Bug 1271971 (CVE-2015-7835, xsa148)

A vulnerability in xen allowing malicious PV guest administrators to escalate privilege, so as to control the whole system, was found.

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.

Xen 3.4 and onward on x86 (both 32 and 64 bit) systems are vulnerable. Only PV guests can exploit the vulnerability.

Mitigation:

Running only HVM guests will avoid this vulnerability. On systems where the guest kernel is controlled by the host rather than guest administrator, running only kernels which do not call these hypercalls will also prevent untrusted guest users from exploiting this issue. However untrusted guest administrators can still trigger it unless further steps are taken to prevent them from loading code into the kernel (e.g. by disabling loadable modules etc) or from using other mechanisms which allow them to run code at kernel privilege.