Date: Wed, 27 Jun 2018 21:03:54 +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-team-members@....org> Subject: Xen Security Advisory 264 (CVE-2018-12891) - preemption checks bypassed in x86 PV MM handling -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2018-12891 / XSA-264 version 3 preemption checks bypassed in x86 PV MM handling UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= Certain PV MMU operations may take a long time to process. For that reason Xen explicitly checks for the need to preempt the current vCPU at certain points. A few rarely taken code paths did bypass such checks. By suitably enforcing the conditions through its own page table contents, a malicious guest may cause such bypasses to be used for an unbounded number of iterations. IMPACT ====== A malicious or buggy PV guest may cause a Denial of Service (DoS) affecting the entire host. Specifically, it may prevent use of a physical CPU for an indeterminate period of time. VULNERABLE SYSTEMS ================== All Xen versions from 3.4 onwards are vulnerable. Xen versions 3.3 and earlier are vulnerable to an even wider class of attacks, due to them lacking preemption checks altogether in the affected code paths. Only x86 systems are affected. ARM systems are not affected. Only multi-vCPU x86 PV guests can leverage the vulnerability. x86 HVM or PVH guests as well as x86 single-vCPU PV ones cannot leverage the vulnerability. MITIGATION ========== Running only HVM, PVH, or single-vCPU PV guests will avoid this vulnerability. For PV guests, the vulnerability can be avoided if the guest kernel is controlled by the host rather than guest administrator, provided that further steps are taken to prevent the guest administrator from loading code into the kernel (e.g. by disabling loadable modules etc) or from using other mechanisms which allow them to run code at kernel privilege. CREDITS ======= This issue was discovered by Jan Beulich of SUSE. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa264.patch xen-unstable xsa264-4.10.patch Xen 4.10.x ... 4.6.x $ sha256sum xsa264* a7d2edf219af3375ac0d49bff9e64628c70e704fcf131ea21684694517aa9210 xsa264.patch 66aca234b168abc01f28fe131b7e07645a73fd5d0f1d141d68343f31914d96cc xsa264-4.10.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 iQEcBAEBCAAGBQJbM+5GAAoJEIP+FMlX6CvZy7cIALkEoEQnHw5O8vYC5KpDA24X P320Gh0OppT2qtQfKtAF7MaCc7VF9Tnhf3CrtNtolXMryM4vrh7KyOn8wk7jbRBy tp28e6ppO8ons9x1kBAmAZrno8LXwOa2t22hQpUv1mYksRkZotViAXS72t4HkOVl SEQVVLElWAIfPbGJwtu1/qgS8dCckA2MeLeN/dKHRm8gD63XsYt37nQnBa2iraKX yN5sdih+WLgXCf55mubFlQfE6+7qgn27khZpMeJAwGk6N+Rz/Q3q1zSFX9YB+P6d 9ppgoRFVxYpekwtCrLkVLxSAoEwCKi6sdYFnvIngHIMlLiVHjNsLd5YKTAsZcEE= =zTq5 -----END PGP SIGNATURE----- Download attachment "xsa264.patch" of type "application/octet-stream" (2835 bytes) Download attachment "xsa264-4.10.patch" of type "application/octet-stream" (1967 bytes)
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