Date: Wed, 16 Feb 2000 07:59:08 +1100
From: Daniel Carosone security-officer@NETBSD.ORG
To: BUGTRAQ@SECURITYFOCUS.COM
Subject: NetBSD Security Advisory 2000-001
NetBSD Security Advisory 2000-001
Topic: procfs security hole
Version: NetBSD 1.4.1 and prior; NetBSD-current until 20000126
Severity: If the proc filesystem is mounted, any user can become
root
Abstract
The procfs filesystem makes the different resources of a process
available under the directory /proc//. One of these resources is
the memory image of the process. Reading to and writing from this
special file is restricted. However, by tricking a setuid binary to
write into this file, this restriction can be circumvented, and the
memory image of another setuid binary can be manipulated in such a
way that it will execute a shell.
Note that the procfs filesystem is not used in default NetBSD
installations.
Technical Details
Access to /proc//mem is protected by the procfs_checkioperm()
function in sys/miscfs/procfs/procfs_mem.c. However, this function
does allow access if the effective uid of the writing process is 0.
If a setuid process can be manipulated in such a way that it writes
to a filedescriptor referring to an open /proc//mem, this check
will not protect the memory written. One way to do this is to open
/proc//mem, dup2() that filedescriptor onto filedescriptor 2, do a
seek on that filedescriptor to an appropriate offset (the right
stack address), execute a setuid binary, and trick it into writing
an error message that contains code to execute a shell. If the main
program, meanwhile, has executed another setuid binary, this will
have its stack overwritten, and execute a shell, giving the user
root access.
Solutions and Workarounds
A patch is available for NetBSD 1.4.1, that revokes all vnodes
referring to procfs files when a process is about to execute a
setuid or setgid binary. It is located at:
ftp://ftp.NetBSD.ORG/pub/NetBSD/misc/security/patches/20000130-procfs
NetBSD-current since 20000126 is not vulnerable. Users of
NetBSD-current should upgrade to a source tree later than
20000126
If this action cannot be taken, an immediate workaround is to
disable the use of the proc filesystem. It is not mounted by
default in NetBSD, and nothing in the NetBSD base tree depends on
it. You can disable it by removing any procfs lines from
/etc/fstab.
Thanks To
Jason Thorpe and Charles Hannum for commenting on the fix.
Revision History
1999/01/29 – initial version
1999/01/31 – corrected spelling of “onto”
1999/02/13 – minor editorial changes for release.
More Information
Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.ORG/ and
http://www.NetBSD.ORG/Security/.
Copyright 1999, The NetBSD Foundation, Inc. All Rights
Reserved.