Re: [Xen-devel] [PATCH v14 07/11] x86/mm: add an extra command to HYPERVISOR_mmu_update...

["Jan Beulich" <JBeulich@xxxxxxxx>]

 >>> On 13.12.17 at 13:06, <Paul.Durrant@xxxxxxxxxx> wrote:
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 12 December 2017 14:39
>> >>> On 12.12.17 at 14:52, <andrew.cooper3@xxxxxxxxxx> wrote:
>> > We are deliberately trying to introducing a mechanism whereby a
>> > toolstack/device-mode/other semi-privileged entity can map resources
>> > belonging to a guest which are not part of the guests physmap.  This is
>> > because we deliberately want to move things like emulator rings out of
>> > the guest physmap for attack surface reduction purposes.
>> 
>> Correct. What I was trying to point out with my reply is that the
>> bypass here removes a check which previously we've been
>> relying on: By finding the page in the guest's physmap, we can
>> at least be certain that access to the page from outside of Xen is
>> expected. With it removed, the only other check is the
>> ownership one; the bypass in get_page_from_l1e() then blindly
>> allows writable mappings to pages owned by the guest, even if
>> they were shared r/o.
>> 
>> So while the relaxation here is deliberate _for the purposes the
>> series intends_, we still need to make sure we don't open a path
>> for device models to gain access to memory which they aren't
>> supposed to be able to write (or just read).
> 
> So, a suggestion would be to use some form of flag on the page (probably a 
> PGC_ flag?) to tag it as a mappable resource. We can then white-list grant 
> frames and ioreq frames with the new flag and then make sure use of 
> MMU_PT_UPDATE_NO_TRANSLATE checks that the mfn is either in the guest P2M 
> anyway, or tagged as a mappable resource?

This doesn't look to be race free: What about a page having the
new flag removed while the page is still mapped, or in the process
of being mapped (but already past the check of the flag)?

Jan


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