Date: Tue, 12 Dec 2017 12:00:09 +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 249 - broken x86 shadow mode refcount overflow check -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory XSA-249 version 2 broken x86 shadow mode refcount overflow check 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. Unfortunately the overflow check when trying to obtain a new reference used a mask one bit wider than the reference count actually is, rendering the entire check ineffective. 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 ================== Xen versions 4.1 and later are affected. Xen versions 4.0 and earlier are not 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 attached patch resolves this issue. xsa249.patch xen-unstable, Xen 4.9.x ... 4.5.x $ sha256sum xsa249* 38a4b8033d634e22939ad42b882c35e46482782619e3e03b968a2f6489e459c9 xsa249.meta e99066b0171d4757c6a66e1223aabe01e990de2d0dc50416936e064e6e750d00 xsa249.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+FMlX6CvZD0AIAJN95Z7d9zV07qt4q1iPyyvmBnhusedCvZmTnJTr +nfYb/mg9H7C8Re3Fsf0RA66P+nA8a76HVC3kqBBuqUvE+QNHteWmVWZ6K7QbtlG cCW6CtjeT0be98G1KyvIhL6rLYjpB/4LWAeXusof6ckcbtxHBRtGL3kQhv3MN91q u/R9nHKUyIYS/G4J39ApHk0XOFJFFg9mx66HhZuMjJMjBDevT+EG516YerXlSWr9 bskfxPICFSC7g8z5I2mYdrAxinJ2QHpzurw2Q3T+adb2ag+ClkZRu3gS9jNHuC3F vqQr0r0LE68t77A2uD7UKyXuU5+kQ61yBE780I6BkhiG4PI= =0o90 -----END PGP SIGNATURE----- Download attachment "xsa249.meta" of type "application/octet-stream" (2153 bytes) Download attachment "xsa249.patch" of type "application/octet-stream" (1656 bytes)
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