Date: Thu, 12 Oct 2017 12:16:30 +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 241 - Stale TLB entry due to page type release race -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory XSA-241 version 3 Stale TLB entry due to page type release race UPDATES IN VERSION 3 ==================== Fix ARM build issue in patches. Public release. ISSUE DESCRIPTION ================= x86 PV guests effect TLB flushes by way of a hypercall. Xen tries to reduce the number of TLB flushes by delaying them as much as possible. When the last type reference of a page is dropped, the need for a TLB flush (before the page is re-used) is recorded. If a guest TLB flush request involves an Inter Processor Interrupt (IPI) to a CPU in which is the process of dropping the last type reference of some page, and if that IPI arrives at exactly the right instruction boundary, a stale time stamp may be recorded, possibly resulting in the later omission of the necessary TLB flush for that page. IMPACT ====== A malicious x86 PV guest may be able to access all of system memory, allowing for all of privilege escalation, host crashes, and information leaks. VULNERABLE SYSTEMS ================== All Xen versions from at least 3.2 onwards are vulnerable. Earlier versions have not been checked. 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. RISK ASSESSMENT =============== A successful attack would require introducing an extended delay between two adjacent operations on one cpu -- long enough for two hypercalls to complete on another cpu. The security team currently has no proof-of-concept for this vulnerability. However, techniques for these sorts of timing-based attacks are continually advancing, so we still recommend users potentially affected by this issue apply the patch as soon as reasonably possible. 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 Jann Horn of Google Project Zero. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa241.patch xen-unstable xsa241-4.9.patch Xen 4.9.x xsa241-4.8.patch Xen 4.8.x, Xen 4.7.x, Xen 4.6.x, Xen 4.5.x $ sha256sum xsa241* 5e239ba4dbd74fd61e59a27f9abc8ea6ba32532bdf81eeb2d7e66f0fd53e40b4 xsa241.meta b8db933d53e7e289652ffda6c46ce284a0254a9f8bc9e1be6793e388009f49ce xsa241.patch 443a5b0818045ada44fad0370ac01af0c96181be5a4078ae3b2575799e4a4e5b xsa241-4.8.patch 927ef14d875556481c38d4065f501211a78eec1c2396a954a4a4abfb9255960f xsa241-4.9.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 iQEcBAEBCAAGBQJZ31v/AAoJEIP+FMlX6CvZsNgIALcJ/DeUN5nv8duBvC3hbAX6 NABBtlVJ6K7qZpAf+04Eztym4bEWXWGtJ1BQVCJ6aPwPZ4aOUodA/zRBEQS7Xp8F 5P5U3Qwa/C+slqLh7QfYdwlkgdMRG67yWIo2xMOEcfORlPjc1wDxohtCQZT9uiMs Y9Xllt/sLhGgYq4+TpNvJyYMzvPp1+oBEuqcR58IZ2aepQJAlPl3LnLdYyN8TAqv MBmli7cRO/vYn5z7aII9NbuF8XEnx0Vfqp7EufLU1LQyG4S9jYXd0xvD6BjjkGWM N/dvJTMq8HXS00VUAoONOv+blq2AdRs9oYD8yeMCglUhpeK8cIaEsYzhOHbCvlI= =1uZK -----END PGP SIGNATURE----- Download attachment "xsa241.meta" of type "application/octet-stream" (2185 bytes) Download attachment "xsa241.patch" of type "application/octet-stream" (6123 bytes) Download attachment "xsa241-4.8.patch" of type "application/octet-stream" (4959 bytes) Download attachment "xsa241-4.9.patch" of type "application/octet-stream" (4981 bytes)
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