Date: Thu, 12 Mar 2015 13:32:43 +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 119 (CVE-2015-2152) - HVM qemu unexpectedly enabling emulated VGA graphics backends -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Xen Security Advisory CVE-2015-2152 / XSA-119 version 3 HVM qemu unexpectedly enabling emulated VGA graphics backends UPDATES IN VERSION 3 ==================== Public release. ISSUE DESCRIPTION ================= When instantiating an emulated VGA device for an x86 HVM guest qemu will by default enable a backend to expose that device, either SDL or VNC depending on the version of qemu and the build time configuration. The libxl toolstack library does not explicitly disable these default backends when they are not enabled, leading to an unexpected backend running. If either SDL or VNC is explicitly enabled in the guest configuration then only the expected backends will be enabled. This affects qemu-xen and qemu-xen-traditional differently. If qemu-xen was compiled with SDL support then this would result in an SDL window being opened if $DISPLAY is valid, or a failure to start the guest if not. If qemu-xen was compiled without SDL support then qemu would instead start a VNC server listening on ::1 (IPv6 localhost) or 127.0.0.1 (IPv4 localhost) with IPv6 preferred if available. A VNC password will not be configured even if one is present in the guest configuration. qemu-xen-traditional will never start a vnc backend unless explicitly configured. However by default it will start an SDL backend if it was built with SDL support and $DISPLAY is valid. IMPACT ====== For qemu-xen compiled without SDL support (unexpected VNC server): Any local user on the domain 0 hosting the VM will be able to access the guest's emulated VGA console. For any qemu compiled with SDL support (unexpected SDL backend): Users who are able to control the DISPLAY environment variable of the toolstack process which creates the VM will be able to direct the SDL output to an X server of their choosing and from there gain access to the guest's emulated console. This is a practical attack only on systems where arrangements have been made for lower-privileged users to execute Xen toolstack code via means which do not sufficiently launder the process environment. This would include some restricted sudo command configurations. In both cases unexpected access to the guest console may then, depending on the guest configuration, grant further privilege or opportunities for attack. Both cases also open up the qemu process to attacks via the VNC or X network protocols. The qemu monitor is not exposed via this means unless it is explicitly enabled in the guest configuration. VULNERABLE SYSTEMS ================== ARM systems are not vulnerable. PV domains are not vulnerable. Systems where either SDL or VNC is explicitly enabled in the guest configuration (eg `sdl=1' or `vnc=1' in the guest config file) are not vulnerable. Systems using qemu-xen-traditional, or systems using qemu-xen where SDL support is built into qemu-xen, are not vulnerable; unless the Xen toolstack code runs in a process environment partially controlled by potential attackers. x86 systems running HVM domains, configured to disable both SDL and VNC access to the emulated VGA device, may be vulnerable. Versions of Xen from 4.2 onwards are known to be affected. Older versions have not been inspected. MITIGATION ========== Running qemu in a stub domain will avoid this issue. Setting nographic to true on the domain (i.e. nographic=1 in an xl configuration file) will completely disable the emulated VGA device and therefore avoid this issue. (NB that publicly visible deployment of this mitigation during the embargo is forbidden.) In order to disable the backends while retaining the emulated VGA then prepending "-vnc none -display none" to the qemu-xen command-line or "-vnc none" to the qemu-xen-traditional command-line, using e.g. a wrapper script will avoid the issue. Note that the "extra_hvm" option exposed by the libxl library is not useful because it appends the given options making them ineffective in this case. CREDITS ======= This issue was discovered by Sander Eikelenboom. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa119-unstable.patch xen-unstable, Xen 4.5.x, Xen 4.4.x, Xen 4.3.x xsa119-4.2.patch Xen 4.2.x $ sha256sum xsa119*.patch ee44c8f6a7cf3ca7b2d9886047b91690aaa2b091baf8629d8ab4c298022c6c47 xsa119-unstable.patch 5470eae3ca776a5100e8da9400ce15a2f4d855177f023430b2462f65e716128f xsa119-4.2.patch $ DEPLOYMENT DURING EMBARGO ========================= Deployment of the patches above (or others which are substantially similar) is permitted during the embargo, even on public-facing systems with untrusted guest users and administrators. Deployment of a revised command qemu line which sets "-vnc none - -display none" or "-vnc none" (as applicable) is also permitted. Mitigation by passing `nographic=1' or equivalent guest configuration, is NOT permitted (except where all the guests are accessible only by members of the Xen Project Security Issues Predisclosure List). Specifically, deployment of such a mitigation on public cloud systems is NOT permitted. This is because the guest-visible configuration change (disappearance of the emulated VGA device as the response to a security issue) would suggest to attackers where to look for the vulnerability. 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.) -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQEcBAEBAgAGBQJVAZVQAAoJEIP+FMlX6CvZ04YIAJZ0goOAXAzc5OwrY/RyTeCp fGHhzkWQ5ZJ3GKR2x+A+uXTb5X+tpo07A/sIS9eGtUDTpzOmfNn/r+vXpicVip8j CW8KMCvNqiMu6BlrF13x7wrYTNSCudLdcg5ermUBasPXadbPspJoLsmEZVDejLEP 7Wp99VoeOJEfR/29JrSTDLAuZ5F5TL9T3TZZ9qnxpWxa4ag7qsKL3AS8akKAj8O5 JDHsCpPdPV0w4BNkLTa9zd9xWfSb1zhPvM1S7OeMwzY1Yv1uEI9vRHwHt2JfUQBD rpP1ED8dZphZfet0xqCzx5iyNLvYzNGenA+DnDslj/ORw07SmQ8vSRzq5SJx/uE= =QKUI -----END PGP SIGNATURE----- Download attachment "xsa119-unstable.patch" of type "application/octet-stream" (3624 bytes) Download attachment "xsa119-4.2.patch" of type "application/octet-stream" (3805 bytes)
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