xen/arm: p2m: Avoid aliasing guest physical frame

xen/arm: p2m: Avoid aliasing guest physical frame

The P2M helpers implementation is quite lax and will end up to ignore
the unused top bits of a guest physical frame.

This effectively means that p2m_set_entry() will create a mapping for a
different frame (it is always equal to gfn & (mask unused bits)). Yet
p2m->max_mapped_gfn will be updated using the original frame.

At the moment, p2m_get_entry() and p2m_resolve_translation_fault()
assume that p2m_get_root_pointer() will always return a non-NULL pointer
when the GFN is smaller than p2m->max_mapped_gfn.

Unfortunately, because of the aliasing described above, it would be
possible to set p2m->max_mapped_gfn high enough so it covers frame that
would lead p2m_get_root_pointer() to return NULL.

As we don't sanity check the guest physical frame provided by a guest, a
malicious guest could craft a series of hypercalls that will hit the
BUG_ON() and therefore DoS Xen.

To prevent aliasing, the function p2m_get_root_pointer() is now reworked
to return NULL If any of the unused top bits are not zero. The caller
can then decide what's the appropriate action to do. Since the two paths
(i.e. P2M_ROOT_PAGES == 1 and P2M_ROOT_PAGES != 1) are now very
similarly, take the opportunity to consolidate them making the code a
bit simpler.

With this change, p2m_get_entry() will not try to insert a mapping as
the root pointer is invalid.

Note that root_table is now switch to unsigned long as unsigned int is
not enough to hold part of a GFN.

This is part of XSA-301.

Reported-by: Julien Grall <Julien.Grall@arm.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>