Bug 1293675 (CVE-2015-8615, xsa169)

A vulnerability in logging was found, allowing malicious guest to cause repeated logging to the hypervisor console, leading to a Denial of Service attack. HYPERVISOR_hvm_op sub-op HVMOP_set_param's HVM_PARAM_CALLBACK_IRQ operation intends to log the new callback method in debug builds only. The full message, however, is split into two parts, the second one of which didn't get suppressed on non-debug builds as would have been intended. These log messages are not rate-limited and can be triggered by guests.

Xen version 4.6 is affected. Older Xen versions are unaffected. ARM systems are not affected. Only x86 HVM guests can expose this vulnerability.

Running only PV guests will avoid this issue.

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 excessively invoke this operation 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.