Date: Thu, 08 Sep 2016 12:00:54 +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 186 (CVE-2016-7093) - x86: Mishandling of instruction pointer truncation during emulation -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2016-7093 / XSA-186 version 4 x86: Mishandling of instruction pointer truncation during emulation UPDATES IN VERSION 4 ==================== Public release. ISSUE DESCRIPTION ================= When emulating HVM instructions, Xen uses a small i-cache for fetches from guest memory. The code that handles cache misses does not check if the address from which it fetched lies within the cache before blindly writing to it. As such it is possible for the guest to overwrite hypervisor memory. It is currently believed that the only way to trigger this bug is to use the way that Xen currently incorrectly wraps CS:IP in 16 bit modes. The included patch prevents such wrapping. IMPACT ====== A malicious HVM guest administrator can escalate their privilege to that of the host. VULNERABLE SYSTEMS ================== Xen versions 4.7.0 and later are vulnerable. Xen releases 4.6.3 and 4.5.3 are vulnerable. Xen releases 4.6.0 to 4.6.2 inclusive are NOT vulnerable. Xen releases 4.5.2 and earlier are NOT vulnerable. The vulnerability is only exposed to HVM guests on x86 hardware. The vulnerability is not exposed to x86 PV guests, or ARM guests. MITIGATION ========== Running only PV guests will avoid this vulnerability. CREDITS ======= This issue was discovered by Brian Marcotte. RESOLUTION ========== Applying the first patch will resolve the issue. Users wishing to independently verify the correctness of the fix may find the second patch helpful. The second patch makes it easier to use the "fep" (Force Emulation Prefix) feature to reproduce the erroneous condition in a test environment. The "fep" feature requires explicit enablement on the hypervisor command line, and is unsuitable for production systems. Accordingly, applying the second patch does not affect production systems and does not improve security. Xen version First patch Second patch xen-unstable: xsa186-0001-*.patch xsa186-0002-*.patch Xen 4.7.x: xsa186-0001-*.patch xsa186-4.7-0002-*.patch Xen 4.6.3: xsa186-0001-*.patch xsa186-4.6-0002-*.patch Xen 4.5.3: xsa186-0001-*.patch xsa186-4.6-0002-*.patch $ sha256sum xsa186* f2082a36d968a47e477bb5082d0e0aaa58e6cb3dc20b26389f043a9b7b595fa6 xsa186-0001-x86-emulate-Correct-boundary-interactions-of-emulate.patch 412fa58edcbd1c7fdbfec7e28898cf98585593e6a24ccfb088dc0b84715286a5 xsa186-0002-hvm-fep-Allow-testing-of-instructions-crossing-the-1.patch 7482a823c3443e26dee1111c4904162845eaa9f826aa7bf8348007406d91bddd xsa186-4.6-0002-hvm-fep-Allow-testing-of-instructions-crossing-the.patch 5a826a32763d82ac83c924f8c89d12aae5f069a4cbc7d5193aa8413a02b6dc05 xsa186-4.7-0002-hvm-fep-Allow-testing-of-instructions-crossing-the.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 iQEcBAEBAgAGBQJX0VLsAAoJEIP+FMlX6CvZoUoIAMvgdMZRYdK5MaaRUAA1hDG3 UFSxZCH8zja6wZG6WPNj7VqvEkQ2350oqb05BGB8jTFCmqtNDDIyHK68WaMpwDMv EEeetosujnlHTtVV7N8e0HO7F497PzZtzfniTyZc/h2Lna552ohMy/UcADtA7xxP IK6qwvxpkx1aLzsDFpHIdrVcttDD/oZcVbBFwcCAqK33eGNC3S6BJvIibCAKfO8h YKiAtvWUNsX/o4L9Zs4M50/pK3TzWsaDjfK3IX5LJPtsrcrKklrALVnDUOpTz1WA 07UIk0BcrzicEuTvuATWSQ3nVxUXAH95io23PCniHHntBtYJHjGA5rIqX+tiN6w= =HT+K -----END PGP SIGNATURE----- Download attachment "xsa186-0001-x86-emulate-Correct-boundary-interactions-of-emulate.patch" of type "application/octet-stream" (2346 bytes) Download attachment "xsa186-0002-hvm-fep-Allow-testing-of-instructions-crossing-the-1.patch" of type "application/octet-stream" (2441 bytes) Download attachment "xsa186-4.6-0002-hvm-fep-Allow-testing-of-instructions-crossing-the.patch" of type "application/octet-stream" (1516 bytes) Download attachment "xsa186-4.7-0002-hvm-fep-Allow-testing-of-instructions-crossing-the.patch" of type "application/octet-stream" (1113 bytes)
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