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[Xen-users] Xen Security Advisory 226 (CVE-2017-12135) - multiple problems with transitive grants

To: xen-announce@xxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxx, xen-users@xxxxxxxxxxxxx, oss-security@xxxxxxxxxxxxxxxxxx

From: Xen.org security team <security@xxxxxxx>

Date: Tue, 29 Aug 2017 12:04:08 +0000

Cc: "Xen.org security team" <security-team-members@xxxxxxx>

Delivery-date: Tue, 29 Aug 2017 12:05:20 +0000

List-id: Xen user discussion <xen-users.lists.xen.org>

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory CVE-2017-12135 / XSA-226 version 7 multiple problems with transitive grants UPDATES IN VERSION 7 ==================== First patch provided in version 6 regressed 32-bit Dom0 or backend domains. The updated patch includes a fix for this. ISSUE DESCRIPTION ================= 1) Code to handle copy operations on transitive grants has built in retry logic, involving a function reinvoking itself with unchanged parameters. Such use assumes that the compiler would also translate this to a so called "tail call" when generating machine code. Empirically, this is not commonly the case, allowing for theoretically unbounded nesting of such function calls. 2) The reference counting and locking discipline for transitive grants is broken. Concurrent use of the transitive grant can leak references on the transitively-referenced grant. IMPACT ====== A malicious or buggy guest may be able to crash Xen. Privilege escalation and information leaks cannot be ruled out. A malicious or buggy guest can leak references on grants it has been given, amounting to a DoS against the grantee. VULNERABLE SYSTEMS ================== All versions of Xen are vulnerable. MITIGATION ========== There is no known mitigation. CREDITS ======= This issue was discovered by Jan Beulich of SUSE. The security team would also like to thank Amazon for helping to identify that the problems with transitive grants were deeper than originally believed. RESOLUTION ========== Applying the appropriate attached pair of patches from the list below addresses this issue: xsa226-unstable/*.patch xen-unstable xsa226-4.9/*.patch Xen 4.9.x, Xen 4.8.x, Xen 4.7.x xsa226-4.6/*.patch Xen 4.6.x xsa226-4.5/*.patch Xen 4.5.x Note that these patches have already been applied to the respective staging trees. Alternatively, applying the appropriate attached patch from the list below works around this issue by disabling transitive grants by default: xsa226.patch xen-unstable, Xen 4.9.x, Xen 4.8.x xsa226-4.7.patch Xen 4.7.x xsa226-4.6.patch Xen 4.6.x xsa226-4.5.patch Xen 4.5.x $ sha256sum xsa226* xsa226*/* b09e07aaf422ae04a4ece5e2c5b5e54036cfae5b5c632bfc6953a0cacd6f60ff xsa226.patch d999767014501d3ac62def06ccd43b97bbbf0ef7d402d3bd70ca96ac9997a14d xsa226-unstable/0001-gnttab-dont-use-possibly-unbounded-tail-calls.patch 4473fd96ce4fdea5e19e0b502d65f20bd279d82473ac34ff404ce2b2cbc10be1 xsa226-unstable/0002-gnttab-fix-transitive-grant-handling.patch ca8b92b2ff58b87e8bec137a34784cbf11e2820659046df6e1d71e23bf7e7dee xsa226-4.5.patch ca77d01172abf263b5b731f26f5e3f74b0b8c75b3e29bee3f65a9318236daba7 xsa226-4.5/0001-gnttab-dont-use-possibly-unbounded-tail-calls.patch de6359e50fd2bb710469da74a596013ce275edb43d3d1c36d41452f88eee9b7d xsa226-4.5/0002-gnttab-fix-transitive-grant-handling.patch 28c7df7edabb91fb2f1fa3fc7d6906bfae75a6e701f1cd335baafaae3e087696 xsa226-4.6.patch 0186f78e99f5f6eec913da8355e0c28946a14a6099a7219bd4e0d385fdf8c306 xsa226-4.6/0001-gnttab-dont-use-possibly-unbounded-tail-calls.patch e34dbba7b94942faeb3e6b7630ba06f01998e2b56be1035d76e67aa47e77457d xsa226-4.6/0002-gnttab-fix-transitive-grant-handling.patch fffcc0a4428723e6aea391ff4f1d27326b5a3763d2308cbde64e6a786502c702 xsa226-4.7.patch 3878c27b77ba24012599289e0e0fb1e5198b1e4efe2f87f7c46def5f335f2fd5 xsa226-4.9/0001-gnttab-dont-use-possibly-unbounded-tail-calls.patch 01d773c5bb4cafe54daf0d14e8a3af899a7c5863513d18927c4a570a74afdb15 xsa226-4.9/0002-gnttab-fix-transitive-grant-handling.patch $ (The .meta file is a prototype machine-readable file for describing which patches are to be applied how.) 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 iQEcBAEBCAAGBQJZpVgpAAoJEIP+FMlX6CvZ228H/jXq5lHGZwtGmbgFY1O6/LBk wrExcAq5iSXVHmfXCR1budkAEYxqCptAbO6FNljvfZVu1bMnGq/ONJs6+UUMCcLb TCLoqqAvSN06dftIcKSCDOW6GpmRs+lEdZYHO6qkEh1hTHY83OjqqQW2jhOGf4iV IS1kytbERXzjzApeTECcUJ4Fxd2sGD8PUMiD4XFagtJu3mjSl5Y1M57z21WBzSuK dHwUzt9sKAd/FOHvpT27GxWw69XR2dI0vKrVtY+Wgudmi4cVt4qnLPirhxkulRVL yVWZeC3dBgjwR1kE2NNuuBXUTHfmyV/kj8s9Jd0z4Z3aGyX/24uZfL1eJq02Sa8= =oTGH -----END PGP SIGNATURE-----

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