Date: Tue, 14 Aug 2018 17:19:35 +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 272 v2 - oxenstored does not apply quota-maxentity -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory XSA-272 version 2 oxenstored does not apply quota-maxentity UPDATES IN VERSION 2 ==================== Ammend patch to reference XSA-272 in the commit message. Public release. ISSUE DESCRIPTION ================= The logic in oxenstored for handling writes depended on the order of evaluation of expressions making up a tuple. As indicated in section 7.7.3 "Operations on data structures" of the OCaml manual: http://caml.inria.fr/pub/docs/manual-ocaml/expr.html the order of evaluation of subexpressions is not specified. In practice, different implementations behave differently. IMPACT ====== oxenstored may not enforce the configured quota-maxentity. This allows a malicious or buggy guest to write as many xenstore entries as it wishes, causing unbounded memory usage in oxenstored. This can lead to a system-wide DoS. VULNERABLE SYSTEMS ================== Xen 4.1 and later are potentially vulnerable. Only systems using the OCaml xenstored implementation are potentially vulnerable. Systems using the C xenstored implementation are not vulnerable. Whether the compiled oxenstored binary is vulnerable depends on which compiler was used. OCaml can be compiled either as bytecode (with ocamlc) or as a native binary (with ocamlopt). The following OCaml program demonstrates the issue, and identifies whether the resulting oxenstored binary will skip the quota enforcement. $ cat order.ml let check () = let flag = ref false in let update _ = flag := true; () in List.iter update [1;2;3], !flag let main () = let _, flag = check () in if flag then print_endline "This code is not vulnerable!" else print_endline "This code is vulnerable!" let () = main () $ ocamlc order.ml -o order.bytecode $ ./order.bytecode This code is vulnerable! $ ocamlopt order.ml -o order.native $ ./order.native This code is not vulnerable! To confirm whether an OCaml binary is bytecode or native, use file. $ file order.bytecode order.bytecode: a /usr/bin/ocamlrun script executable (binary data) $ file order.native order.native: ELF 64-bit LSB executable, ... NOTE: These results are applicable to OCaml 4.01.0-5 as distributed in Debian Jessie. These results are not representative of other versions of OCaml, or of other OS distributions. MITIGATION ========== There are no mitigations available. CREDITS ======= This issue was discovered by Christian Lindig of Citrix. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa272.patch All versions of Xen $ sha256sum xsa272* 0da953ca48d0cf0688ecff6a074304a9d2217871809a76ef26b9addeb66ecb3e xsa272.meta 6e0359d89bf65794f16d39198cc90f5c3137bce4eb850e54625ab00e2c568c2c xsa272.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 iQEcBAEBCAAGBQJbcw8fAAoJEIP+FMlX6CvZ1VYIALce26h9Sf0P0joLd/fhUwf4 JcCIaTWvHsy0ucJgpi7i+SCMa7Iz60CriK6dSYlwIuPvka8XU5MDmZ56gbENApDZ ibWMwvyCrgb0BH3VIwJZfk7eaKM7OwKeEnnIrIWaVGsT2StwoZOHgdLRLCTSFJ/K iss3ALSzZ8z7/WqEkBE3JeJ7skrh5nmNp428fJXWYhOyYbqkqyggn6XzBQg/EzGD vabxz4CdYCr1ox7sq42Q/UFeLoWB6CKCLgRgqOGyCrm7K324ymBzRXtXpPUrLEaq ugR27W/zr09e8N/fOhH4dBNCzkktuqclwrfMlFr1WUfiltSDmVwNZkURkvVGeu0= =TPZD -----END PGP SIGNATURE----- Download attachment "xsa272.meta" of type "application/octet-stream" (2083 bytes) Download attachment "xsa272.patch" of type "application/octet-stream" (1271 bytes)
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