linux-kernel - Re: [RFC PATCH 3/3] KVM: x86: Use KVM_BUG/KVM_BUG_ON to handle bugs that are fatal to the VM

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Date:   Mon, 28 Sep 2020 20:52:57 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org,
        Huacai Chen <chenhc@...ote.com>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        linux-mips@...r.kernel.org, Paul Mackerras <paulus@...abs.org>,
        kvm-ppc@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.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

On Fri, Sep 25, 2020 at 11:06:10PM +0200, Paolo Bonzini wrote:
> On 25/09/20 19:12, Sean Christopherson wrote:
> >> 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.
> 
> I think usage should be limited to dangerous cases, basically WARN_ON
> level.  However I agree with Vitaly that KVM_GET_* should be allowed.

Makes sense.

On the topic of feedback from Vitaly, while dredging through my mailbox I
rediscovered his suggestion of kvm->kvm_internal_bug (or maybe just
kvm->internal_bug) instead of kvm->vm_bugged[*].  Like past me, I like the
"internal" variants better.

[*] https://lkml.kernel.org/r/20190930153358.GD14693@linux.intel.com

> The other question is whether to return -EIO or KVM_EXIT_INTERNAL_ERROR.
>  The latter is more likely to be handled already by userspace.

And probably less confusing for unsuspecting users.  E.g. -EIO is most
likely to be interpreted as "I screwed up", whereas KVM_EXIT_INTERNAL_ERROR
will correctly be read as "KVM screwed up".