Date: Thu, 08 Sep 2016 12:00:50 +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 185 (CVE-2016-7092) - x86: Disallow L3 recursive pagetable for 32-bit PV guests -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2016-7092 / XSA-185 version 3 x86: Disallow L3 recursive pagetable for 32-bit PV guests UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= On real hardware, a 32-bit PAE guest must leave the USER and RW bit clear in L3 pagetable entries, but the pagetable walk behaves as if they were set. (The L3 entries are cached in processor registers, and don't actually form part of the pagewalk.) When running a 32-bit PV guest on a 64-bit Xen, Xen must always OR in the USER and RW bits for L3 updates for the guest to observe architectural behaviour. This is unsafe in combination with recursive pagetables. As there is no way to construct an L3 recursive pagetable in native 32-bit PAE mode, disallow this option in 32-bit PV guests. IMPACT ====== A malicious 32-bit PV guest administrator can escalate their privilege to that of the host. VULNERABLE SYSTEMS ================== All versions of Xen are vulnerable. Only 64-bit builds of the hypervisor are vulnerable. For Xen 4.3 and earlier, 32-bit builds of the hypervisor are not vulnerable. The vulnerability is only exposed to 32-bit PV guests on x86 hardware. The vulnerability is not exposed to 64-bit PV guests, x86 HVM guests, or ARM guests. MITIGATION ========== Running only 64-bit PV or HVM guests will avoid this vulnerability. CREDITS ======= This issue was found in parallel by multiple discoverers, who each disclosed it to the Xen Project Security Team. The first report to us was made by Jérémie Boutoille of Quarkslab. The second report, one working day later, by Shangcong Luan of Alibaba Cloud. RESOLUTION ========== Applying the attached patch resolves this issue. xsa185.patch xen-unstable - Xen 4.4 $ sha256sum xsa185* 3328a1953ecdf4de35462ea8396b0927171d718e95f73a87a7f651427bd8f8b4 xsa185.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 iQEcBAEBAgAGBQJX0VLpAAoJEIP+FMlX6CvZ/koH/0hN8oXOpBPVgsr5d+ylYFBU We948VVN/0uthy9IgI1DBnjM2tjoGgy0w7c7dKWUD3ACTvdIq4hWZywA+6uMIwb5 aneB7hgZZ1i/ie1kAwMl96hdWgPGaXjL1r19WxslgOnr2TkH/9zlAaBvhFkbL+/c cw2lI+AOmhB/VOtNfXYd81qxdSUBUPz2DfiOEjgVx8e8E+q/S5dJO1L41kqRt1bM ENG8NtaxBrXAtZzilxOPVPmQmvSSegTjZMshGhx29wIgUy4R/HnsoYW7OklZQDhU 6DV7WUSlrUU5vlIhwQVIZidXpyhzLBLnR5GS0R4CKcYSb6pRQ8FO3TG81TmO/6Q= =NDX0 -----END PGP SIGNATURE----- Download attachment "xsa185.patch" of type "application/octet-stream" (1312 bytes)
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