Date: Tue, 26 Jul 2016 12:05:17 +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 183 (CVE-2016-6259) - x86: Missing SMAP whitelisting in 32-bit exception / event delivery -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2016-6259 / XSA-183 version 5 x86: Missing SMAP whitelisting in 32-bit exception / event delivery UPDATES IN VERSION 5 ==================== Public release. ISSUE DESCRIPTION ================= Supervisor Mode Access Prevention is a hardware feature designed to make an Operating System more robust, by raising a pagefault rather than accidentally following a pointer into userspace. However, legitimate accesses into userspace require whitelisting, and the exception delivery mechanism for 32bit PV guests wasn't whitelisted. IMPACT ====== A malicious 32-bit PV guest kernel can trigger a safety check, crashing the hypervisor and causing a denial of service to other VMs on the host. VULNERABLE SYSTEMS ================== Xen version 4.5 and newer are vulnerable. Versions 4.4 and older are not, due to not having software support for SMAP. The vulnerability is only exposed on x86 hardware supporting the SMAP feature (Intel Broadwell and later CPUs). The vulnerability is not exposed on ARM hardware, or x86 hardware which do not support SMAP. The vulnerability is only exposed to x86 32bit PV guests. The vulnerability is not exposed to 64bit PV guests or HVM guests. MITIGATION ========== Running only HVM guests or 64-bit PV guests, avoids the vulnerability. Disabling SMAP in the hypervisor by booting Xen with "smap=0" on the command line will avoid this vulnerability. (Depending on the circumstances this workaround may pose a small risk of increasing the impact of other, possibly unknown, vulnerabilities.) CREDITS ======= This issue was discovered by Andrew Cooper of Citrix. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa183.patch xen-unstable, 4.7.x xsa183-4.6.patch Xen 4.6.x, 4.5.x $ sha256sum xsa183* ea0ea4b294332814330f222e6d78eea3b19c394eac8ae22feb4a5bd21e90331f xsa183-unstable.patch 0fee41f21a3eb4af1487590098047f4625688bcef7419572a8f418f9fb728468 xsa183-4.6.patch $ DEPLOYMENT DURING EMBARGO ========================= Deployment of the patches 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: Deployment of the "smap=0" mitigation 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 this produces a guest-visible change which could lead to rediscovery of the vulnerability. And: 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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJXl0M9AAoJEIP+FMlX6CvZYB4IAIkCjnrkDBqYcPJrnAAjNDGL v/qJiE6NAKlvqyi/pRkDodAk+5CLvvjDHmTBtqvT+7SU3ixt4C80MLiVMCuJVsUw kMcp95KsJne1TSoivAqSXED+J3gkIWXG8PYvpUOwwOqr0aJViuN9Uv52g0+MVUsW OnkHzYzyyMkIRi0bIzXmhvGeHTUxVhcz8RjMWsjD9FPb+i6lu/kfNUvpiecVa0mx 0J7ByS5l4iEefCH+beT35NFg1BfQINU3cMmDM/i8pklRuJI+HKCYFzPGJyl2+Ccr 0Zd7Lgub2jGsJjgXjBBPCHw/CCdlmX7RiiAvnIQU5adBtCIk6p0T0ugcGXwTIAw= =ydwH -----END PGP SIGNATURE----- Download attachment "xsa183-unstable.patch" of type "application/octet-stream" (2681 bytes) Download attachment "xsa183-4.6.patch" of type "application/octet-stream" (2701 bytes)
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