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TUCoPS :: Linux :: General :: ciack064.htm

Linux Kernel Capability Vulnerability



Linux Kernel Capability Vulnerability Privacy and Legal Notice

CIAC INFORMATION BULLETIN

K-064: Linux Kernel Capability Vulnerability

August 15, 2000 21:00 GMT


PROBLEM:       A vulnerability exists in the setcap(2) call. This call will
               attempt to break down root permisssions into a series of capabilities.
               It may be possible to set the capabilities so that
               a setuid program cannot fully give up its root privileges thus allowing
               a normal user elevated privileges.
PLATFORM:      All linux kernels 2.XXX through 2.2.15.
DAMAGE:        Local users can gain root privileges.
SOLUTION:      Due to the many programs using the linux kernel capability, it
               is strongly suggested that the linux kernel be upgraded to
               2.2.16. Information on workarounds for sendmail, SGI Linux and
               Sendmail, Inc. switches follow.


VULNERABILITY  The risk is MEDIUM, the vulnerability has been publicly
ASSESSMENT:    discussed. 


[******  Begin Sendmail Advisory ******]

-----BEGIN PGP SIGNED MESSAGE-----

                SENDMAIL SECURITY TEAM ADVISORY

        Sendmail Workaround for Linux Capabilities Bug

The Sendmail Consortium and Sendmail, Inc. has been informed of a
serious problem in the Linux kernel that can be used to get root
access.  This is not a sendmail security problem, although sendmail
is one of the vectors for this attack.

PROBLEM

        There is a bug in the Linux kernel capability model for versions
        through 2.2.15 that allows local users to get root.  Sendmail is
        one of the programs that can be attacked this way.  This problem
        may occur in other capabilities-based kernels.

SOLUTION

        The correct fix is to update your Linux kernel to version
        2.2.16.  This is the only way to ensure that other programs
        running on Linux cannot be attacked by this bug.

WORKAROUND

        Sendmail 8.10.2 has added a check to see if the kernel has
        this bug, and if so will refuse to run.  This version also
        does more detailed checks on certain system calls, notably
        setuid(2), to detect other possible attacks.  Although there
        are no known attacks, this version is strongly recommended,
        whether or not you have a vulnerable kernel.

        Sendmail 8.10.2 can be obtained from:

        ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.10.2.tar.gz
        ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.10.2.tar.Z
        ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.10.2.tar.sig

        and has MD5 signatures:

        acb8b6f50869a058a9baaa4fb4692c4b sendmail.8.10.2.tar.Z
        00705e5ca3412604cebd052e0d7aefcd sendmail.8.10.2.tar.gz
        92dca37fb68a2a44f02c292656c123b6 sendmail.8.10.2.tar.sig

        You only need one of the first two files (either the gzip'ed
        version or the compressed version).  The .sig file is a PGP
        signatures of the tar file (after uncompressing it).  It is
        signed with the Sendmail Signing Key/2000, available on the web
        site (http://www.sendmail.org/) or on the public key servers.

        Note however that installing this sendmail patch does not
        fully protect you from attack.  Other programs are probably
        vulnerable.

ACKNOWLEDGEMENTS

        Several people contributed to this advisory.  Wojciech Purczynski
        of Elzab Soft first identified the problem.  Alan Cox verified
        and patched the Linux kernel bug.  Gregory Neil Shapiro and other
        members of the Sendmail Consortium helped identify the problem
        and produce the sendmail workaround.

DETAILS OF THE VULNERABILITY

        The problem lies in the setcap(2) call, which is not documented
        on most Linux-based systems (we think there might be a man page
        on Mandrake).  This call, based on the unratified Posix 1e draft,
        attempts to break down root permissions into a series of
        capabilities.  Normally root has all capabilities and normal
        users have none of the capabilities.

        One such capability is the ability of a process to do an
        arbitrary setuid(2) call.  As documented in ISO/IEC 9945-1
        (ANSI/IEEE Std 1003.1) POSIX Part 1:

                4.2.2.2 Description
                ...
                   If {_POSIX_SAVED_IDS} is defined:

                   (1) If the process has appropriate privileges, the
                       setuid() function sets the real user ID, effective
                       user ID, and the saved set-user-ID to uid.

