Re: [Xen-devel] xc_evtchn_status fails with EFAULT on HVM, the same on PV works

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/06/2017 09:23, Marek Marczykowski-Górecki wrote:
> [resurrecting old thread...]
>
> On Mon, Jan 16, 2017 at 11:41:55PM +0000, Andrew Cooper wrote:
>> On 16/01/2017 23:06, Marek Marczykowski-Górecki wrote:
>>> On Mon, Jan 16, 2017 at 05:17:59AM -0700, Jan Beulich wrote:
>>>> 2) When the guest issues stac()/clac(), it indicates to Xen _its own_
>>>> intended view, without affecting Xen's. That is, as soon as hypervisor
>>>> context is being entered again, SMAP protection would be in effect
>>>> again (albeit as per point 1 guarding only against accessing PV guest
>>>> mappings).
>>>>
>>>> So the driver adjustment suggested by Andrew has an effect on only
>>>> page walks done by Xen during copy_{to,from}_guest(), but not on
>>>> actual memory accesses.
>>> Ok, so indeed the kernel patch makes the most sense here. Is the change
>>> in this shape (if works - I'll test it shortly) good to include
>>> upstream, or is it "ugly hack"?
>> If it works (which I suspect it will), then it will be the correct
>> proper upstream fix, and will of course CC stable@.
> Should I submit it?

Yes please.

>
>> In the meantime until it percolates into downstream kernels, disabling
>> SMAP for affected guests is probably the best stopgap solution.
> How to disable SMAP for selected guests only?

The toolstack definitely has that kind of control, but I don't know how
well it works in practice in libxl.  You want to look into the CPUID=
configuration option.

~Andrew

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