Date: Tue, 22 Nov 2016 12:02:35 +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 195 (CVE-2016-9383) - x86 64-bit bit test instruction emulation broken -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2016-9383 / XSA-195 version 3 x86 64-bit bit test instruction emulation broken UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= The x86 instructions BT, BTC, BTR, and BTS, when used with a destination memory operand and a source register rather than an immediate operand, access a memory location offset from that specified by the memory operand as specified by the high bits of the register source. When Xen needs to emulate such an instruction, to efficiently handle the emulation, the memory address and register operand are recalculated internally to Xen. In this process, the high bits of an intermediate expression were discarded, leading to both the memory location and the register operand being wrong. The wrong memory location would have only a guest local effect (either access to an unintended location, or a fault delivered to the guest), whereas the wrong register value could lead to either a host crash or an unintended host memory access. IMPACT ====== A malicious guest can modify arbitrary memory, allowing for arbitrary code execution (and therefore privilege escalation affecting the whole host), a crash of the host (leading to a DoS), or information leaks. The vulnerability is sometimes exploitable by unprivileged guest user processes. VULNERABLE SYSTEMS ================== All Xen versions are affected. The vulnerability is only exposed to x86 guests running in 64-bit mode. On Xen 4.6 and earlier the vulnerability is exposed to all guest user processes, including unprivileged processes, in such guests. On Xen 4.7 and later, the vulnerability is exposed only to guest user processes granted a degree of privilege (such as direct hardware access) by the guest administrator; or, to all user processes when the when the VM has been explicitly configured with a non-default cpu vendor string (in xm/xl, this would be done with a `cpuid=' domain config option). The vulnerability is not exposed to 32-bit PV guests. ARM systems are not vulnerable. MITIGATION ========== There is no known mitigation. CREDITS ======= This issue was discovered by George Dunlap of Citrix, using American Fuzzy Lop v2.35b. RESOLUTION ========== Applying the attached patch resolves this issue. xsa195.patch xen-unstable, Xen 4.7.x, Xen 4.6.x, Xen 4.5.x, Xen 4.4.x $ sha256sum xsa195* 6ab5f13b81e3bbf6096020f4c3beeffaff67a075cab67e033ba27d199b41cec1 xsa195.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 iQEcBAEBAgAGBQJYNDL4AAoJEIP+FMlX6CvZnzYH/RtmqS8kpqLKShvrQx5Ueh+M LaHBWJiU0z1m9FaF9RvEgfvWpUCcD/qyC4rLHmkwhkyS6aIToh2XVXzQyebIqw/7 CCDXaY8TkYlLPYRdNseX5X5blpu1EnqW5yQMJz6QkgDK+Qu4F1jDimSd5JffrFkJ WkpWwsoppNHwYyaENq59lg7R1WxNq0uSLxMPTnk/RpMmizKyU8gK7RrQWHJNoy6n l3vSTKx9sCDo+AgMQgbDMdpvv1l1It+QcRXXBrBp7qAdz+0H7VRkUFOnBUFMQQo3 OjmjStKxnE9E7Uh6+373xj2Z6Nts+wkD72vRHHg/1KTZ5FN5XnS2CvPDNuGZD50= =AtOu -----END PGP SIGNATURE----- Download attachment "xsa195.patch" of type "application/octet-stream" (1734 bytes)
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