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