Missing check for grant table

ISSUE DESCRIPTION

The function __gnttab_cache_flush handles GNTTABOP_cache_flush grant table operations. It checks to see if the calling domain is the owner of the page that is to be operated on. If it is not, the owner’s grant table is checked to see if a grant mapping to the calling domain exists for the page in question.
However, the function does not check to see if the owning domain actually has a grant table or not. Some special domains, such as DOMID_XEN, DOMID_IO and DOMID_COW are created without grant tables. Hence, if __gnttab_cache_flush operates on a page owned by these special domains, it will attempt to dereference a null pointer in the domain struct.

IMPACT

The guest can get Xen to dereference a NULL pointer.
For ARM guests and x86 PV guests on systems with SMAP enabled, this will cause a host crash (denial-of-service).
For x86 PV guests on systems without SMAP enabled, an attacker can map a crafted grant structure at virtual address 0. This can be leveraged to increment an arbitrary virtual address, which can then probably be leveraged into a full privilege escalation.

VULNERABLE SYSTEMS

All versions of Xen since Xen 4.5 are vulnerable.
x86 HVM guests do not expose the vulnerability.
ARM guests and x86 PV guests on systems with SMAP enabled are only vulnerable to a Denial-of-Service (host crash).
x86 PV guests on systems without SMAP running are vulnerable to a privilege escalation.