linux-kernel - Re: symlinks with permissions

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Message-ID: <20091027102734.GA32063@shareable.org>
Date:	Tue, 27 Oct 2009 10:27:34 +0000
From:	Jamie Lokier <jamie@...reable.org>
To:	Pavel Machek <pavel@....cz>
Cc:	Trond Myklebust <trond.myklebust@....uio.no>,
	Jan Kara <jack@...e.cz>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk
Subject: Re: symlinks with permissions

Pavel Machek wrote:
> On Mon 2009-10-26 14:22:16, Trond Myklebust wrote:
> > On Sun, 2009-10-25 at 10:36 +0100, Pavel Machek wrote:
> > > Well, it is unexpected and mild security hole.
> > > 
> > > Part of the problem is that even  if you have read-only
> > > filedescriptor, you can upgrade it to read-write, even if path is
> > > inaccessible to you.
> > > 
> > > So if someone passes you read-only filedescriptor, you can still write
> > > to it.
> > 
> > If someone passes you a file descriptor, can't you in any case play
> > games with, openat(fd,"",O_RDWR), in order to achieve the same thing? I
> > must admit I haven't tried it yet, but at a first glance I can't see
> > anything that prevents me from doing this...
> 
> According to my documentation, openat needs directory fd.

Correct.  There has been something about fstatat() and similar
allowing a non-directory when passed a NULL path, but openat() does
not.  (It's probably ok to extend openat() to allow a NULL path, if it
does the equivalent of re-opening /proc/self/fd/NN).

I think this whole issue is neatly solved by enforcing the file access
mode for open(/proc/PID/fd/NN) to be a safe subset of the original
file access mode.

It should use the original file access mode so that O_APPEND can
be enforced too.  Checking symlink permissions wouldn't do that.

Anything you can change with fcntl(F_SETFL) is fair game for changing.

The ptrace permission check is nice, but even with ptrace you can't
convert a read-only descriptor to a writable one (or write-only to
readable, or append-only to writable, etc.)

-- Jamie
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