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Date: Thu, 14 Jul 2011 19:06:02 +0400
From: Solar Designer <solar@...nwall.com>
To: NeilBrown <neilb@...e.de>
Cc: James Morris <jmorris@...ei.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Vasiliy Kulikov <segoon@...nwall.com>, linux-kernel@...r.kernel.org,
Greg Kroah-Hartman <gregkh@...e.de>,
Andrew Morton <akpm@...ux-foundation.org>,
"David S. Miller" <davem@...emloft.net>,
kernel-hardening@...ts.openwall.com, Jiri Slaby <jslaby@...e.cz>,
Alexander Viro <viro@...iv.linux.org.uk>,
linux-fsdevel@...r.kernel.org,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Eric Paris <eparis@...hat.com>, Stephen Smalley <sds@...ho.nsa.gov>,
Willy Tarreau <w@....eu>, Sebastian Krahmer <krahmer@...e.de>
Subject: Re: [PATCH] move RLIMIT_NPROC check from set_user() to do_execve_common()
On Thu, Jul 14, 2011 at 11:27:51AM +1000, NeilBrown wrote:
> I'm still trying to understand the full consequences, but I agree that there
> is no rush - the code has been this way for quite a while and their is no
> obvious threat that would expedite things (as far as I know).
I don't insist on getting this in sooner than in the next merge window,
although I would have liked that. Relevant userspace vulnerabilities
are being found quite often - I'll include some examples below.
> However I'm not convinced that testing will help all that much - if there are
> problems they will be is rare corner cases that testing is unlikely to find.
This makes sense.
> The only case where this change will improve safety is where:
> 1/ a process has RLIMIT_NPROC set
> 2/ the process is running with root privileges
> 3/ the process calls setuid() and doesn't handle errors.
Yes, and this is a pretty common case.
> I think the only times that a root process would have RLIMIT_NPROC set are:
> 1/ if it explicitly set up rlimits before calling setuid. In this case
> we should be able to expect that the process checks setuid .. maybe
> this is naive(?)
RLIMIT_NPROC could be set by the parent process or by pam_limits. The
machine I am typing this on has:
* hard nproc 200
(as well as other limits) in /etc/security/limits.conf, so if this
machine's kernel let setuid() fail on RLIMIT_NPROC, I would be taking
extra risk of a security compromise by reducing the risk of system
crashes from inadvertent excessive resource consumption by runaway
processes - a tradeoff I'd rather avoid.
> 2/ if the process was setuid-root and inherited rlimits from before, and
> never re-set them. In this case it is easy to imagine that a setuid()
> would not be checked.
Right. (In practice, all kinds of programs tend to forget to check
setuid() return value, though.)
Actually, for the problem to apply to setuid-root programs, they need to
switch their real uid first (more fully become root), then try to switch
to a user - but this is common.
Here are some examples for 2011-2010:
"... missing setuid() retval check in opielogin which leads to easy root
compromise":
http://www.openwall.com/lists/oss-security/2011/06/22/6
"The /usr/lib/libgnomesu/gnomesu-pam-backend suid binary which belongs
to the libgnomesu package is not checking setuid() return values.
As a result, two cooperating users, or users with access to guest,
cgi or web accounts can run arbitrary commands as root very easily."
http://www.openwall.com/lists/oss-security/2011/05/30/2
pam_xauth (exploitable if pam_limits is also used):
http://www.openwall.com/lists/oss-security/2010/08/16/2
A collection of examples from 2006:
http://lists.openwall.net/linux-kernel/2006/08/20/156
> So maybe an alternate 'fix' would be to reset all rlimits to match init_task
> when a setuid-root happens. There are other corner cases where inappropriate
> rlimits can cause setuid programs to behave in ways they don't expect.
> Obviously such programs are buggy, but so are programs that don't check
> 'setuid'. (There is a CVE about mount potentially corrupting mtab.)
Right, but to me possibly resetting rlimits is not an "alternative" to
moving the RLIMIT_NPROC check. setuid-root exec is not the only case
where having setuid() fail on RLIMIT_NPROC is undesirable. We also
don't want such failures with pam_limits, nor on daemon startup:
http://www.openwall.com/lists/oss-security/2009/07/14/2
As to resetting rlimits on SUID/SGID exec, I think this would make
sense for RLIMIT_FSIZE, which would mitigate the mount mtab issue
(thank you for bringing it up!) But it's to be discussed separately.
Alexander
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