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

Neil Brown
2010-08-22T23:17:27Z

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?

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.

Was it one of these that you were referring to?

Thanks,
NeilBrown

>
> On Sat, Aug 21, 2010 at 10:09:00AM +1000, Neil Brown wrote:
> > [[email address for Nick Piggin changed to npiggin@kernel.dk]]
> >
> > On Fri, 20 Aug 2010 12:51:35 +0100
> > Al Viro <viro@ZenIV.linux.org.uk> wrote:
> >
> > > On Fri, Aug 20, 2010 at 07:53:03PM +1000, Neil Brown wrote:
> > > > On Fri, 20 Aug 2010 04:30:57 -0400
> > > > Christoph Hellwig <hch@infradead.org> wrote:
> > > >
> > > > > Suddenly getting an file pointer for a symlink which could never happen
> > > > > before is a really bad idea. Just add a proper readlink_by_handle
> > > > > system call, similar to what's done in the XFS interface.
> > > >
> > > > Why is that?
> > > > With futexes we suddenly get a file descriptor for something we could never
> > > > get a file descriptor on before and that doesn't seem to be a problem.
> > > >
> > > > Why should symlinks be special as the only thing that you cannot have a file
> > > > descriptor for? Uniformity of interface is a very valuable property.
> > >
> > > You are welcome to review the codepaths around pathname resolution for
> > > assumptions of presense of ->follow_link() and friends; there _are_
> > > subtle cases and dumping your "opened symlinks" in there is far from
> > > a trivial change. Note that it affects more than just the starting
> > > points of lookups; /proc/*/fd/* stuff is also involved.
> >
> > So as I understand it you aren't rejecting the concept in principle, but you
> > believe non-trivial code review is required before it can be considered an
> > acceptable change?
> > That's quite reasonable. I hope to find time to have a look at the code.
> >
> > >
> > > BTW, speaking of NULL pathname, linkat() variant that allows creating a link
> > > to an opened file is also a very dubious thing; at the very least, you get
> > > non-trivial security implications, since now a process that got an opened
> > > descriptor passed to it by somebody else may create hardlinks to the sucker.
> >
> > Fair comment, and while one could imagine ways around this (such as requiring
> > some Capability to link an fd) they wouldn't be very elegant.
> > But then nor is inventing a pile of new syscalls for doing different things
> > with handles such as the list Aneesh posted.
> >
> > Maybe a different approach is needed.
> >
> > How about a new AT flag: AT_FILE_HANDLE
> >
> > Meaning is that the 'dirfd' is used only to identify a filesystem (vfsmnt) and
> > the 'name' pointer actually points to a filehandle fragment interpreted in
> > that filesystem.
> >
> > One problem is that there is no way to pass the length...
> > Options:
> > fragment is at most 64 bytes nul padded at the end
> > fragment is hex encoded and nul terminated
> > ??
> >
> > I think I prefer the hex encoding, but I'm hoping someone else has a better
> > idea.
> >
> > NeilBrown
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