actually want to prevent *all* ioctls? E.g. when 'vm bugged'
> condition is triggered userspace may want to extract some information to
> assist debugging but even things like KVM_GET_[S]REGS will just return
> -EIO. I'm not sure it is generally safe to enable *everything* (except
> for KVM_RUN which should definitely be forbidden) so maybe your approach
> is preferable.
The answer to this probably depends on the answer to the first question of
when it's appropriate to use KVM_BUG(). E.g. if we limit usage to fatal or
dangrous cases, then blocking all ioctls() is probably the right thing do do.
next prev parent reply other threads:[~2020-09-25 17:25 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-23 22:45 [RFC PATCH 0/3] KVM: Introduce "VM bugged" concept Sean Christopherson
2020-09-23 22:45 ` [RFC PATCH 1/3] KVM: Export kvm_make_all_cpus_request() for use in marking VMs as bugged Sean Christopherson
2020-09-23 22:45 ` [RFC PATCH 2/3] KVM: Add infrastructure and macro to mark VM " Sean Christopherson
2020-09-23 22:45 ` [RFC PATCH 3/3] KVM: x86: Use KVM_BUG/KVM_BUG_ON to handle bugs that are fatal to the VM Sean Christopherson
2020-09-24 12:34 ` Vitaly Kuznetsov
[not found] ` <20200924181134.GB9649@linux.intel.com>
2020-09-25 9:50 ` Vitaly Kuznetsov
2020-09-25 17:12 ` Sean Christopherson [this message]
2020-09-25 21:06 ` Paolo Bonzini
2020-09-29 3:52 ` Sean Christopherson
2020-09-29 9:15 ` Paolo Bonzini
2020-09-24 6:37 ` [RFC PATCH 0/3] KVM: Introduce "VM bugged" concept Christian Borntraeger
2020-09-25 16:32 ` Marc Zyngier
2020-09-25 17:00 ` Sean Christopherson
2020-09-25 21:05 ` Paolo Bonzini
2020-09-29 9:27 ` Cornelia Huck
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