                   (2) If the process does not have the appropriate
                       privileges, but uid is equal to the real user ID
                       or the saved set-user-ID, the setuid() function
                       sets the effective user ID to uid; the real user
                       ID and saved set-user-ID remain unchanged by this
                       function call.

        The CAP_SETUID capability represents the "appropriate privileges".

        Normally this would not be an issue, since a setuid root program
        would simply have capability reinstated.  However, Linux has
        an added capability CAP_SETPCAP that controls the ability of a
        process to inherit capabilities; this capability does affect
        setuid programs.  It is possible to set the capabilities such
        that a setuid program does not have "appropriate privileges."
        The effect of this is that a root program cannot fully give up
        its root privileges (since the saved set-user-ID cannot be
        reset).

        Note that checking the return value from setuid() is insufficient;
        the setuid(getuid()) succeeds even when the process does not have
        "appropriate privileges."

        The sendmail patch attempts a setuid(0) after a setuid(getuid());
        under normal circumstances this should fail (unless of course
        the real uid is root).  If this setuid(0) succeeds, then the
        kernel has failed to properly give up permissions and sendmail
        will refuse to continue running.

        This problem can, of course, appear in any setuid root program
        that attempts to cede special permissions.

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[******  End Sendmail Advisory ******]

[******  Begin SGI Advisory ******]

-----BEGIN PGP SIGNED MESSAGE-----

______________________________________________________________________________
                          SGI Security Advisory

        Title:   Linux Kernel Capability Vulnerability
        Number:  20000802-01-P
        Date:    August 14, 2000
______________________________________________________________________________

SGI provides this information freely to the SGI user community for its
consideration, interpretation, implementation and use.   SGI recommends
that this information be acted upon as soon as possible.

SGI provides the information in this Security Advisory on an "AS-IS" basis
only, and disclaims all warranties with respect thereto, express, implied
or otherwise, including, without limitation, any warranty of merchantability
or fitness for a particular purpose.  In no event shall SGI be liable for
any loss of profits, loss of business, loss of data or for any indirect,
special, exemplary, incidental or consequential damages of any kind arising
from your use of, failure to use or improper use of any of the instructions
or information in this Security Advisory.
______________________________________________________________________________


- -----------------------
- --- Issue Specifics ---
- -----------------------

Sendmail, Inc. (http://www.sendmail.com/) reported a vulnerability in the
capability model of the Linux kernel which can lead to a root compromise.

SGI ProPack for Linux uses a modified kernel optimized for SGI Linux
hardware.  This kernel is based on the standard kernel available from
http://www.kernel.org/ with SGI patches from http://oss.sgi.com/

SGI has investigated the issue and recommends the following steps for
neutralizing the exposure.  It is HIGHLY RECOMMENDED that these measures
be implemented on ALL vulnerable SGI systems.  This issue will be
corrected in future releases of SGI ProPack for Linux.


- --------------
- --- Impact ---
- --------------

SGI ProPack for Linux comes with a modified Linux kernel and is installed
by default on various Linux distributions.  For more information, see:
http://oss.sgi.com/projects/ or http://support.sgi.com/linux/

A local user account on the vulnerable system is required in order to
exploit capability model of the Linux kernel.

This Linux kernel vulnerability can lead to a root compromise.

This Linux kernel vulnerability was reported by Sendmail, Inc.:
http://www2.sendmail.com/alert/ and
http://sendmail.net/?feed=000607linuxbug

This Linux kernel vulnerability has been publicly discussed in Usenet
newsgroups and mailing lists.


- --------------------------
- --- Temporary Solution ---
- --------------------------

Unfortunately, there are no immediate or temporary workarounds for
this issue.  This issue can only be addressed with the installation
of an RPM.


