linux-kernel - Re: [PATCH] KVM: x86: remove bogus user-triggerable WARN

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Message-ID: <87tv2i7uv1.fsf@nanos.tec.linutronix.de>
Date:   Fri, 20 Mar 2020 19:18:42 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     syzbot+00be5da1d75f1cc95f6b@...kaller.appspotmail.com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH] KVM: x86: remove bogus user-triggerable WARN_ON

Paolo Bonzini <pbonzini@...hat.com> writes:
> On 20/03/20 16:23, Thomas Gleixner wrote:
>> Thomas Gleixner <tglx@...utronix.de> writes:
>> 
>>> Paolo Bonzini <pbonzini@...hat.com> writes:
>>>> The WARN_ON is essentially comparing a user-provided value with 0.  It is
>>>> trivial to trigger it just by passing garbage to KVM_SET_CLOCK.  Guests
>>>> can break if you do so, but if it hurts when you do like this just do not
>>>> do it.
>>>
>>> Yes, it's a user provided value and it's completely unchecked. If that
>>> value is bogus then the guest will go sideways because timekeeping is
>>> completely busted. At least you should explain WHY you don't care.
>> 
>> Or why it does not matter....
>
> I can change the commit message to "Guests can break if you do so, but
> the same applies to every KVM_SET_* ioctl".  It's impossible to be sure
> that userspace doesn't ever send a bogus KVM_SET_CLOCK and later
> rectifies it with the right value.

Yes please.