linux-kernel - Re: [patch v2] move RLIMIT_NPROC check from set_user() to do_execve

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Date:	Wed, 27 Jul 2011 12:15:49 +1000
From:	NeilBrown <neilb@...e.de>
To:	Vasiliy Kulikov <segoon@...nwall.com>
Cc:	kernel-hardening@...ts.openwall.com,
	Solar Designer <solar@...nwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	James Morris <jmorris@...ei.org>, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	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>, Willy Tarreau <w@....eu>,
	Sebastian Krahmer <krahmer@...e.de>
Subject: Re: [patch v2] move RLIMIT_NPROC check from set_user() to
 do_execve_common()

On Tue, 26 Jul 2011 18:48:48 +0400 Vasiliy Kulikov <segoon@...nwall.com>
wrote:

> Neil, Solar,
> 
> On Tue, Jul 26, 2011 at 14:11 +1000, NeilBrown wrote:
> > I don't really see that failing mmap is any more hackish than failing execve.
> > 
> > Both are certainly hacks.  It is setuid that should fail, but that is
> > problematic.
> > 
> > We seem to agree that it is acceptable to delay the failure until the process
> > actually tries to run some code for the user.  I just think that
> > mapping-a-file-for-exec is a more direct measure of "trying to run some code
> > for the user" than "execve" is.
> > 
> > So they are both hacks, but one it more thorough than the other.  In the
> > world of security I would hope that "thorough" would win.
> 
> Well, I don't mind against something more generic than the check in
> execve(), however, the usefulness of the check in mmap() is unclear to
> me.  You want to make more programs fail after setuid(), but does mmap
> stops really many programs?  Do you know any program doing mmap/dlopen
> after setuid() call?  What if the program will not do any mmap/dlopen
> and e.g. start to handle network connections or do some computations?
> I suppose the latter case is much more often than mmap/dlopen.

I think I didn't make myself clear.
I don't mean we should intercept the mmap system call.

I mean we could intercept the internal kernel function do_mmap_pgoff.

This is used by the mmap system call but also (and more importantly) by the
execve system call and the uselib system call.

So any attempt to map a file and execute the code in that file - whether via
exec or via mapping a shared object - will go through do_mmap_pgoff.

So if we disable do_mmap_pgoff() requests which ask for execute permission
when a setuid has caused RLIMIT_NPROC to be exceeded, then we catch every
attempt to run the user's code as the user.

I won't catch a situation where an interpreter is already loaded into the
root-owned process and the setuid is followed by loading a script and running
that, it is isn't perfect.  But I think it is more general than just trapping
in execve.

NeilBrown

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