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

Nick Piggin
2010-08-24T07:30:10Z

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@kernel.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.