Re: [FYI PATCH 0/7] Mitigation for CVE-2018-12207

[Pawan Gupta]

On Thu, Nov 14, 2019 at 09:13:22AM +0100, Jan Kiszka wrote:
> On 14.11.19 00:25, Pawan Gupta wrote:
> > On Wed, Nov 13, 2019 at 09:23:30AM +0100, Paolo Bonzini wrote:
> > > On 13/11/19 07:38, Jan Kiszka wrote:
> > > > When reading MCE, error code 0150h, ie. SRAR, I was wondering if that
> > > > couldn't simply be handled by the host. But I suppose the symptom of
> > > > that erratum is not "just" regular recoverable MCE, rather
> > > > sometimes/always an unrecoverable CPU state, despite the error code, right?
> > > 
> > > The erratum documentation talks explicitly about hanging the system, but
> > > it's not clear if it's just a result of the OS mishandling the MCE, or
> > > something worse.  So I don't know. :(  Pawan, do you?
> > 
> > As Dave mentioned in the other email its "something worse".
> > 
> > Although this erratum results in a machine check with the same MCACOD
> > signature as an SRAR error (0x150) the MCi_STATUS.PCC bit will be set to
> > one. The Intel Software Developers manual says that PCC=1 errors are
> > fatal and cannot be recovered.
> > 
> > 	15.10.4.1 Machine-Check Exception Handler for Error Recovery [1]
> > 
> > 	[...]
> > 	The PCC flag in each IA32_MCi_STATUS register indicates whether recovery
> > 	from the error is possible for uncorrected errors (UC=1). If the PCC
> > 	flag is set for enabled uncorrected errors (UC=1 and EN=1), recovery is
> > 	not possible.
> > 
> 
> And, as Dave observed, even that event is not delivered to software (maybe
> just logged by firmware for post-reset analysis) but can or does cause a
> machine lock-up, right?

It can either cause a machine lock-up or a reset and the event delivery
to the software is not guaranteed.

Thanks,
Pawan