linux-kernel - Re: WARNING in vcpu_enter

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Message-ID: <cd32ee6d-f30d-b221-8126-cf995ffca52e@redhat.com>
Date:   Thu, 19 Mar 2020 18:44:26 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     syzbot <syzbot+00be5da1d75f1cc95f6b@...kaller.appspotmail.com>,
        bp@...en8.de, hpa@...or.com, jmattson@...gle.com, joro@...tes.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, rkrcmar@...hat.com,
        syzkaller-bugs@...glegroups.com, tglx@...utronix.de,
        vkuznets@...hat.com, wanpengli@...cent.com, x86@...nel.org
Subject: Re: WARNING in vcpu_enter_guest

On 19/03/20 18:39, Sean Christopherson wrote:
>> Yep.  I worked through logic/math, mostly to gain a wee bit of knowledge
>> about the clock stuff, and it's sound.  The KVM_SET_CLOCK from syzkaller
>> is simply making time go backwards.
> Actually, would it make sense to return -EINVAL for KVM_SET_CLOCK if the
> user tries to make kvmclock_offset go backwards?

No, it is possible to do that depending on the clock setup on the live
migration source.  You could cause the warning anyway by setting the
clock to a very high (signed) value so that kernel_ns + kvmclock_offset
overflows.

Paolo