NetBSD Security Advisory 2012-003: Intel processors sysret to non-canonical address behaviour

[NetBSD Security Officer <security-officer%NetBSD.org@localhost>]

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                NetBSD Security Advisory 2012-003
                =================================

Topic:          Intel processors sysret to non-canonical address behaviour


Version:        NetBSD-current:         source prior to June 11, 2012
                NetBSD 6.0 Beta:        affected
                NetBSD 5.1.*:           affected
                NetBSD 5.1:             affected
                NetBSD 5.0.*:           affected
                NetBSD 5.0:             affected
                NetBSD 4.0.*:           affected
                NetBSD 4.0:             affected

Severity:       local DoS, possibly local user privilege escalation

Fixed:          NetBSD-current:         June 12th, 2012
                NetBSD-6 branch:        June 12th, 2012
                NetBSD-5 branch:        June 12th, 2012
                NetBSD-5-1 branch:      June 12th, 2012
                NetBSD-5-0 branch:      June 12th, 2012
                NetBSD-4 branch:        June 12th, 2012
                NetBSD-4-0 branch:      June 12th, 2012

Please note that NetBSD releases prior to 4.0 are no longer supported.
It is recommended that all users upgrade to a supported release.


Abstract
========

On Intel CPUs, the sysret instruction can be manipulated into returning 
to specific non-canonical addresses, which may yield a CPU reset.
We cannot currently rule out with utter confidence that this vulnerability
could not also be used to execute code with kernel privilege instead
of crashing the system.

This vulnerability has been assigned CVE-2012-0217.


Technical Details
=================

This is a vulnerability following from a difference of behaviour of
sysret in Intel's version of the amd64 architecture, em64t.

System calls may be implemented as using the em64t syscall/sysret
instruction pair.

syscall saves the context of the calling unprivileged process before
executing a system call in kernel mode; sysret restores it and resumes
ordinary operations in user mode.

In the Intel implementation of sysret, if you have invalid information
about the "next instruction" address in your saved context, the sysret
instruction will trigger a trap in kernel space. However the sysret
instruction is executed with the user stack pointer already loaded,
so the kernel fault frame is written to the user stack.
The kernel is unable to safely recover from this, so must ensure
that the trap doesn't happen.

If your invalid "next instruction" address is in kernel space or
in user space (and in the latter case, not where your program is)
the program will segfault.
If it is in the gap between user space and kernel space, the CPU
will reset, except if someone managed to seed the address location
with a valid instruction.


Solutions and Workarounds
=========================

The fix in the following versions will disallow sysret to
an invalid address.

src/sys/arch/amd64/amd64/machdep.c
HEAD            1.184
netbsd-6        1.175.2.6
netbsd-5        1.102.4.14
netbsd-5-1      1.102.4.13.2.1
netbsd-5-0      1.102.4.10.2.2
netbsd-4        1.44.2.7
netbsd-4-0      1.44.2.3.6.1

src/sys/arch/amd64/amd64/netbsd32_machdep.c
HEAD            1.77
netbsd-6        1.74.10.2
netbsd-5        1.55.4.4
netbsd-5-1      1.55.4.3.2.1
netbsd-5-0      1.55.6.3
netbsd-4        1.30.2.4
netbsd-4-0      1.30.2.1.6.1


Thanks To
=========

Thanks to Manuel Bouyer for providing the fix, and David Laight for
help with writing the advisory. Also, thanks to Rafal Wojtczuk for
bringing the issue to our attention.


Revision History
================

        2012-06-14      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-SA2012-003.txt.asc

Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.org/ and http://www.NetBSD.org/Security/ .


Copyright 2012, The NetBSD Foundation, Inc.  All Rights Reserved.
Redistribution permitted only in full, unmodified form.

$NetBSD $

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