Date: Tue, 22 Nov 2016 12:02:38 +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 196 (CVE-2016-9377,CVE-2016-9378) - x86 software interrupt injection mis-handled -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2016-9377,CVE-2016-9378 / XSA-196 version 3 x86 software interrupt injection mis-handled UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= There are two closely-related bugs. When Xen emulates instructions which generate software interrupts it needs to perform a privilege check involving an IDT lookup. This check is sometimes erroneously conducted as if the IDT had the format for a 32-bit guest, when in fact it is in the 64-bit format. Xen will then read the wrong part of the IDT and interpret it in an unintended manner. (CVE-2016-9377) When Xen emulates instructions which generate software interrupts, and chooses to deliver the software interrupt, it may try to use the method intended for injecting exceptions. This is incorrect, and results in a guest crash. (CVE-2016-9378) These instructions are not ususally handled by the emulator. Exploiting the bug requires ability to force use of the emulator. IMPACT ====== An unprivileged guest user program may be able to crash the guest. VULNERABLE SYSTEMS ================== Xen versions 4.5 and newer are vulnerable. Older versions are not vulnerable. The vulnerability is only exposed on AMD hardware lacking the NRip feature. AMD hardware with the NRip feature, and all Intel hardware, is not vulnerable. Xen prints information about CPU features on boot. If you see this: (XEN) SVM: Supported advanced features: ... (XEN) - Next-RIP Saved on #VMEXIT then you are not vulnerable because you have an AMD CPU with NRip. If you see this: (XEN) VMX: Supported advanced features: then you are not vulnerable because you have an Intel CPU. The vulnerability is only exposed on HVM guests. ARM systems are NOT vulnerable. MITIGATION ========== Running only PV guests will avoid this issue. CREDITS ======= This issue was discovered by Andrew Cooper of Citrix. RESOLUTION ========== Applying the attached patches resolves this issue. xsa196-000*.patch xen-unstable, Xen 4.7.x, Xen 4.6.x, Xen 4.5.x $ sha256sum xsa196* c4122280f3786416231ae5f0660123446d29e9ac5cd3ffb92784ed36edeec8b7 xsa196-0001-x86-emul-Correct-the-IDT-entry-calculation-in-inject.patch 25671c44c746d4d0e8f7e2b109926c013b440e0bf225156282052ec38536e347 xsa196-0002-x86-svm-Fix-injection-of-software-interrupts.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 iQEcBAEBAgAGBQJYNDMVAAoJEIP+FMlX6CvZZ7MH/36KnwbAxmRHtUDIpQF/Syoh Lc8s6gNV1oOzcCpFgz+gSyIOMzp7KWieKQiVX1HbI0lnLYK/sRa77VNV/Y9bUt+Y y9b9QOZRDHoO92dZ4Ym/hzdtaNkdOQX/JAfy+E5pCGuqPtH/Jy5NuwVL8W7V8PNM QTHmvbgB4/Y2U6QqWpIP+S7oC0A9iuIf9eekd6ZTpqTadPFylTe2WX22mns1TEtN 3Z0NX737AjQLyUVnUoJ32sITCBk6tGutvvEmOc2Y+4eMrUvKSoafVy+5IZcTGwLp 3ke5sDNN1tOpzmqbXgWXBsVkpjWf2i0NW0dl5jh8/tN5FtrTuByd193dJGSKzEE= =IE45 -----END PGP SIGNATURE----- Download attachment "xsa196-0001-x86-emul-Correct-the-IDT-entry-calculation-in-inject.patch" of type "application/octet-stream" (2812 bytes) Download attachment "xsa196-0002-x86-svm-Fix-injection-of-software-interrupts.patch" of type "application/octet-stream" (3476 bytes)
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