linux-kernel - Re: [PATCH] procfs: make /proc style symlinks behave like "normal" symlinks

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Date:	Thu, 19 Nov 2009 14:35:55 -0500
From:	Jeff Layton <jlayton@...hat.com>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	pavel@....cz
Subject: Re: [PATCH] procfs: make /proc style symlinks behave like "normal"
 symlinks

On Thu, 19 Nov 2009 10:57:08 -0800
ebiederm@...ssion.com (Eric W. Biederman) wrote:

> Jeff Layton <jlayton@...hat.com> writes:
> 
> > On Thu, 19 Nov 2009 09:07:16 -0800
> > ebiederm@...ssion.com (Eric W. Biederman) wrote:
> >
> >> 
> >> Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> >> 
> >> This is broken.  If the referenced file is in a different mount namespace
> >> the path returned could point to a completely different path in your
> >> own mount namespace.  Even in your own mount namespace this makes the
> >> proc symlinks racy and not guaranteed to return the file of interest.
> >> 
> >> I don't see any hope of this approach ever working.
> >> 
> >> Eric
> >> 
> >
> > Then is proc_pid_readlink broken in the same way?
> 
> proc_pid_readlink has the same deficiencies.  The race is fundamental
> to all readlink operations, the difference is that for normal symlinks
> it is a don't care, and for proc it is incorrect behavior if you follow
> the symlink to the wrong file.   If you are dealing with a file in a
> different namespace or a socket what you get back doesn't actually
> work as a file in your local namespace but that is the best we can do
> with a pathname, and if you know the context of what is going on readlink
> is still useful.
> 
> Adding all of the short comings to followlink that readlink has is a problem,
> especially as followlink does much better now.
> 
> At a practical level I think your changes are much easier to exploit than
> Pavels contrived example.
> 
> I really don't have any problems with your first patch to proc to add the
> missing revalidate.
> 

Thanks, that makes sense. The raciness was evident once you pointed it
out, so I think you're correct that we can't take this approach.

Adding the missing revalidations is fine, but I don't believe that
helps to fix Pavel's issue. I'll go back and take a more careful look
at the suggestion that Miklos made and see whether it makes sense to
implement a new FS_* flag for this, and see what it'll take to fix
Pavel's issue.

-- 
Jeff Layton <jlayton@...hat.com>
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