The netback driver in Xen, when using certain Linux versions that do not allow sleeping in softirq context, allows local guest administrators to cause a denial of service ("scheduling while atomic" error and host crash) via a malformed packet, which causes a mutex to be taken when trying to disable the interface.
http://xenbits.xen.org/xsa/advisory-90.html
http://www.securitytracker.com/id/1029949
http://www.securityfocus.com/bid/66386