[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

(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


> There's one adjustment to the logic here that I've been
> considering, but I'm still undecided due to the downsides:
> Without exposing the value, we could make the decision to zap
> the #GP dependent upon us being able to read the MSR.
> The other possible adjustment would be to avoid issuing two
> log messages for the same operation (affecting debug builds
> only). But the code structure (which isn't really consistent
> about when the pre-existing message would get issued)
> doesn't directly lend itself to such an adjustment without
> altering the behavior for some of the MSRs explicitly
> handled.
> As a tangent, while discussing this situation, please let's
> not forget about this code in Linux:
> static u64 xen_read_msr(unsigned int msr)
> {
>       /*
>        * This will silently swallow a #GP from RDMSR.  It may be worth
>        * changing that.
>        */
>       int err;
>       return xen_read_msr_safe(msr, &err);
> }
> static void xen_write_msr(unsigned int msr, unsigned low, unsigned high)
> {
>       /*
>        * This will silently swallow a #GP from WRMSR.  It may be worth
>        * changing that.
>        */
>       xen_write_msr_safe(msr, low, high);
> }
> Imo this "silent swallowing" has always been the wrong thing
> to do, and hence ought to be dropped. Of course right now it
> saves the kernel from dying on the HWCR read.
> Jan



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.