Date: Tue, 27 Feb 2018 12:00:11 +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 256 - x86 PVH guest without LAPIC may DoS the host -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Xen Security Advisory XSA-256 version 2 x86 PVH guest without LAPIC may DoS the host UPDATES IN VERSION 2 ==================== Public release. ISSUE DESCRIPTION ================= So far, x86 PVH guests can be configured with or without Local APICs. Configurations with Local APICs are identical to x86 HVM guests, and will use as much hardware acceleration support as possible. Configurations without Local APICs try to turn off all hardware acceleration, and disable all software emulation. Multiple paths in Xen assume the presence of a Local APIC without sufficient checks, and can fall over a NULL pointer. On Intel hardware, the logic to turn off hardware acceleration is incomplete and leaves the guest with full control of the real Task Priority Register. IMPACT ====== A malicious or buggy guest may cause a hypervisor crash, resulting in a Denial of Service (DoS) affecting the entire host. VULNERABLE SYSTEMS ================== Xen version 4.8 and onwards are vulnerable. Only x86 systems are vulnerable. ARM systems are not vulnerable. Only x86 PVH guests can exploit the vulnerability. x86 PV and HVM guests cannot exploit the vulnerability. MITIGATION ========== Running only PV or HVM guests avoids the vulnerability. Running all PVH guests with "apic=1" in the guest configuration file (or equivalent thereof) also avoids the vulnerability. CREDITS ======= This issue was discovered by Ian Jackson of Citrix. RESOLUTION ========== Applying the appropriate attached patch resolves this issue. xsa256.patch xen-unstable, Xen 4.10.x, Xen 4.9.x xsa256-4.8.patch Xen 4.8.x $ sha256sum xsa256* 3e45cc3f2ea516e7470083592041e238c0dfe32324790b2fba0e47c9efe38865 xsa256.patch c029fcb67ff7c3c9a2adcb8e6f5e245a0d347acc8a9b3530591a639cbf321349 xsa256-4.8.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 iQEcBAEBCAAGBQJalUe0AAoJEIP+FMlX6CvZsmIH/3B9QnpiL1+NRkGIE62xljEG NfV/vL6gE2ytNMs8PRdhycovQum7qj+l9S53EswiwgiaUFw9VW5Jq9pg1UQlAQ/q 7aIIke33TgkVKwZnb+7ercGfLNWsJAIldGc5emc9lBSBkPOUhFtxmTytdudB6dy1 VMI+MVM1f4xgxEizNN7QstmlaMB34m0WH0nEdoCR8evXlAcmcBi+HwYDouUNnR5x 21DkEBxyslvheX6SI8sbocfrZpT/K2b8B3zdLmd3nO3TF5ypC1daowIk0vl8o4Yj TSx4nsBlJ4V0G0gYa1UDBktUfDbVrpoEcdGb5zO3RhoMhcagzWVD6P6F25aYbiU= =PLNS -----END PGP SIGNATURE----- Download attachment "xsa256.patch" of type "application/octet-stream" (1711 bytes) Download attachment "xsa256-4.8.patch" of type "application/octet-stream" (2062 bytes)
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