Date: Tue, 12 Dec 2017 12:00:11 +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-team-members@....org> Subject: Xen Security Advisory 250 - improper x86 shadow mode refcount error handling -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory XSA-250 version 2 improper x86 shadow mode refcount error handling UPDATES IN VERSION 2 ==================== Public release. Provide metadata file. ISSUE DESCRIPTION ================= Pages being used to run x86 guests in shadow mode are reference counted to track their uses. When another reference cannot be acquired, the corresponding page table entry must not be inserted. Due to incorrect error handling, this constraint could be violated. IMPACT ====== A malicious or buggy guest may cause a hypervisor crash, resulting in a Denial of Service (DoS) affecting the entire host, or cause hypervisor memory corruption. We cannot rule out a guest being able to escalate its privilege. VULNERABLE SYSTEMS ================== All Xen versions are affected. x86 systems are vulnerable. ARM systems are not vulnerable. Only guests run in shadow mode can exploit the vulnerability. PV guests typically only run in shadow mode during live migration, as well as for features like VM snapshot. Note that save / restore does *not* use shadow mode, and so does not expose this vulnerability. Some downstreams also include a "non-live migration" feature, which also does not use shadow mode (and thus does not expose this vulnerability). HVM guests run in shadow mode on hardware without HAP support, or when HAP is disabled (globally or in the VM configuration file). Live migration does not affect an HVM guest's use of shadow mode. MITIGATION ========== For HVM guest explicitly configured to use shadow paging (e.g. via the `hap=0' xl domain configuration file parameter), changing to HAP (e.g. by setting `hap=1') will avoid exposing the vulnerability to those guests. HAP is the default (in upstream Xen), where the hardware supports it; so this mitigation is only applicable if HAP has been disabled by configuration. For PV guests, avoiding their live migration avoids the vulnerability. CREDITS ======= This issue was discovered by Jan Beulich of SUSE. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa250.patch xen-unstable, Xen 4.9.x ... 4.6.x xsa250-4.5.patch Xen 4.5.x $ sha256sum xsa250* c15c1c3e64cfb7ab2e2c48970214aa8c3881deb7e11c498526554bb74535b601 xsa250.meta adf4d8242dbddb4ec52fe1effc1f8b233d33d8d6a59c1bb677dcc6e2ed2bf711 xsa250.patch d123a58308db606185c4e48dcf4a114ac29bb988ffc0eeb04ded213ec474e0f2 xsa250-4.5.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----- iQFABAEBCAAqFiEEI+MiLBRfRHX6gGCng/4UyVfoK9kFAlovuNkMHHBncEB4ZW4u b3JnAAoJEIP+FMlX6CvZx+4H/2ADwtz7LzqBd7aZ9BnODa3L+KM/hO05tG0t+feh eunJSfxAY3jRep4NxWUgK8zerAusw3zZi9lRzmhdLMHYtmslJPDWy5ul0N09E6Y5 KH2Ky8zkFb2puzHZs2oMKywW25aRI6Bs7VdFK44KxWPRrLAFTNup6xOCVNWJ4VWw YhNTu4g/+mUUa+KLRPL/s6sKjIw07/sbh/koHWSwlAksxmlUfdHaFuLbsvspPRe0 vq8Q8zN/n9Att6i8RrjeWLAb36mYXhKYIzkZhmJXNlwQx9dkhuLdlRaJ4zb7uERb wDYYlT9wib8CB5tsKxX+ozLQ0mr43DAFfsLJpzi7TudYplE= =+/I2 -----END PGP SIGNATURE----- Download attachment "xsa250.meta" of type "application/octet-stream" (2276 bytes) Download attachment "xsa250.patch" of type "application/octet-stream" (2277 bytes) Download attachment "xsa250-4.5.patch" of type "application/octet-stream" (2304 bytes)
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