Xen Security Advisory CVE-2015-5307 / XSA-156 x86: CPU lockup during fault delivery *** EMBARGOED UNTIL 2015-11-10 00:01 UTC *** ISSUE DESCRIPTION ================= When a benign exception occurs while delivering another benign exception, it is architecturally specified that these would be delivered sequentially. There are, however, cases where this results in an infinite loop inside the CPU, which (in the virtualized case) can be broken only by intercepting delivery of the respective exception. Architecturally, at least some of these cases should also be resolvable by an arriving NMI or external interrupt, but empirically this has been determined to not be the case. The cases affecting Xen are: #AC (Alignment Check Exception): When a 32-bit guest sets up the IDT entry corresponding to this exception to reference a ring-3 handler, and when ring 3 code triggers the exception while running with an unaligned stack pointer, delivering the exception will re-encounter #AC, ending in an infinite loop. #DB (Debug Exception): When a guest sets up a hardware breakpoint covering a data structure involved in delivering #DB, upon completion of the delivery of the first exception another #DB will need to be delivered. The effects slightly differ depending on further guest characteristics: - Guests running in 32-bit mode would be expected to sooner or later encounter another fault due to the stack pointer decreasing during each iteration of the loop. The most likely case would be #PF (Page Fault) due to running into unmapped virtual space. However, an infinite loop cannot be excluded (e.g. when the guest is running with paging disabled). - Guests running in long mode, but not using the IST (Interrupt Stack Table) feature for the IDT entry corresponding to #DB would behave similarly to guests running in 32-bit mode, just that the larger virtual address space allows for a much longer loop. The loop can't, however, be infinite, as eventually the stack pointer would move into non-canonical address space, causing #SS (Stack Fault) instead. - Guests running in long mode and using the IST for the IDT entry corresponding to #DB would enter an infinite loop, as the stack pointer wouldn't change between #DB instances. IMPACT ====== A malicious HVM guest administrator can cause a denial of service. Specifically, prevent use of a physical CPU for a significant, perhaps indefinite period. If a host watchdog (Xen or dom0) is in use, this can lead to a watchdog timeout and consequently a reboot of the host. If another, innocent, guest, is configured with a watchdog, this issue can lead to a reboot of such a guest. It is possible that a guest kernel might expose the #AC vulnerability to malicious unprivileged guest users (by permitting #AC to be handled in guest user mode). However, we believe that almost all ordinary operating system kernels do not permit this; we are not aware of any exceptions. (A guest kernel which exposed the #AC vulnerability to guest userspace would be vulnerable when running on baremetal, without Xen involved.) VULNERABLE SYSTEMS ================== The vulnerability is exposed to any x86 HVM guest. ARM is not vulnerable. x86 PV VMs are not vulnerable. All versions of Xen are affected. x86 CPUs from all manufacturers are affected. MITIGATION ========== Running only PV guests will avoid this issue. Running only kernels which avoid exposing the #AC problem to userspace (as discussed in Impact) will prevent untrusted guest users from exploiting this issue. With such good kernels, the vulnerability can be avoided altogether if the guest kernel is controlled by the host rather than guest administrator, provided that further steps are taken to prevent the guest administrator 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. In Xen HVM, controlling the guest's kernel would involve locking down the bootloader. RESOLUTION ========== To correctly support the intended uses of the relevant CPU features would require architectural changes to the CPU specification, design and implementation. This is not practical as a security response. Applying the appropriate attached patch works around the issue in software. xsa156.patch xen-unstable, Xen 4.6.x