linux-kernel - Re: [PATCH -V18 04/13] vfs: Allow handle based open on symlinks

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Date:	Tue, 24 Aug 2010 17:29:58 +1000
From:	Nick Piggin <npiggin@...nel.dk>
To:	Neil Brown <neilb@...e.de>
Cc:	Nick Piggin <npiggin@...nel.dk>, Al Viro <viro@...IV.linux.org.uk>,
	Christoph Hellwig <hch@...radead.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	adilger@....com, corbet@....net, hooanon05@...oo.co.jp,
	bfields@...ldses.org, miklos@...redi.hu,
	linux-fsdevel@...r.kernel.org, sfrench@...ibm.com,
	philippe.deniel@....FR, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V18 04/13] vfs: Allow handle based open on symlinks

On Mon, Aug 23, 2010 at 09:17:08AM +1000, Neil Brown wrote:
> On Sat, 21 Aug 2010 18:30:24 +1000
> Nick Piggin <npiggin@...nel.dk> wrote:
> 
> > Thanks, I had both of the same concerns as Christoph with API
> > change and exposing symlink fds last time I looked at the patces,
> > actually.
> > 
> > But they can probably be worked around or avoided. I think the more
> > important thing is whether it is worth supporting. This is
> > all restricted to root (or CAP_DAC_READ_SEARCH) only, right, and
> > what exact semantics they want. I would like to see more discussion
> > of what this enables and some results.
> 
> They allow a credible user-space implementation of the server for some
> network filesystem protocols such as NFS and apparently 9P.
> 
> > 
> > For the case of avoiding expensive network revalidations in path name
> > lookup, do we even need to open symlinks? Could the security issues be
> > avoided by always having handle attached to an open fd?
> 
> I don't see what you are getting at here... which particular security isses,
> and what fd would you use?

Well the issue that you need escalated privilges to use it. The other
use case for it I understand is Andreas's file-handle-server which
avoids a lot of path lookup costs on non-local filesystems. I'm
wondering is that really useful if it's not availale to unprivileged
users?

> 
> As I understand it there are only two security issues that have been noted.
> 1/ lookup-by-filehandle can bypass any 'search' permission tests on ancestor
>    directories.  I cannot see any way to avoid this except require
>    CAP_DAC_READ_SEARCH
> 2/ Creating a hardlink to an 'fd'  allows a process that was given an 'fd'
>    that it could not have opened itself to prevent that file from being
>    removed (and space reclaimed) by creating a private hardlink.
>    This could be avoided by requiring CAP_DAC_READ_SEARCH for that particular
>    operation (and probably requiring i_nlink > 0 anyway) but that feels like
>    a very special-case restriction.

Just so long as the process could have created a hardlink to the file
otherwise via traditional operations, I think it's OK.

> 
> Was it one of these that you were referring to?

Just the general problem of security and inherent restrictions to using
the syscalls.

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