linux-kernel - Re: symlinks with permissions

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Date:	Thu, 29 Oct 2009 12:03:44 +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!

> >> 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?
> 
> Thinking about this proc fundamentally gives you the ability to create
> (via open) a new file descriptor for a file you already have open.

Yes. Problem is that by using /proc, I can work-around open(READONLY)
restriction and work-around open(APPEND_ONLY) restriction.

> I do see a security issue in your example, but the security issue I
> see is how you have chosen to use the linux facilities, that have been
> there for ages.  Facilities cloned from plan 9 and apparently
> available in slightly different forms on many unix variants existence.
> /dev/fd/N is not a linuxism.
> 
> To close this whole would require some sort of stacking inode that
> when opened opened the real fs inode.  With all kinds of convolutions
> and complications.  Just to close the issue that some idiot might
> give someone a fd to a world writeable file that they don't want
> them to open.

Ok, so you agree issue is there. Good.

Now, fix for READONLY issue should be fairly simple: follow link in
/proc/*/fd/* should check the link permissions,  and return
read-only/write-only descriptors as neccessary.

Basically, that follow link should behave as dup(), not as open().

> I certainly am not interested in debugging or maintaining the stacking
> inode code that would be necessary to close this theoretical corner
> case.  There are much more real bugs that need attention.

But if we can get trivial 10-liner, that should be acceptable, right?
									Pavel

-- 
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