Date: Tue, 22 Nov 2016 12:02:22 +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 193 (CVE-2016-9385) - x86 segment base write emulation lacking canonical address checks -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2016-9385 / XSA-193 version 3 x86 segment base write emulation lacking canonical address checks UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= Both writes to the FS and GS register base MSRs as well as the WRFSBASE and WRGSBASE instructions require their input values to be canonical, or a #GP fault will be raised. When the use of those instructions by the hypervisor was enabled, the previous guard against #GP faults (having recovery code attached) was accidentally removed. IMPACT ====== A malicious guest administrator can crash the host, leading to a DoS. VULNERABLE SYSTEMS ================== Xen versions 4.4 and onwards are affected. Xen versions 4.3 and earlier are not affected. The vulnerability is only exposed to x86 PV guests. The vulnerability is NOT exposed to x86 HVM guests. ARM systems are NOT vulnerable. MITIGATION ========== Running only HVM guests will avoid this vulnerability. For PV guests the vulnerability can be avoided 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. CREDITS ======= This issue was discovered by Andrew Cooper of Citrix. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa193.patch xen-unstable xsa193-4.7.patch Xen 4.7.x, Xen 4.6.x xsa193-4.5.patch Xen 4.5.x, Xen 4.4.x $ sha256sum xsa193* 401df29b462a3430403a4f5bb36fd7824e692c9b5bac650e1a9d70bd440a55a1 xsa193.patch b3494b1fe5fefc0d032bd603340e364c880ec0d3ae3fb8aa3a773038e956f955 xsa193-4.5.patch f1b0092c585ebffe83d6ed7df94885ec5dfcb4227bdb33f421bad9febb8135a1 xsa193-4.7.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 iQEcBAEBAgAGBQJYNDK2AAoJEIP+FMlX6CvZswsIAI17sWqaGeP8GvtddxR08G2J 3Nb7Lnb/4cq8Hdc5XmUnX/zuDqobT5AGJEgKAuhRc9zs2TOv8FwcABc+/odKG6ak tcMAaLThMcKbB0b0ZYEkcrU+jaCDDVE3rYVGjKv0hHKZNRY/SmWOdl180xcHksXG pj5OQn6/+db6nqMlhyOcOyjM3w1/1AUe/O0EDsdUSNrY1mZi4/MjUXlDaJTZbDCc KW9XUeRSq66iZELawBaosViTenOm/R+8DJGiR8fmJlXx+gzpEywtsEUCrxeKlTDo tT68gwy0aHdlqKbIthkKr5qaT5FtKPyX0UpIXu7qtldbdEZG61iIlNOEG8tyPhU= =fjbt -----END PGP SIGNATURE----- Download attachment "xsa193.patch" of type "application/octet-stream" (2798 bytes) Download attachment "xsa193-4.5.patch" of type "application/octet-stream" (2886 bytes) Download attachment "xsa193-4.7.patch" of type "application/octet-stream" (2966 bytes)
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