Re: [RFC] Expose a memory poison detector ioctl to user space.

[Jue Wang <juew@google.com>]

On Mon, May 2, 2022 at 10:33 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 02.05.22 19:30, Jue Wang wrote:
> > On Mon, May 2, 2022 at 10:19 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 26.04.22 21:39, Dave Hansen wrote:
> >>> On 4/26/22 12:23, Jue Wang wrote:
> >>>> On Tue, Apr 26, 2022 at 11:18 AM Dave Hansen <dave.hansen@intel.com> wrote:
> >>>>> What if you're in a normal (non-TDX) guest and some of the physical
> >>>>> address space has been ballooned away?
> >>>>
> >>>> Accessing to memory that gets ballooned away will cause extra EPT
> >>>> violations and have the memory faulted in on the host side, which is
> >>>> transparent to the guest.
> >>>
> >>> Yeah, but it completely subverts the whole purpose of ballooning.  In
> >>> other words, this is for all intents and purposes also mutually
> >>> exclusive with ballooning.
> >>
> >> Some balloon (or balloon-like) implementations don't support reading
> >> memory that's mapped into the direct map. For example, with never
> >> virtio-mem devices in the hypervisor, reading unplugged memory can
> >> result in undefined behavior (in the worst case, you'll get your VM zapped).
> >>
> >> Reading random physical memory ranges without further checks is a very
> >> bad idea. There are more corner cases, that we e.g., exclude when
> >> reading /proc/kcore.
> >>
> >> Take a look at read_kcore() KCORE_RAM case, where we e.g., exclude
> >> reading PageOffline(), is_page_hwpoison() and !pfn_is_ram(). Unaccepted
> >> memory might be another case we want to exclude there in the future.
> >>
> >>
> >> I assume something as you imagine could be implemented in user space
> >> just by relying on /proc/iomem and /proc/kcore right now in an unsafe
> >> way. So you might want something similar, however, obviously without
> >> exporting page content to user space and requiring root permissions.
> >
> > Thanks.
> >
> > Are the following cases benign if the scan only happens on the host side?
> >
> > . virtio-mem - unplugged memory
> > . Unaccepted memory
>
> No, only in virtualized worlds.
>
> I assume GART memory that implements the pfn_is_ram() callback is around
> on physical machines.

I think host E820 provides an accurate view of which address range is
ram or not?
>
>
> --
> Thanks,
>
> David / dhildenb
>