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Re: Ping²: [PATCH] x86/PV: conditionally avoid raising #GP for early guest MSR accesses

On 05.02.21 11:14, Jan Beulich wrote:
(simply re-sending what was sent over 2 months ago)

On 04.11.2020 11:50, Jan Beulich wrote:
On 03.11.2020 18:31, Andrew Cooper wrote:
On 03/11/2020 17:06, Jan Beulich wrote:
Prior to 4.15 Linux, when running in PV mode, did not install a #GP
handler early enough to cover for example the rdmsrl_safe() of
MSR_K8_TSEG_ADDR in bsp_init_amd() (not to speak of the unguarded read
of MSR_K7_HWCR later in the same function). The respective change
(42b3a4cb5609 "x86/xen: Support early interrupts in xen pv guests") was
backported to 4.14, but no further - presumably since it wasn't really
easy because of other dependencies.

Therefore, to prevent our change in the handling of guest MSR accesses
to render PV Linux 4.13 and older unusable on at least AMD systems, make
the raising of #GP on these paths conditional upon the guest having
installed a handler. Producing zero for reads and discarding writes
isn't necessarily correct and may trip code trying to detect presence of
MSRs early, but since such detection logic won't work without a #GP
handler anyway, this ought to be a fair workaround.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

I appreciate that we probably have to do something, but I don't think
this is a wise move.

I wouldn't call it wise either, but I'm afraid something along
these lines is necessary.

Linux is fundamentally buggy.  It is deliberately looking for a
potential #GP fault given its use of rdmsrl_safe().  The reason this bug
stayed hidden for so long was as a consequence of Xen's inappropriate
MSR handling for guests, and the reasons for changing Xen's behaviour
still stand.

I agree.

This change, in particular, does not apply to any explicitly handled
MSRs, and therefore is not a comprehensive fix.

But it's intentional that this deals with the situation in a
generic way, not on a per-MSR basis. If we did as you suggest
further down, we'd have to audit all Linux versions up to 4.14
for similar issues with other MSRs. I don't think this would
be a practical thing to do, and I also don't think that leaving
things as they are until we have concrete reports of problems
is a viable option either.

Adding explicit handling for the two offending MSRs (and any
possible further ones we discover) would imo only be to avoid
issuing the respective log messages.

   Nor is it robust to
someone adding code to explicitly handling the impacted MSRs at a later
date (which are are likely to need to do for HWCR), and which would
reintroduce this failure to boot.

I'm afraid I don't understand. Looking at the two functions
the patch alters, only X86EMUL_OKAY is used in return statements
other than the final one. If this model is to be followed by
future additions (which I think it ought to be; perhaps we
should add comments to this effect), the code introduced here
will take care of the situation nevertheless.

We should have the impacted MSRs handled explicitly, with a note stating
this was a bug in Linux 4.14 and older.  We already have workaround for
similar bugs in Windows, and it also gives us a timeline to eventually
removing support for obsolete workarounds, rather than having a "now and
in the future, we'll explicitly tolerate broken PV behaviour for one bug
back in ancient linux".

Comparing with Windows isn't very helpful; the patch here is
specifically about PV, and would help other OSes as well in
case they would have missed setting up exceptions early in
just the PV-on-Xen case. For the HVM case I'd indeed rather
see us go the route we've gone for Windows, if need be.

As can be seen from this reply, we're not in agreement what to
do here. But we need to do something. I'm not sure how to get
unstuck discussions like this one ...

Besides this suggestion of yours I also continue to have
trouble seeing what good it will do to record an exception to
inject into a guest when we know it didn't install a handler

As we need to consider backports of processor bug mitigations
in old guests, too, I think we need to have a "catch-all"

Not being able to run an old updated guest until we add handling
for a new MSR isn't a viable option IMO.


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