- ----------------
- --- Solution ---
- ----------------

This issue is fixed in 2.2.16 Linux kernel. For more information see:
http://www.linux.org.uk/VERSION/relnotes.2216.html

SGI ProPack 1.4 plans to use the 2.2.16 Linux kernel as its base.
SGI has back-ported the fix to older Linux kernels in ProPack 1.2 and 1.3.
SGI ProPack 1.2 is based on the 2.2.13 Linux kernel.
SGI ProPack 1.3 is based on the 2.2.15 Linux kernel.


   Version           Vulnerable      Patch #      Other Actions
   ---------------   -----------     -------      -------------
   SGI ProPack 1.2      yes           3993
   SGI ProPack 1.3      yes           3991
   SGI ProPack 1.4      no


Patches are available via the web, anonymous FTP and from your SGI
service/support provider.

SGI patches and RPMs for Linux can be found at:
http://support.sgi.com/linux/ and click on patches link and
http://oss.sgi.com/projects/sgilinux-combined/download/security-fixes/

SGI patches for Windows NT or 2000 can be found at:
http://support.sgi.com/nt/

IRIX 5.2-6.4 Recommended/Required Patch Sets can be found at:
http://support.sgi.com/irix/ and ftp://patches.sgi.com/support/patchset/

IRIX 6.5 Maintenance Release Streams can be found at:
http://support.sgi.com/irix/ and ftp://patches.sgi.com/support/relstream/

IRIX Security Patchsets can be found at:
http://www.sgi.com/support/security/ and ftp://sgigate.sgi.com/patches/

SGI Security Advisories can be found at:
http://www.sgi.com/support/security/ and ftp://sgigate.sgi.com/security/

The primary SGI anonymous FTP site for security information and patches
is sgigate.sgi.com (204.94.209.1). Security information and patches can be
found in the ~ftp/security and ~ftp/patches directories, respectively.

For security and patch management reasons, ftp.sgi.com (mirror of sgigate) lags
behind and does not do a real-time update of ~ftp/security and ~ftp/patches


- ------------------------
- --- Acknowledgments ----
- ------------------------

SGI wishes to thank the users of the Internet Community at large for their
assistance in this matter.


- -----------------------------------------
- --- SGI Security Information/Contacts ---
- -----------------------------------------

If there are questions about this document, email can be sent to
cse-security-alert@sgi.com.

                      ------oOo------

SGI provides security information and patches for use by the entire SGI
community.  This information is freely available to any person needing
the information and is available via anonymous FTP and the Web.

The primary SGI anonymous FTP site for security information and patches
is sgigate.sgi.com (204.94.209.1).  Security information and patches
are located under the directories ~ftp/security and ~ftp/patches,
respectively.

The SGI Security Headquarters Web page is accessible at the URL:
http://www.sgi.com/support/security/

For issues with the patches on the FTP sites, email can be sent to
cse-security-alert@sgi.com.

For assistance obtaining or working with security patches, please
contact your SGI support provider.

                      ------oOo------

SGI provides a free security mailing list service called wiretap and
encourages interested parties to self-subscribe to receive (via email) all
SGI Security Advisories when they are released. Subscribing to the mailing
list can be done via the Web (http://www.sgi.com/support/security/wiretap.html)
or by sending email to SGI as outlined below.

% mail wiretap-request@sgi.com
subscribe wiretap 
end
^d

In the example above,  is the email address that you
wish the mailing list information sent to.  The word end must be on a
separate line to indicate the end of the body of the message. The
control-d (^d) is used to indicate to the mail program that you are
finished composing the mail message.


                      ------oOo------

SGI provides a comprehensive customer World Wide Web site. This site is
located at http://www.sgi.com/support/security/ .

                      ------oOo------

For reporting *NEW* SGI security issues, email can be sent to
security-alert@sgi.com or contact your SGI support provider.  A
support contract is not required for submitting a security report.

______________________________________________________________________________
      This information is provided freely to all interested parties
      and may be redistributed provided that it is not altered in any
      way, SGI is appropriately credited and the document retains and
      includes its valid PGP signature.


