Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N

From: Pavel Machek <pavel@ucw.cz>
To: David Herrmann <dh.herrmann@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"Theodore Ts'o" <tytso@mit.edu>
Subject: Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N
Date: Tue, 22 Apr 2014 20:58:02 +0200	[thread overview]
Message-ID: <20140422185802.GA31201@amd.pavel.ucw.cz> (raw)
In-Reply-To: <CANq1E4TPPcdD-8FF_Kc=r44Vrd51Vmt-OVu+EtCC-oEzXkvJvg@mail.gmail.com>

On Tue 2014-04-22 17:19:42, David Herrmann wrote:
> Hi
> 
> On Tue, Apr 22, 2014 at 4:31 PM, Pavel Machek <pavel@ucw.cz> wrote:
> > Such as here?
> >
> > http://www.securityfocus.com/archive/1/507386
> 
> Thanks, that's the first real example someone mentioned.
> 
> Quoted from your link:
> 
> > The reopen does check the inode permission, but it does not require
> > you have any reachable path to the file. Someone _might_ use that as
> > a traditional unix security mechanism, but if so it's probably quite rare.
> 
> In other words, the bug you describe is that /proc/pid/fd/ allows
> access to objects without a reachable path to the only _real_
> filesystem link. But isn't the same true for openat()?

I don't think openat helps you. This is what we are talking about, it
is easy to reproduce. Can you reproduce it without /proc mounted?

I think that chmod 700 . should stop you. Openat seems no worse than
just placing cwd there...

pavel@toy:/tmp$ uname -a
Linux toy.ucw.cz 2.6.32-rc3 #21 Mon Oct 19 07:32:02 CEST 2009 armv5tel
GNU/Linux
pavel@toy:/tmp mkdir my_priv; cd my_priv
pavel@toy:/tmp/my_priv$ echo this file should never be writable >
unwritable_file
# lock down directory
pavel@toy:/tmp/my_priv$ chmod 700 .
# relax file permissions, directory is private, so this is safe
# check link count on unwritable_file. We would not want someone 
# to have a hard link to work around our permissions, would we?
pavel@toy:/tmp/my_priv$ chmod 666 unwritable_file 
pavel@toy:/tmp/my_priv$ cat unwritable_file 
this file should never be writable
pavel@toy:/tmp/my_priv$ cat unwritable_file 
got you
# Security problem here
									Pavel
-- 
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