Date: Thu, 12 Oct 2017 12:16:32 +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 242 - page type reference leak on x86 -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory XSA-242 version 2 page type reference leak on x86 UPDATES IN VERSION 2 ==================== Public release. ISSUE DESCRIPTION ================= The page type system of Xen requires cleanup when the last reference for a given page is being dropped. In order to exclude simultaneous updates to a given page by multiple parties, pages which are updated are locked beforehand. This locking includes temporarily increasing the type reference count by one. When the page is later unlocked, the context precludes cleanup, so the reference that is then dropped must not be the last one. This was not properly enforced. IMPACT ====== A malicious or buggy PV guest may cause a memory leak upon shutdown of the guest, ultimately perhaps resulting in Denial of Service (DoS) affecting the entire host. VULNERABLE SYSTEMS ================== All Xen versions from 3.4 onwards are vulnerable. Xen versions 3.3 and earlier are not vulnerable. Only x86 systems are affected. ARM systems are not affected. Only x86 PV guests can leverage the vulnerability. x86 HVM guests cannot leverage the vulnerability. MITIGATION ========== Running only HVM 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. xsa242.patch xen-unstable xsa242-4.9.patch Xen 4.9.x, Xen 4.8.x, Xen 4.7.x, Xen 4.6.x, Xen 4.5.x $ sha256sum xsa242* 168db3aef00806025afa255dee35cd0c042706a27a0256744e4d63f3ee86a2e8 xsa242.meta 16848f71311c2fd6a38afd7602e59211c89a3daf29b874097dba0b1e31ba6eec xsa242.patch 5e66b6b1d1cd400905d3abd3478144539c3afa24f5a744a11809d9c5eb517b98 xsa242-4.9.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 iQEcBAEBCAAGBQJZ31wBAAoJEIP+FMlX6CvZs4YH+QH5lTpge4JLyHQRJbLry52Z 70oB+1vZIsoWg9/XONE9/l1kei0WOGPh4Pt2AWUZOXy8I/euHlMUeGZchl7cQ73M 6EOPjQ1+EXv+vIePwyjZiZmjKQJYQDZ5IsNZ3lz2oV27SkppSW6KKPFlj9G3Dc+E Fv0JwawHNBruGQu9RYWukLbCKn9g4Z0OD/4OwpzF0PY3c/zqk9aYjg318i2Na5zu tWDI9+srfzgvT9N2+om/hVBQYHp48OOIUIGtMz7M4A33LBySsETigpBaCiNmyNeG +l3ONWKF8XNeJbpYGtd3jClgXLg8Hy5MgalSCKOyB2XAgl0y2BSX3tyhOnQZKcs= =tqOh -----END PGP SIGNATURE----- Download attachment "xsa242.meta" of type "application/octet-stream" (2287 bytes) Download attachment "xsa242.patch" of type "application/octet-stream" (1715 bytes) Download attachment "xsa242-4.9.patch" of type "application/octet-stream" (1459 bytes)
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