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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 =========================================================================== AUSCERT External Security Bulletin Redistribution ESB-2015.2482 Xen Security Advisory CVE-2015-7311 / XSA-142 24 September 2015 =========================================================================== AusCERT Security Bulletin Summary --------------------------------- Product: Xen Publisher: Xen Operating System: Xen UNIX variants (UNIX, Linux, OSX) Impact/Access: Modify Arbitrary Files -- Existing Account Resolution: Patch/Upgrade CVE Names: CVE-2015-7311 Original Bulletin: http://xenbits.xen.org/xsa/advisory-142.html - --------------------------BEGIN INCLUDED TEXT-------------------- - -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2015-7311 / XSA-142 version 2 libxl fails to honour readonly flag on disks with qemu-xen UPDATES IN VERSION 2 ==================== CVE assigned. ISSUE DESCRIPTION ================= Callers of libxl can specify that a disk should be read-only to the guest. However, there is no code in libxl to pass this information to qemu-xen (the upstream-based qemu); and indeed there is no way in qemu to make a disk read-only. The vulnerability is exploitable only via devices emulated by the device model, not the parallel PV devices for supporting PVHVM. Normally the PVHVM device unplug protocol renders the emulated devices inaccessible early in boot. IMPACT ====== Malicious guest administrators or (in some situations) users may be able to write to supposedly read-only disk images. CDROM devices (that is, devices specified to be presented to the guest as CDROMs, regardless of the nature of the backing storage on the host) are not affected. VULNERABLE SYSTEMS ================== Only systems using qemu-xen (rather than qemu-xen-traditional) as the device model version are vulnerable. Only systems using libxl or libxl-based toolstacks are vulnerable. (This includes xl, and libvirt with the libxl driver.) All versions of libxl which support qemu-xen are vulnerable. The affected code was introduced in Xen 4.1. If the host and guest together usually support PVHVM, the issue is exploitable only if the malicious guest administrator has control of the guest kernel or guest kernel command line. MITIGATION ========== Switching to qemu-xen-traditional will avoid this vulnerability. This can be done with device_model_version="qemu-xen-traditional" in the xl configuration file. Using stub domain device models (which necessarily involves switching to qemu-xen-traditional) will also avoid this vulnerability. This can be done with device_model_stubdomain_override=true in the xl configuration file. Either of these mitigations is liable to have other guest-visible effects or even regressions. It may be possible, depending on the configuration, to make the underlying storage object readonly, or to make it reject writes. RESOLUTION ========== There is no reasonable resolution because Qemu does not (at the time of writing) support presenting a read-only block device to a guest as a disk. The attached patch corrects the weakness in the libxl code, by rejecting the unsupported configurations, rather than allowing them to run but with the device perhaps writeable by the guest. Applying it should increase confidence and avoid future configuration errors, but will break affected configurations specifying read-only disk devices. xsa142-4.6.patch Xen 4.6.x and later xsa142-4.5.patch Xen 4.3.x to 4.5.x inclusive $ sha256sum xsa142*.patch 9ec0649f39720bc692be03c87ebea0506d6ec574f339fc745e41b31643240124 xsa142-4.5.patch 65f01167bfc141048261f56b99ed9b48ec7ff6e98155454ced938a17ec20e7d1 xsa142-4.6.patch $ NOTE REGARDING LACK OF EMBARGO ============================== This issue was discussed in public in the Red Hat bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1257893 CREDITS ======= Thanks to Michael Young of Durham University for bring this problem to our attention. - -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQEcBAEBAgAGBQJWAXCcAAoJEIP+FMlX6CvZ1asH/0yJQ9+33gZtE69Bxicms3C2 uSepfkZVBUym+eEBqGKd2hiapngIAInotOTk+iI7DDo41wvfnJxq1eaEaQ9XurKK kylHOb8eHmYw+HwTW2kJV2g6ffeGBMIcI5mpK35yBa5NnNHHJz0b9ZeRzddR9rSR 0eQpuP4DlN1/2/z6obXmYms84Q1oiIzMDz+MzJA/zPtfL7Q/tBjUmMfPj67zNKwe vIfIstI5IbCRgnXSEL9EjTckqNFszyr3pH4z/Y97UXWlbTg233ewAS11Wz/CwJKT yzS4uJGpckqTRC3YKyS1unKCP39yAVIBTx4QoPu9hrWyzUJpZUD/FvmrIHhr8co= =kHPH - -----END PGP SIGNATURE----- - --------------------------END INCLUDED TEXT-------------------- You have received this e-mail bulletin as a result of your organisation's registration with AusCERT. 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It may not be updated when updates to the original are made. If downloading at a later date, it is recommended that the bulletin is retrieved directly from the author's website to ensure that the information is still current. Contact information for the authors of the original document is included in the Security Bulletin above. If you have any questions or need further information, please contact them directly. Previous advisories and external security bulletins can be retrieved from: http://www.auscert.org.au/render.html?cid=1980 =========================================================================== Australian Computer Emergency Response Team The University of Queensland Brisbane Qld 4072 Internet Email: auscert@auscert.org.au Facsimile: (07) 3365 7031 Telephone: (07) 3365 4417 (International: +61 7 3365 4417) AusCERT personnel answer during Queensland business hours which are GMT+10:00 (AEST). On call after hours for member emergencies only. =========================================================================== -----BEGIN PGP SIGNATURE----- Comment: http://www.auscert.org.au/render.html?it=1967 iQIVAwUBVgNfBX6ZAP0PgtI9AQI4FxAAmJumJP8L3/AWOfS1I8I/KZ+kVfLlyEyh ZhaWU7bP+icNtcIfBfm11DivWXvIj2eivY7qtzcLesvF+tnBvLkFGUbwzpNgRcUf J3ozCWqTbNlUWiFPFLuoJEcWN4TTw1E5GdhUzcKcql/YwZpAuN0jKZnHrM7kuHqy mu6U3X7KJCWZXe9SfG0ZI9mRD4Xwm+zqqUicyk8wX11dviwfPkjJJaOF0AyAPOJE YM9DHgJ12+zgwJFX6EVk7GKVmt3U2pxG7H3W8kRFYJXTTZkt+m4gTOZcZTtMZQmb QQQ4Ip7ylkXJxQt90h8LSGZggmwLL5r4AgGtmNoTgsRK3NH8stLzGNz0vwkTdeql +iqFkFV4YPvGi1PN1As2ELGXWfAShfJ0tBWwGmZfVvp2Y6utCBXq/vHiXz2LQV0h UQltj89PZZPpmq/FKs5Zfq5Z6MOaWGiOCcJ0FXXl3+h8iKF0JgAOHSGEmHgFXl65 0sUR5aeSHAL1M6sNZokTDSftBs2V/19lq1wn/ZujYf/DrcN4rKfukDrOrALjUGkQ l6rXW2xRQlGe9HTJ+ZwVP5FlnSzzRFRSD7sDQvR3Idp6RxFx+oL5sCS0Uptcc3uE WAHYRzvVbQ/QxXMN3i7CE3dAW5WXrOIGH2o+oRuWnaR//B0iFUy2rxG8yFATSOqV fGkcWLV4EBY= =kV34 -----END PGP SIGNATURE-----