Created attachment 434658 [details] xsa176 Patch - as in Document Xen Security Advisory CVE-2016-4480 / XSA-176 version 3 x86 software guest page walk PS bit handling flaw UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= The Page Size (PS) page table entry bit exists at all page table levels other than L1. Its meaning is reserved in L4, and conditionally reserved in L3 and L2 (depending on hardware capabilities). The software page table walker in the hypervisor, however, so far ignored that bit in L4 and (on respective hardware) L3 entries, resulting in pages to be treated as page tables which the guest OS may not have designated as such. If the page in question is writable by an unprivileged user, then that user will be able to map arbitrary guest memory. IMPACT ====== On vulnerable OSes, guest user mode code may be able to establish mappings of arbitrary memory inside the guest, allowing it to elevate its privileges inside the guest. VULNERABLE SYSTEMS ================== All Xen versions expose the vulnerability. ARM systems are not vulnerable. x86 PV guests are not vulnerable. To be vulnerable, a system must have both a vulnerable hypervisor, and a vulnerable guest operating system, i.e. ones which make non-standard use of the PS bit. We are not aware of any vulnerable guest operating systems, but we cannot rule it out. We have checked with maintainers of the following operating systems, all of whom have said that to the best of their knowledge their operating system is not vulnerable: Linux, FreeBSD, NetBSD, OpenBSD, and Solaris. Nor has it been observed in common proprietary operating systems. MITIGATION ========== Running only PV guests will avoid this issue. CREDITS ======= This issue was discovered by Jan Beulich from SUSE. RESOLUTION ========== Applying the attached patch resolves this issue. Note, however, that on hosts supporting 1Gb page mappings, for guests which get this capability hidden via CPUID override in their config file, fully correct behavior cannot be provided when using HAP paging. This is a result of hardware behavior, which software cannot mitigate. If that is a concern, such guests would need to be run in shadow paging mode. xsa176.patch xen-unstable, Xen 4.6.x, Xen 4.5.x, Xen 4.4.x, Xen 4.3.x $ sha256sum xsa176* e61c52477a8d8aa79111d686b103202ff8a558d8b3356635288c1290789b7eb3 xsa176.patch
commit bc21ec5985e878110034366d860b451cf9102a2e Author: Ian Delaney <idella4@gentoo.org> Date: Fri May 20 12:39:15 2016 +0800 app-emulation/xen-tools: revbump to 4.6.1-r3 Add sec patch xsa-176 patch, re security bug Holding off revbump to 4.6.0, considering also purging Gentoo-bug: #583462 Package-Manager: portage-2.3.0_rc1 commit e8b0c88c33a3a45e75af232291e826289c22f7e4 Author: Ian Delaney <idella4@gentoo.org> Date: Fri May 20 11:50:17 2016 +0800 app-emulation/xen: revbump to 4.6.1-r2 Add sec patch xsa-176 patch, re security bug Holding off revbump to 4.6.0, considering also purging Gentoo-bug: #583462
GLSA Vote: No