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[******  End SGI Advisory ******]

[******  Begin Sendmail Inc Alert ******]

Sendmail Workaround for Linux Capabilities Bug

PROBLEM

There is a bug in the Linux kernel capability model for versions from 2.XXX through 2.2.15 that
allows local users to get root. Sendmail is one of the programs that can be attacked this way.
Please note that this is NOT a Sendmail security issues, but rather a Linux issue that can manifest
itself in the sendmail program. This problem may also occur in other capabilities-based kernels.

FIX

The correct fix is to update your Linux kernel to version 2.2.16. This is the only way to ensure that
other programs running on Linux cannot be attacked.

WORKAROUND

If you are currently unable to obtain an upgrade from your vendor, we strongly recommend that
you download the Sendmail 2.0.5 patch. Sendmail Switch 2.0.5 has added a check to see if the
kernel has this bug, and if so will refuse to run. This version also does more detailed checks on
certain system calls, notably setuid(2), to detect other possible attacks. Although there are no
known attacks, this version is strongly recommended, whether or not you have a vulnerable kernel.
Please download the correct product-specified version of the patch indicated below:

      Sendmail Single Switch 2.0.5 patch
      Sendmail Secure Switch 2.0.5 patch
      Sendmail Multi Switch with Openswitch MTA 
      Sendmail Multi Switch with Cryptoswitch MTA
      Open Source sendmail 

DETAILS OF THE VULNERABILITY

The problem lies in the setcap(2) call, which is not documented on most Linux-based systems (we
think there might be a man page on Mandrake). This call, based on the unratified Posix 1e draft,
attempts to break down root permissions into a series of capabilities. Normally root has all
capabilities and normal users have none of the capabilities.

One such capability is the ability of a process to do an arbitrary setuid(2) call. As documented in
ISO/IEC 9945-1 (ANSI/IEEE Std 1003.1) POSIX Part 1:

      4.2.2.2 Description
      ...

           If {_POSIX_SAVED_IDS} is defined:

        1.If the process has appropriate privileges, the setuid() function sets the real user
           ID, effective user ID, and the saved set-user-ID to uid.
        2.If the process does not have the appropriate privileges, but uid is equal to the
           real user ID or the saved set-user-ID, the setuid() function sets the effective
           user ID to uid; the real user ID and saved set-user-ID remain unchanged by this
           function call.

The CAP_SETUID XXX capability represents the "appropriate privileges".

Normally this would not be an issue, since a setuid root program would simply have capability
reinstated. However, Linux has an added capability CAP_XXX that controls the ability of a process
to inherit capabilities; this capability does affect setuid programs. It is possible to set the
capabilities such that a setuid program does not have "appropriate privileges." The effect of this is
that a root program cannot fully give up its root privileges (since the saved set-user-ID cannot be
reset).

Note that checking the return value from setuid() is insufficient; the setuid(getuid()) succeeds even
when the process does not have "appropriate privileges."

The Sendmail Switch 2.0.5 patch attempts a setuid(0) after a setuid(getuid()); under normal
circumstances this should fail (unless of course the real uid is root). If this setuid(0) succeeds, then
the kernel has failed to properly give up permissions and sendmail will refuse to continue running.

This problem can, of course, appear in any setuid root program that attempts to cede special
permissions.

Copyright  1999-2000 Sendmail Inc., all rights reserved.

[******  End Sendmail Inc Alert ******]


CIAC wishes to acknowledge the contributions of Sendmail, Inc., SGI, and sendmail.org for the information contained in this bulletin.



CIAC services are available to DOE, DOE Contractors, and the NIH. CIAC can be contacted at:

    Voice:          +1 925-422-8193 (7 x 24)
    FAX:            +1 925-423-8002
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    E-mail:          ciac@llnl.gov
    World Wide Web:  http://www.ciac.org/
                     http://ciac.llnl.gov
                     (same machine -- either one will work)
    Anonymous FTP:   ftp.ciac.org
                     ciac.llnl.gov
                     (same machine -- either one will work)

This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising or product endorsement purposes.
UCRL-MI-119788
[Privacy and Legal Notice]


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