Date: Wed, 16 Mar 2016 19:04:28 +0000 From: Xen.org security team <security@....org> To: xen-announce@...ts.xen.org, xen-devel@...ts.xen.org, xen-users@...ts.xen.org, oss-security@...ts.openwall.com CC: Xen.org security team <security@....org> Subject: Xen Security Advisory 171 (CVE-2016-3157) - I/O port access privilege escalation in x86-64 Linux -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2016-3157 / XSA-171 version 4 I/O port access privilege escalation in x86-64 Linux UPDATES IN VERSION 4 ==================== Clarify Vulnerable Systems section. Public release. ISSUE DESCRIPTION ================= IRET and POPF do not modify EFLAGS.IOPL when executed by code at a privilege level other than zero. Since PV Xen guests run at privilege level 3 (for 64-bit ones; 32-bit ones run at privilege level 1), to compensate for this the context switching of EFLAGS.IOPL requires the guest to make use of a dedicated hypercall (PHYSDEVOP_set_iopl). The invocation of this hypercall, while present in the 32-bit context switch path, is missing from its 64-bit counterpart. IMPACT ====== User mode processes not supposed to be able to access I/O ports may be granted such permission, potentially resulting in one or more of in-guest privilege escalation, guest crashes (Denial of Service), or in-guest information leaks. VULNERABLE SYSTEMS ================== All upstream x86-64 Linux versions operating as PV Xen guests are vulnerable. ARM systems are not vulnerable. x86 HVM guests are not vulnerable. 32-bit Linux guests are not vulnerable. x86-64 Linux versions derived from linux-2.6.18-xen.hg (XenoLinux) are not vulnerable. We believe that non-Linux guests are not vulnerable, as we are not aware of any with an analogous bug. MITIGATION ========== Running only HVM or 32-bit PV guests will avoid this issue. CREDITS ======= This issue was discovered by Andy Lutomirski. RESOLUTION ========== Applying the attached patch resolves this issue for the indicated Linux versions. xsa171.patch Linux 4.5-rc7, Linux 4.4.x $ sha256sum xsa171* 5d47ead1212c735b444ac8f82e7f311cda3473fe3847e576c3772ce020265dfd xsa171.patch $ DEPLOYMENT DURING EMBARGO ========================= The patch is a change to the domU, ie, to the guest, not to hosts. Where the guest kernel is provided by the host administrator - ------------------------------------------------------------ Deployment of the patch by the host administrator is NOT permitted (except where all the affected systems and VMs are administered and used only by organisations which are members of the Xen Project Security Issues Predisclosure List). Specifically, deployment on public cloud systems is NOT permitted. This is because a the cloud guest administrator is almost certainly in a position to see the changes that are made by to the kernel even if the kernel is provided by the host administrator. Deployment is permitted only AFTER the embargo ends. Where the guest kernel is provided by the guest administrator - ------------------------------------------------------------- Deployment of the patch (or another which is substantially similar) by the guest administrator is permitted during the embargo ONLY if (i) the host administrator organisation is also a member of the Xen Project Security Issues Predisclosure List. (ii) all the guest's users are also members of predisclosure list. (guest users includes administrators of Linux containers running within the guest). Restriction (i) is because the host administrator can see changes that made to the kernel by a guest administrator. Restriction (ii) is because it is difficult to fully conceal the Linux kernel from unprivileged guest user processes. If the host is not operated by a member of the predisclosure list, or the guest has users outside the predisclousre list, deployment is permitted only AFTER the embargo ends. In any case - ----------- 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, or whose situation is not clearly covered above, 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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQEcBAEBAgAGBQJW6a4ZAAoJEIP+FMlX6CvZEs4H/12hKU3NzqfHZb/wOW9PeT4Z yhGQ2mkVE6FATW15b+/+Lr4N2nIUHa40BtWjPyEOQR4UXJrZr3R5HL/wINRO7c6M 5XNjDyHqmfhOAsHWsrTB0a3CP2wWNNQ6LiBN5AuiUwoqiJiZPLhKCeEi99F+rFFK IINyOgd4XSeGRkb96GfZcPbizbO3wqiREfBIAjECYchBARv7JVGr3my6R3YBYdTn VtBratEPdkEmAEn0LtdiQlnjPib5O3paiaIDk41IPbPu1WPiozt3RJSqJUSwu+al A3qe9cBGz0NyghdYkXQjvaPP+1Q3BjyJC4hgGLo+yqyODPdaFAJZ0mjR/e0uajs= =F9Nz -----END PGP SIGNATURE----- Download attachment "xsa171.patch" of type "application/octet-stream" (2095 bytes)
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