-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 NetBSD Security Advisory 2018-005 ================================= Topic: Privilege separation bug in Xen-amd64 Version: NetBSD-current: source prior to Sun, Dec 31st 2017 NetBSD 7.1.2: not affected NetBSD 7.1 - 7.1.1: affected NetBSD 7.0 - 7.0.2: affected NetBSD 6.1 - 6.1.5: affected NetBSD 6.0 - 6.0.6: affected Severity: Privilege escalation / Local DoS Fixed: NetBSD-current: Sun, Dec 31st 2017 NetBSD-7-1 branch: Mon, Jan 22nd 2018 NetBSD-7-0 branch: Mon, Jan 22nd 2018 NetBSD-7 branch: Mon, Jan 22nd 2018 NetBSD-6-1 branch: Mon, Feb 19th 2018 NetBSD-6-0 branch: Mon, Feb 19th 2018 NetBSD-6 branch: Mon, Feb 19th 2018 Teeny versions released later than the fix date will contain the fix. Please note that NetBSD releases prior to 6.0 are no longer supported. It is recommended that all users upgrade to a supported release. Abstract ======== A mistake the Xen-amd64 port of NetBSD allowed unprivileged users to read from and write to the CPU's I/O ports. This could be used to escalate privileges. Technical Details ================= The kernel uses several flags that define CPU protections, and in particular, SEL_KPL and SEL_UPL, that respectively define "kernel" privileges and "user" privileges in the %cs register. 64bit Xen PV guests run, by design, in ring3, the same protection level as userland. As a result, SEL_KPL equals SEL_UPL. Xen uses a specific iopl privilege mechanism to control access rights to the CPU I/O ports: it expects the iopl value to match the intended privilege, and not the hardware privilege. Therefore, if the kernel wanted to prevent userland from accessing the CPU I/O ports, it had to set the iopl to ring0, even if the kernel actually runs in ring3. A mistake existed in NetBSD, where iopl was unintentionally set to ring3, allowing userland to access CPU I/O ports. The mistake in question was a confusion with the privilege flags: iopl was set to SEL_KPL, but in the case of Xen-amd64 this was equal to SEL_UPL, which meant ring3. Solutions and Workarounds ========================= For all NetBSD versions, you need to obtain fixed kernel sources, rebuild and install the new kernel, and reboot the system. The fixed source may be obtained from the NetBSD CVS repository. The following instructions briefly summarize how to upgrade your kernel. In these instructions, replace: ARCH with your architecture (from uname -m), KERNCONF with the name of your kernel configuration file and VERSION with the file version below File versions containing the fixes: FILE HEAD netbsd-7 netbsd-7-0 netbsd-7-1 ---- ---- -------- ---------- ---------- src/sys/arch/amd64/amd64/machdep.c 1.280 1.211.2.2 1.211.6.2 1.211.10.2 src/sys/arch/amd64/include/segments.h 1.34 1.24.12.1 1.24.16.1 1.24.22.1 src/sys/arch/i386/i386/machdep.c 1.800 1.752.4.2 1.752.8.2 1.752.12.2 src/sys/arch/i386/include/segments.h 1.64 1.54.30.1 1.54.34.1 1.54.38.1 src/sys/arch/x86/x86/vm_machdep.c 1.30 1.25.4.2 1.25.8.2 1.25.4.1.2.1 FILE netbsd-6 netbsd-6-0 netbsd-6-1 ---- -------- ---------- ---------- src/sys/arch/amd64/amd64/machdep.c 1.175.2.10 1.175.2.7.2.3 1.175.2.8.2.2 src/sys/arch/amd64/include/segments.h 1.22.10.1 1.22.14.1 1.22.16.1 src/sys/arch/i386/i386/machdep.c 1.717.2.9 1.717.2.7.4.2 1.717.2.7.6.2 src/sys/arch/i386/include/segments.h 1.54.10.1 1.54.16.1 1.54.24.1 src/sys/arch/x86/x86/vm_machdep.c 1.14.2.1 1.14.6.1 1.14.8.1 To update from CVS, re-build, and re-install the kernel: # cd src # cvs update -d -P -r VERSION sys/arch/amd64/amd64/machdep.c # cvs update -d -P -r VERSION sys/arch/amd64/include/segments.h # cvs update -d -P -r VERSION sys/arch/i386/i386/machdep.c # cvs update -d -P -r VERSION sys/arch/i386/include/segments.h # cvs update -d -P -r VERSION sys/arch/x86/x86/vm_machdep.c # ./build.sh kernel=KERNCONF # mv /netbsd /netbsd.old # cp sys/arch/ARCH/compile/obj/KERNCONF/netbsd /netbsd # shutdown -r now For more information on how to do this, see: http://www.NetBSD.org/guide/en/chap-kernel.html Thanks To ========= Maxime Villard for finding and fixing the issue. Revision History ================ 2018-04-09 Initial release More Information ================ Advisories may be updated as new information becomes available. The most recent version of this advisory (PGP signed) can be found at http://ftp.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2018-005.txt.asc Information about NetBSD and NetBSD security can be found at http://www.NetBSD.org/ and http://www.NetBSD.org/Security/ . Copyright 2018, The NetBSD Foundation, Inc. All Rights Reserved. Redistribution permitted only in full, unmodified form. -----BEGIN PGP SIGNATURE----- iQIcBAEBAgAGBQJay9YPAAoJEAZJc6xMSnBuvvkP/i++oukmYnYwveWd20jYIQ7r /6K5FrdfslPkk8Wk0Rm8MV5QlPkp+aouJ/id/3ywQ7QoUx8qQOITo3yjTrVGqKkK ksF2I5JLh+VwrOr4sitvanRST2b7FN6XqLL+UZf5aQg70ck/pobrI2nnH03Cs18I snDno1hIly4WnBTu7H4+LvlUPgr+Ta6Ipix7/EwpQPKY6NImYG+LEIX8sEAe3D/e RDyXV0O7/H6sf6cUmO4mNCnt1SbjblkS2LBMvYgrmrAWY5K6P93KYnQJ3HfIGwac xM9FrQveZ2FYfoVx/OhIAmLrQXKTUde/YRhwwS8uyz4WLwaI1K31YjrVs6I2lcXn K8AhGLzhHYr7d/UO9KgNufHlWtR1KEuqEj0LLOk3gFONmHkE37isZ6vnCR4+J3Ol aVWGzd53pOxAz4i4e7KFfEDd91Xfxex1A561A6S7HGJtiTZX9UdvXraIU7/srC32 TjLbR7tHbiGm8O5Ajj9AvbCYaEcHaRNF6BImzz5SlZSsov9alCMuiotswgucyTkF yu8rObLYJTGhNKHPrqMAN18jGQn9pap+KyT+DMcsDLQxtqEzuoqaSU6pSS76vt2E bUb+RFhELRFCYjUr+UfWhCLRXlyK/2d6p7t3NAra0b0EVWQ0CoRSwEXwYKq76I8E Lqk8n1FjrRmAqthVEqHi =pr1D -----END PGP SIGNATURE-----