Re: [PATCH v2 1/7] sysfs/cpu: Add "Unknown" vulnerability state

[Dave Martin]

On Thu, Jan 03, 2019 at 05:48:31PM +0100, Greg Kroah-Hartman wrote:
> On Thu, Jan 03, 2019 at 10:38:16AM -0600, Jeremy Linton wrote:
> > On 01/03/2019 03:38 AM, Greg Kroah-Hartman wrote:
> > > On Wed, Jan 02, 2019 at 06:49:15PM -0600, Jeremy Linton wrote:
> > > > There is a lot of variation in the Arm ecosystem. Because of this,
> > > > there exist possible cases where the kernel cannot authoritatively
> > > > determine if a machine is vulnerable.
> > > 
> > > Really?  Why not?  What keeps you from "knowing" this?  Can't the
> > > developer of the chip tell you?
> > 
> > There tends to be a few cases, possibly incomplete white/black lists,
> 
> Then fix the lists :)
> 
> > firmware that isn't responding correctly, or the user didn't build in the
> > code to check the mitigation (possibly because its an embedded system and
> > they know its not vulnerable?).
> 
> If the firmware doesn't respond, that would imply it is vulnerable :)
> 
> And if the code isn't built in, again, it's vulnerable.
> 
> > I would hope that it is an exceptional case.
> 
> Then have the default be vulnerable, don't give people false hope.
> 
> > > > Rather than guess the vulnerability status in cases where
> > > > the mitigation is disabled or the firmware isn't responding
> > > > correctly, we need to display an "Unknown" state.
> > > 
> > > Shouldn't "Unknown" really be the same thing as "Vulnerable"?  A user
> > > should treat it the same way, "Unknown" makes it feel like "maybe I can
> > > just ignore this and hope I really am safe", which is not a good idea at
> > > all.
> > 
> > I tend to agree its not clear what to do with "unknown".
> > 
> > OTOH, I think there is a hesitation to declare something vulnerable when it
> > isn't. Meltdown for example, is fairly rare given that it currently only
> > affects a few arm parts, so declaring someone vulnerable when they likely
> > aren't is going to be just as difficult to explain.
> 
> If you know it is rare, then you know how to properly detect it so
> "unknown" is not needed, correct?
> 
> Again, "unknown" is not going to help anyone out here, please don't do
> it.

Thinking about it, "unknown" is actually the common case.

Kernels that predate the sysfs vulnerabilities interface effectively
report this for all vulnerabilities by omitting the sysfs entries
entirely.

Current kernels also don't know anything about future vulnerabilities
that may be added in sysfs later on (but which may nevertheless be
discovered subsequently to affect current hardware).

So, can we simply omit the sysfs entries for which we can't provide a
good answer?

IMHO the kernel should make best efforts to provide answers for every
vulnerability that it knows about, so the checks should not be Kconfig-
dependent without a good reason.

There will be cases where whitelists/blacklists are the only source of
answers, and we are ultimately reliant on vendors to provide that
information.  Upstream Linux is likely to lag, there's not much we can
do about that.

Cheers
---Dave