Re: [PATCH v2 3/6] kvm: x86: Emulate split-lock access as a write

[Xiaoyao Li]

On 2/11/2020 9:34 PM, Paolo Bonzini wrote:
> On 11/02/20 14:22, Thomas Gleixner wrote:
> > Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:
> > > On 03/02/20 16:16, Xiaoyao Li wrote:
> > > > A sane guest should never tigger emulation on a split-lock access, but
> > > > it cannot prevent malicous guest from doing this. So just emulating the
> > > > access as a write if it's a split-lock access to avoid malicous guest
> > > > polluting the kernel log.
> > >
> > > Saying that anything doing a split lock access is malicious makes little
> > > sense.
> >
> > Correct, but we also have to accept, that split lock access can be used
> > in a malicious way, aka. DoS.
>
> Indeed, a more accurate emulation such as temporarily disabling
> split-lock detection in the emulator would allow the guest to use split
> lock access as a vehicle for DoS, but that's not what the commit message
> says.  If it were only about polluting the kernel log, there's
> printk_ratelimited for that.  (In fact, if we went for incorrect
> emulation as in this patch, a rate-limited pr_warn would be a good idea).
>
> It is much more convincing to say that since this is pretty much a
> theoretical case, we can assume that it is only done with the purpose of
> DoS-ing the host or something like that, and therefore we kill the guest.

So you think there is no need to emulate this feature and return #AC to 
guest?
Anyway, I'm fine with killing the guest.

BTW, Can it really be used for DoS purpose by malicious guest? Since 
it's in kvm emulator so it needs vm-exit first and won't the die() in 
kernel handler kill KVM? (Actually I'm not clear about KVM after die())

> > > Split lock detection is essentially a debugging feature, there's a
> > > reason why the MSR is called "TEST_CTL".  So you don't want to make the
> >
> > The fact that it ended up in MSR_TEST_CTL does not say anything. That's
> > where they it ended up to be as it was hastily cobbled together for
> > whatever reason.
>
> Or perhaps it was there all the time in test silicon or something like
> that...  That would be a very plausible reason for all the quirks behind it.

Alright, I don't know the history of TEST_CTRL, there is a bit 31 in it 
which means "Disable LOCK# assertion for split locked access" when set. 
Bit 31 exists for a long period, but linux seems not use it so I guess 
it may be a testing purpose bit.

However, when it comes to bit 29, split lock #AC, the main purpose is to 
prevent any split lock more than debugging.

BTW, I guess the reason putting it in MSR_TEST_CTRL is that it's related 
with split lock as bit 31.

> Paolo