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

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Message-ID: <ef15a35b-3e69-7922-ceda-4a4867054a44@redhat.com>
Date:   Fri, 20 Mar 2020 18:47:43 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>, 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

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.

Paolo