Re: [PATCH] KVM: x86: remove bogus user-triggerable WARN_ON

[Paolo Bonzini]

On 20/03/20 16:23, Thomas Gleixner wrote:
> Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
> 
>> Paolo Bonzini <pbonzini@xxxxxxxxxx> 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