x86: Uncontrolled creation of large page mappings by PV guests

ISSUE DESCRIPTION

The code to validate level 2 page table entries is bypassed when certain conditions are satisfied. This means that a PV guest can create writeable mappings using super page mappings.
Such writeable mappings can violate Xen intended invariants for pages which Xen is supposed to keep read-only.
This is possible even if the “allowsuperpage” command line option is not used.

IMPACT

Malicious PV guest administrators can escalate privilege so as to control the whole system.

VULNERABLE SYSTEMS

Xen 3.4 and onward are vulnerable.
Only x86 systems are vulnerable. ARM systems are not vulnerable.
Only PV guests can exploit the vulnerability. Both 32-bit and 64-bit PV guests can do so.