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NetBSD Security Advisory 2000-001: procfs

Feb 17, 2000, 20:44 (1 Talkback[s])
(Other stories by Daniel Carosone)

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.