Bug 1271626 (CVE-2015-7813, xsa146)

A vulnerability allowing malicious guest to cause Denial of Service.

The HYPERVISOR_physdev_op hypercall and most suboperations of the HYPERVISOR_hvm_op hypercall are not currently implemented by Xen on ARM and when called will log the use to the hypervisor console. However these guest accessible log messages are not rate-limited. A malicious guest could cause repeated logging to the hypervisor console, leading to a Denial of Service attack.

Xen 4.4 and later systems running on ARM hardware are vulnerable.

Mitigation:

The problematic log messages are issued with priority Warning.

Therefore they can be rate limited by adding "loglvl=error/warning" to the hypervisor command line or suppressed entirely by adding "loglvl=error".

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.