linux-kernel - Re: symlinks with permissions

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Date:	Wed, 28 Oct 2009 22:03:24 +0100
From:	Pavel Machek <pavel@....cz>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
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,
	jamie@...reable.org
Subject: Re: symlinks with permissions

Hi!

> >> > Well, it is unexpected and mild security hole.
> >> 
> >> /proc/<pid>/fd is only viewable by the owner of the process or by
> >> someone with CAP_DAC_OVERRIDE.  So there appears to be no security
> >> hole exploitable by people who don't have the file open.
> >
> > Please see bugtraq discussion at
> > http://seclists.org/bugtraq/2009/Oct/179 .
> >
> > (In short, you get read-only fd, and you can upgrade it to read-write
> > fd. Yes, you are the owner of the process, but you are not owner of
> > the file the fd refers to.)
> 
> Assuming you have permission to open it read-write.

Please see the bugtraq discussion.

It works even if you would not have permission to write to it with
/proc unmounted.

> >> Openly if you actually have permission to open the file again.  The actual
> >> permissions on the file should not be ignored.
> >
> > The actual permissions of the file are not ignored, but permissions of
> > the containing directory _are_. If there's 666 file in 700 directory,
> > you can reopen it read-write, in violation of directory's 700
> > permissions.
> 
> I can see how all of this can come as a surprise.  However I don't see
> how any coder who is taking security seriously and being paranoid about
> security would actually write code that would have a problem with
> this.

So, there's "surprise" that gives _you_ write access to my files. You
agree that it is surprising, and you would not have write access to my
file if /proc was not mounted.

Call it "security surprise" if you prefer. But many people call it
"security hole".

> Do you know of any cases where this difference matters in practice?

No. Do you have a proof that it does not matter anywhere?

> It looks to me like it has been this way for better than a decade
> without problems so there is no point in changing it now.  

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