Date: Tue, 04 Oct 2016 12:51:08 +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 190 (CVE-2016-7777) - CR0.TS and CR0.EM not always honored for x86 HVM guests -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2016-7777 / XSA-190 version 5 CR0.TS and CR0.EM not always honored for x86 HVM guests UPDATES IN VERSION 5 ==================== Public release. ISSUE DESCRIPTION ================= Instructions touching FPU, MMX, or XMM registers are required to raise a Device Not Available Exception (#NM) when either CR0.EM or CR0.TS are set. (Their AVX or AVX-512 extensions would consider only CR0.TS.) While during normal operation this is ensured by the hardware, if a guest modifies instructions while the hypervisor is preparing to emulate them, the #NM delivery could be missed. Guest code in one task may thus (unintentionally or maliciously) read or modify register state belonging to another task in the same VM. IMPACT ====== A malicious unprivileged guest user may be able to obtain or corrupt sensitive information (including cryptographic material) in other programs in the same guest. VULNERABLE SYSTEMS ================== All versions of Xen expose the vulnerabilty to their x86 HVM guests. In order to exploit the vulnerability, the attacker needs to be able to trigger the Xen instruction emulator. On Xen 4.7 the emulator can only be triggered: by user mode tasks which have been given access to memory-mapped IO; in guests which have been migrated between systems with CPUs from different vendors; or in guests which have been configured with a CPU vendor different from the host's. On Xen 4.6 and earlier, all HVM guests can trigger the emulator by attempting to execute an invalid opcode, exposing the vulnerability. The vulnerability is only exposed to x86 HVM guests. The vulnerability is not exposed to x86 PV or ARM guests. MITIGATION ========== On Xen 4.7, not migrating across CPU vendors will avoid this vulnerability. (Unless the guest grants mmio access to unprivileged tasks, or has been configured with a specific CPU vendor, eg using the xl "cpuid" configuraton option.) CREDITS ======= This issue was discovered by Jan Beulich from SUSE. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa190.patch xen-unstable, Xen 4.7.x xsa190-4.6.patch Xen 4.6.x xsa190-4.5.patch Xen 4.5.x, Xen 4.4.x $ sha256sum xsa190* 21e7b1d08874527ab2e4cd23d467e9945afcd753dd3390ab2aaf9d24d231916c xsa190.patch 477d56c41cc2101432459ab79e4d5663aade779c36285f5c1d6d6ed4e34e1009 xsa190-4.5.patch dbfc4b36132c841959847dfbb85a188ee6489ad3b8d7ecec43c55a303a43df21 xsa190-4.6.patch $ 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 -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJX86WyAAoJEIP+FMlX6CvZZOQH/0rLFSZeiGeWDlKzQJoB3VLy zDvpDKjfhuwPyWT9+oyfwUHxARWuJkYSy85bpVuNWmxtb1tGy+QTjbSZgyVrsRXY 4t09MzhTF9CuNhqTghEGbFeGdh20ht3EoDjiwkjlbfb4TQ439e189qo9Oe0J/LvD 4XjL/oHza0YMI/wFviANUZvvTzAcjTAw1Zwk6dpnM17cwK4HduPYBncUyfDrSa3G 97nOraBXh/CiwWlm6goRSOI73ORUkYYBwJLGcq3a50HJPJ7pCbBaRJpDCalMPZ2B Lf+HO38HROEGBbTfkOjyZKkbTjQ2njTu0kHaBl+IVK8LI3PLv35n5MQ6qStYL/U= =7/xB -----END PGP SIGNATURE----- Download attachment "xsa190.patch" of type "application/octet-stream" (5643 bytes) Download attachment "xsa190-4.5.patch" of type "application/octet-stream" (5223 bytes) Download attachment "xsa190-4.6.patch" of type "application/octet-stream" (5223 bytes)
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