|From:||Solar Designer <solar-AT-openwall.com>|
|Subject:||Re: [PATCH] move RLIMIT_NPROC check from set_user() to do_execve_common()|
|Date:||Thu, 14 Jul 2011 19:06:02 +0400|
|Cc:||James Morris <jmorris-AT-namei.org>, Linus Torvalds <torvalds-AT-linux-foundation.org>, Vasiliy Kulikov <segoon-AT-openwall.com>, linux-kernel-AT-vger.kernel.org, Greg Kroah-Hartman <gregkh-AT-suse.de>, Andrew Morton <akpm-AT-linux-foundation.org>, "David S. Miller" <davem-AT-davemloft.net>, kernel-hardening-AT-lists.openwall.com, Jiri Slaby <jslaby-AT-suse.cz>, Alexander Viro <viro-AT-zeniv.linux.org.uk>, linux-fsdevel-AT-vger.kernel.org, KOSAKI Motohiro <kosaki.motohiro-AT-jp.fujitsu.com>, Eric Paris <eparis-AT-redhat.com>, Stephen Smalley <sds-AT-tycho.nsa.gov>, Willy Tarreau <w-AT-1wt.eu>, Sebastian Krahmer <krahmer-AT-suse.de>|
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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to email@example.com More majordomo info at http://vger.kernel.org/majordomo-info.html
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