NetBSD Security Advisory 2017-005: x86: vulnerabilities in context handling
8 September, 2017 by security-officer@netbsd.org | netbsd
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NetBSD Security Advisory 2017-005
=================================
Topic: x86: vulnerabilities in context handling
Version: NetBSD-current: source prior to Sun, Jul 1st 2017
NetBSD 7.1: affected
NetBSD 7.0 - 7.0.2: affected
NetBSD 6.1 - 6.1.4: affected
NetBSD 6.0 - 6.0.5: affected
Severity: Privilege escalation / Local DoS
Fixed: NetBSD-current: Sun, Jul 1st 2017
NetBSD-7-1 branch: Thu, Jul 20th 2017
NetBSD-7-0 branch: Thu, Jul 20th 2017
NetBSD-7 branch: Thu, Jul 20th 2017
NetBSD-6-1 branch: Tue, Aug 8th 2017
NetBSD-6-0 branch: Tue, Aug 8th 2017
NetBSD-6 branch: Tue, Aug 8th 2017
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
========
Three issues were discovered in the i386 and amd64 ports of NetBSD:
1) A missing check in the trap handler could allow userland to panic an i386
kernel.
2) A race condition could allow userland to panic an i386 kernel, and
escalate privileges on amd64.
3) A missing check in the Linux compatibility layer could allow userland to
exploit the Intel Sysret Vulnerability on amd64.
Technical Details
=================
1) Under certain circumstances, userland can legitimately make the kernel
generate a stack fault when executing 'iret'. However, in the trap handler,
the appropriate check was missing, and this fault could lead to a panic.
2) The same call gate existed on i386 and amd64 that allowed binaries from
previous releases to perform syscalls. However, call gates do not
automatically disable interrupts when entering the target function.
Therefore, there was a condition where the kernel would be executed with
userland registers loaded and interrupts enabled, which breaks a certain
number of assumptions in the i386 and amd64 implementations. On i386 this
can lead to a panic, and on amd64 to a privilege escalation.
3) A missing check in the trap frame could allow userland to have the kernel
execute 'sysret' with a fully-controllable %rip, thereby allowing the
exploitation of the Intel Sysret Vulnerability (see NetBSD-SA2012-003).
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 summarise 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
---- ---- -------- ---------- ----------
sys/arch/i386/i386/trap.c
1.288 1.272.4.2 1.272.6.2 1.272.10.2
sys/arch/i386/i386/locore.S
1.146 1.112.4.1 1.112.6.1 1.112.10.1
sys/arch/i386/i386/machdep.c
1.783 1.752.4.1 1.752.8.1 1.752.12.1
sys/arch/amd64/amd64/trap.c
1.96 1.78.4.3 1.78.6.3 1.78.10.3
sys/arch/amd64/amd64/locore.S
1.124 1.76.2.2 1.76.4.2 1.76.8.2
sys/arch/amd64/amd64/machdep.c
1.254 1.211.2.1 1.211.6.1 1.211.10.1
sys/compat/linux/arch/amd64/linux_machdep.c
1.51 1.48.4.1 1.48.8.1 1.48.4.1
FILE netbsd-6 netbsd-6-0 netbsd-6-1
---- -------- ---------- ----------
sys/arch/i386/i386/trap.c
1.262.8.2 1.262.12.2 1.262.14.2
sys/arch/i386/i386/locore.S
1.95.10.4 1.95.10.2.4.1 1.95.10.3.2.1
sys/arch/i386/i386/machdep.c
1.717.2.8 1.717.2.7.4.1 1.717.2.7.6.1
sys/arch/amd64/amd64/trap.c
1.69.2.3 1.69.2.1.4.2 1.69.2.1.6.2
sys/arch/amd64/amd64/locore.S
1.66.2.2 1.66.2.1.4.1 1.66.2.1.6.1
sys/arch/amd64/amd64/machdep.c
1.175.2.9 1.175.2.7.2.2 1.175.2.8.2.1
sys/compat/linux/arch/amd64/linux_machdep.c
1.39.6.1 1.39.10.1 1.39.12.1
To update from CVS, re-build, and re-install the kernel:
# cd src
# cvs update -d -P -r VERSION sys/arch/i386/i386/trap.c
# cvs update -d -P -r VERSION sys/arch/i386/i386/locore.S
# cvs update -d -P -r VERSION sys/arch/i386/i386/machdep.c
# cvs update -d -P -r VERSION sys/arch/amd64/amd64/trap.c
# cvs update -d -P -r VERSION sys/arch/amd64/amd64/locore.S
# cvs update -d -P -r VERSION sys/arch/amd64/amd64/machdep.c
# cvs update -d -P -r VERSION sys/compat/linux/arch/amd64/linux_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 issues, Chuck Silvers for proposing
a better fix for issue 2).
Revision History
================
2017-09-08 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-SA2017-00X.txt.asc
Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.org/ and http://www.NetBSD.org/Security/ .
Copyright 2017, The NetBSD Foundation, Inc. All Rights Reserved.
Redistribution permitted only in full, unmodified form.
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