Bug 1272525 (CVE-2015-7971, xsa152)

HYPERCALL_xenoprof_op and HYPERVISOR_xenpmu_op log some errors and attempts at invalid operations. These log messages are not rate-limited, even though they can be triggered by guests.

A malicious guest could cause repeated logging to the hypervisor console, leading to a Denial of Service attack.

Xen versions 3.2.x and later are affected. (The VPMU part of the vulnerability is applicable only to Xen 4.6 and later.) ARM systems are not affected. (The pmu hypercall is x86-specific, and xenoprof is not supported on ARM.)

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.