Date: Tue, 25 Mar 2014 12:13:49 +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 89 - HVMOP_set_mem_access is not preemptible -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory XSA-89 version 2 HVMOP_set_mem_access is not preemptible UPDATES IN VERSION 2 ==================== Public release. ISSUE DESCRIPTION ================= Processing of the HVMOP_set_mem_access HVM control operations does not check the size of its input and can tie up a physical CPU for extended periods of time. IMPACT ====== In a configuration where device models run with limited privilege (for example, stubdom device models), a guest attacker who successfully finds and exploits an unfixed security flaw in qemu-dm could leverage the other flaw into a Denial of Service affecting the whole host. In the more general case, in more abstract terms: a malicious administrator of a domain privileged with regard to an HVM guest can cause Xen to become unresponsive leading to a Denial of Service. VULNERABLE SYSTEMS ================== All Xen versions from 4.1 onwards are vulnerable. In 4.2 only 64-bit versions of the hypervisor are vulnerable (HVMOP_set_mem_access is not available in 32-bit hypervisors). The vulnerability is only exposed to service domains for HVM guests which have privilege over the guest. In a usual configuration that means only device model emulators (qemu-dm). In the case of HVM guests whose device model is running in an unrestricted dom0 process, qemu-dm already has the ability to cause problems for the whole system. So in that case the vulnerability is not applicable. The situation is more subtle for an HVM guest with a stub qemu-dm. That is, where the device model runs in a separate domain (in the case of xl, as requested by "device_model_stubdomain_override=1" in the xl domain configuration file). The same applies with a qemu-dm in a dom0 process subjected to some kind kernel-based process privilege limitation (eg the chroot technique as found in some versions of XCP/XenServer). In those latter situations this issue means that the extra isolation does not provide as good a defence (against denial of service) as intended. That is the essence of this vulnerability. However, the security is still better than with a qemu-dm running as an unrestricted dom0 process. Therefore users with these configurations should not switch to an unrestricted dom0 qemu-dm. Finally, in a radically disaggregated system: where the HVM service domain software (probably, the device model domain image) is not always supplied by the host administrator, a malicious service domain administrator can excercise this vulnerability. MITIGATION ========== Running only PV guests will avoid this vulnerability. In a radically disaggregated system, restricting HVM service domains to software images approved by the host administrator will avoid the vulnerability. CREDITS ======= This issue was discovered by Jan Beulich. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa89.patch xen-unstable, Xen 4.4.x, Xen 4.3.x, Xen 4.2.x xsa89-4.1.patch Xen 4.1.x $ sha256sum xsa89*.patch 741c8fbbfa8e425d8debba17135d4c2e1e962d15717769bc93d68a65b5dc5ea6 xsa89.patch 7d965e9bf1894b7d909bfaddbc6b7bdcee0ba91b86942ce85e0ae80464f2463e xsa89-4.1.patch $ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQEcBAEBAgAGBQJTMXLgAAoJEIP+FMlX6CvZZ78H/RbnQJwEHxKxn3zhaEULpm57 zBPG1D2cGP12UCkFQLqR8tWvPYmEtm3/x/FQHjzTCBBCM3GMFJ9BiKOX+u5+h2Bu 17xPD3K8cH1tBkZpnQTkTBTz7XrfwV+C78kaNxo3TBvlgTIljaGCHxkXt0PmR1Vq DPZEQdYXj/v8pblmyHYuhd6zf3n6V07ABLqHyPc9n6yZ4/o2LFjqQPZJpYFiFZI+ NGPw18+WCYlXc9w9ZtpGlNOo7Y5O2lraLLu7Gyi+JjC/BHXnb1XLgmgOSTyj2X5M 5v6zIMXy3vqaXHyjqw7uX6EzhCPfPhXAXVjpVGDin+RY/Ykp0QBDweUxZb4U71U= =u+aG -----END PGP SIGNATURE----- Download attachment "xsa89.patch" of type "application/octet-stream" (3011 bytes) Download attachment "xsa89-4.1.patch" of type "application/octet-stream" (1247 bytes)
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