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Date: Wed, 30 Apr 2014 09:45:00 +0100
From: John Haxby <john.haxby@...cle.com>
To: oss-security@...ts.openwall.com
Subject: Re: local privilege escalation due to capng_lock as
used in seunshare
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On 29/04/14 23:12, Solar Designer wrote:
> On Tue, Apr 29, 2014 at 05:49:04PM -0400, Steve Grubb wrote:
>> On Tuesday, April 29, 2014 02:20:47 PM Andy Lutomirski wrote:
>>> if (setuid(getuid()) != 0) err(1, "setuid(getuid())");
>>
>> If you do not want the saved uid to be available, you need to use
>> setresuid. That removes it. I would classify this as a bug in the
>> test program.
>
> Not quite.
>
> Per POSIX.1-2001, setuid() "shall set the real user ID, effective
> user ID, and the saved set-user-ID of the calling process to uid"
> if the process has "appropriate privileges". On traditional Unix
> systems, without capabilities, running as root historically does
> constitute "appropriate privileges". If we want current systems to
> support safely running programs written for traditional Unix
> (including SUID root programs), which I think is taken for granted
> by many of us, we must not deviate from those semantics in
> dangerous ways. Any such deviation is a vulnerability in our
> current kernel code or configuration.
Linux's own man page (even going back as far as man-pages-1.67; 3.53
seems to be about current) says:
setuid sets the effective user ID of the current process. If the
effective userid of the caller is root, the real and saved user ID’s
are also set.
Under Linux, setuid is implemented like the POSIX version with the
_POSIX_SAVED_IDS feature. This allows a setuid (other than root) pro-
gram to drop all of its user privileges, do some un-privileged work,
and then re-engage the original effective user ID in a secure manner.
If the user is root or the program is setuid root, special care must
be taken. The setuid function checks the effective uid of the caller
and if it is the superuser, all process related user ID’s are set to
uid. After this has occurred, it is impossible for the program to
regain root privileges.
Thus, a setuid-root program wishing to temporarily drop root privi-
leges, assume the identity of a non-root user, and then regain root
privileges afterwards cannot use setuid. You can accomplish this with
the (non-POSIX, BSD) call seteuid.
If the setuid program were using any uid other than zero, then the
effect that sesploit.c demonstrates is exactly what you would expect.
Going back to Steve's comment about setresuid and using getresuid()
instead of getuid/geteuid neatly demonstrates the problem as well:
$ ./sesploit
Initial privs: ruid=1000 euid=0 suid=0
Dropped privs: ruid=1000 euid=1000 suid=1000
sesploit: seteuid: Operation not permitted
Phew, safe.
$ seunshare -t . $PWD/sesploit
Initial privs: ruid=1000 euid=0 suid=0
Dropped privs: ruid=1000 euid=1000 suid=0
It's baaaack!
When I replace setuid(getuid()) with setresuid(ruid, ruid, ruid) then
sesploit behaves the same in both cases, but that's not what Posix
mandates and what the linux man page says. It's not a bug in the test
program.
>
> Distributions must not ship with settings or programs that allow
> anyone other than an administrator to alter the definition of
> "appropriate privileges" in a way that, while compliant with this
> vague wording in POSIX, introduces a vulnerability for correct
> programs written for traditional Unix systems.
>
> What we have here is a reincarnation of:
>
> "Sendmail Workaround for Linux Capabilities Bug"
> https://www.sendmail.com/sm/open_source/security/security_docs/sendmail.8.10.1.LINUX-SECURITY.txt
>
> albeit in slightly different shape (not entirely in the kernel,
> but with a userland "helper").
>
> I think that sendmail-exposed vulnerability was in the kernel (not
> in sendmail), and I think the vulnerability Andy is reporting is in
> some distros (apparently, Red Hat's).
>
> Of course, it is possible that I have missed something important as
> I did not look into this issue closely, but the above is my
> current understanding based on Andy's message.
>
> Alexander
>
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