xsa156-4.5.patch Xen 4.5.x xsa156-4.4.patch Xen 4.4.x xsa156-4.3.patch Xen 4.3.x $ sha256sum xsa156*.patch a7f52c56d2f6a89c337a0a6e90c5783bdb07097d806b073043df12a5d43effed xsa156-4.3.patch 33e8cf12e680f3db7254a177eccf1d1e95c588cba23b858886f1baf26f3eca89 xsa156-4.4.patch 050a10835802e7728f1a3dfe90659ea9938c8becf43264fe5bedfed46777236b xsa156-4.5.patch a456ce1f63c92c36772915fa5990403f119cb34e3a336ee08d54230502d70905 xsa156.patch $ NOTE REGARDING CVE ================== The #AC issue is CVE-2015-5307. We believe that the #DB issue will be assigned a different CVE but we have no confirmation of this at this time. NOTE REGARDING EMBARGO DURATION =============================== We have released this advisory as soon as possible after we obtained firm confirmation of the embargo end date from the discoverer. DEPLOYMENT DURING EMBARGO ========================= Deployment of the patches and/or mitigations described above (or others which are substantially similar) is permitted during the embargo, even on public-facing systems with untrusted guest users and administrators. But: Distribution of updated software is prohibited (except to other members of the predisclosure list). Predisclosure list members who wish to deploy significantly different patches and/or mitigations, please contact the Xen Project Security Team. (Note: this during-embargo deployment notice is retained in post-embargo publicly released Xen Project advisories, even though it is then no longer applicable. This is to enable the community to have oversight of the Xen Project Security Team's decisionmaking.) For more information about permissible uses of embargoed information, consult the Xen Project community's agreed Security Policy: http://www.xenproject.org/security-policy.html
The issue is now public
commit 3c606b8d93fcaff04c463764bb5bab96780654ea Author: Ian Delaney <idella4@gentoo.org> Date: Tue Nov 10 18:05:41 2015 +0800 app-emulation/xen: revbumps; add xsa-156 patches in 4.5 4.5.2-r1, 4.6.0-r2 Required by gentoo security bug. These are embargoed patches now free for public release. Gentoo bug: #564932 The addition might need re-doing since it doesn't fit dlan's form but it's trivia. Each new addition requires making of a new .conf file which am not familiar with. These work
Arches, please stabilize =app-emulation/xen-4.5.2-r1 Stable targets: amd64 x86
actually x86 was dropped in xen some time ago: KEYWORDS="amd64 ~arm ~arm64 -x86" leaving amd64. However I see no reason why arm and arm64 should not be made stable but that should be kept for the next stabilising of 4.6.n
(In reply to Ian Delaney from comment #4) > actually x86 was dropped in xen some time ago: > > KEYWORDS="amd64 ~arm ~arm64 -x86" > > leaving amd64. However I see no reason why arm and arm64 should not be made > stable but that should be kept for the next stabilising of 4.6.n x86 is still stable for 4.2 series, has a due diligence been performed to establish that it is not affected by the security vulnerability, and if so a patch backported?
@maintainer, since the xsa says: > All versions of Xen are affected. Please clarify if you want to drop 4.2.x or we need to wait the fixed version.
amd64 stable
(In reply to Agostino Sarubbo from comment #6) > Please clarify if you want to drop 4.2.x or we need to wait the fixed > version. we'll drop 4.2.x, do not waste the effort to stable it
(In reply to Yixun Lan from comment #8) > (In reply to Agostino Sarubbo from comment #6) > > Please clarify if you want to drop 4.2.x or we need to wait the fixed > > version. > > we'll drop 4.2.x, do not waste the effort to stable it It should likely be p.masked in that case
(In reply to Yixun Lan from comment #8) > we'll drop 4.2.x, do not waste the effort to stable it ok, then x86 has nothing to do here. Please cleanup.
commit 8f7e07bb5fc8f742d97e22fa659f044ebd5cc570 Author: Ian Delaney <idella4@gentoo.org> Date: Sun Nov 29 15:53:09 2015 +0800 app-emulation/xen: clean old vns.: 4.5.x, 4.6.0-r1
Arches and Maintainer(s), Thank you for your work. Added to an existing GLSA Request.
This issue was resolved and addressed in GLSA 201604-03 at https://security.gentoo.org/glsa/201604-03 by GLSA coordinator Yury German (BlueKnight).
