User mode linux ebuild r12 contains the uml patch which includes a resurfacing of the failed to forward" bug fixed in the uml patch in ebuild r13. However r13 is not recognized by emerge, though a manual install of the ebuild I verified as working. Not sure why 'emerge search usermode' says usermode-sources latest version available 2.4.19-r12 when the r13 ebuild is in usr/portage/sys-kernel/usermode-sources . No usermode items are in package.mask.
its not fully tested ... you need 'ACCEPT_KEYWORDS="~x86"' in your make.conf file ... the ~ means its released for testing