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Message-ID: <20110713170657.59dae548@notabene.brown>
Date: Wed, 13 Jul 2011 17:06:57 +1000
From: NeilBrown <neilb@...e.de>
To: Solar Designer <solar@...nwall.com>
Cc: 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>,
James Morris <jmorris@...ei.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
linux-fsdevel@...r.kernel.org,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH] move RLIMIT_NPROC check from set_user() to
do_execve_common()
On Wed, 13 Jul 2011 10:31:42 +0400 Solar Designer <solar@...nwall.com> wrote:
> Linus, Neil, Motohiro - thank you for your comments!
>
> On Wed, Jul 13, 2011 at 09:14:08AM +1000, NeilBrown wrote:
> > The contrast is really "failing when trying to use reduced privileges is
> > safer than failing to reduce privileges - if the reduced privileges are not
> > available".
>
> Right.
>
> > Note that there is room for a race that could have unintended consequences.
> >
> > Between the 'setuid(ordinary-user)' and a subsequent 'exit()' after execve()
> > has failed, any other process owned by the same user (and we know where are
> > quite a few) would fail an execve() where it really should not.
>
> It is not obvious to me that this is unintended, and that dealing with
> it in some way makes much of a difference. (Also, it's not exactly "any
> other process owned by the same user" - this only affects processes that
> also run with similar or lower RLIMIT_NPROC. So, for example, if a web
> server is set to use RLIMIT_NPROC of 30, but interactive logins use 40,
> then the latter may succeed and allow for shell commands to succeed.
> This is actually a common combination of settings that we've been using
> on some systems for years.)
I don't think it can be intended to cause 'execve' to fail when a user is at
the NPROC limit - except in the specific case that the process has previously
called setuid. So I feel justified in calling it an unintended consequence.
It my not be a very common consequence but but we all know that uncommon
things do happen.
I agree that having different limits for different cases could make this much
less of a problem, but it doesn't necessarily remove it.
>
> > I think it would be safer to add a test for PF_SUPERPRIV and PF_FORKNOEXEC
> > in current->flags and only fail the execve if both are set.
> > i.e.
> > (current->flags & (PF_SUPERPRIV|PF_FORKNOEXEC)) == (PF_SUPERPRIV|PF_FORKNOEXEC)
> >
> > That should narrow it down to only failing in the particular case that we are
> > interested in.
>
> That's a curious idea, and apparently this is what NetBSD does, but
> unfortunately it does not match a common use case that we are interested
> in - specifically, Apache with suEXEC (which is part of the Apache
> distribution). Here's what happens:
>
> httpd runs as non-root. It forks, execs suexec (SUID root). suexec
> calls setuid() to the target non-root user and execve() on the CGI
> program (script, interpreter, whatever).
>
> Notice how the fork() and the setuid() are separated by execve() of
> suexec itself. Thus, we need to apply the RLIMIT_NPROC check on
> execve() unconditionally (well, we may allow processes with
> CAP_SYS_RESOURCE to proceed despite of the failed check, like it's
> done in -ow patches), or at least not on the condition proposed above.
>
> Alexander
Yes, the PF_FORKNOEXEC test causes problems in that case.
Using just the PF_SUPERPRIV test would still be a good idea I think, but would
not be quite as thorough a check.
Adding a new PF flag would be possible (there seem to be 3 unused) but is
probably not justified.
NeilBrown
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