linux-kernel - Re: [RFC 2/2] fs,proc: Respect FMODE

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Date:	Wed, 23 Apr 2014 00:12:13 +0200
From:	Pavel Machek <pavel@....cz>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening
 /proc/pid/fd/N

On Tue 2014-04-22 23:31:11, David Herrmann wrote:
> Hi
> 
> On Tue, Apr 22, 2014 at 8:58 PM, Pavel Machek <pavel@....cz> wrote:
> > 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...
> 
> Example1:
> $ mkdir -p subdir/next
> $ chmod 000 subdir
> $ touch subdir/next/test
> => EACCES
> $ cd subdir
> => EACCES
> 
> Example2:
> $ mkdir -p subdir/next
> $ cd subdir/next
> $ chmod 000 ..
> $ touch test
> => SUCCESS
> 
> This is the exact same situation. The filesystem tree is exactly the
> same in both situations, but in the first example CWD is outside of
> "subdir", in the second example CWD is inside of "subdir". Thus, they
> can create files in that directory, even though they have no access to
> _any_ absolute path to that directory.
> 
> This is the exact same race that you describe via /proc/self/fd/. But
> instead of keeping a ref to the dir via CWD, in your example you keep
> the ref via a FD in that exact same directory and access it via
> /proc.

Yes, that's how permissions work. You snipped my example. Can you show
how to write to "unwritable_file" below without /proc? Because I
believe I got my chmods right.

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


> (Hint: instead of using CWD, you can also keep an FD via open(O_PATH)
> and pass it to openat())

Feel free to use openat(), too.

Thanks,
								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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