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    SubjectRe: [PATCH 07/10] ovl: add initial revalidate support
    On Thu, 16 Sep 2010 16:47:29 +0200
    Miklos Szeredi <miklos@szeredi.hu> wrote:

    > On Mon, 06 Sep 2010, NeilBrown wrote:
    > > Add dentry_revalidate method and fail validation of either the
    > > upper or lower dentry has been renamed or unlinked directly in the
    > > otherlying filesystem.
    > > This allows such changes to appear promptly in the overlay providing
    > > the file isn't currently in use.
    >
    > I fixed up some things in the revalidation logic and tested it out.
    > There are some unexpected effects, but they boil down to the fact that
    > busy directories can't be invalidated. Mostly it works as expected.
    >
    > However, the "rearange directories so that a/b becomes b/a" trick
    > still strikes in evil ways. Consider the following script:
    >
    > mkdir /upper/a/b
    > cd /mnt/overlay/a
    > while true; do
    > cd b
    > mv /upper/a/b /upper/b
    > mv /upper/a /upper/b/a
    > cd a
    > mv /upper/b/a /upper/a
    > mv /upper/b /upper/a/b
    > done
    >
    > It will create an ever deeper directory tree on the overlay.
    >
    > Can this be prevented? Probably, e.g. lookup should make sure that
    > each new directory gets a *unique* set of lower and upper dentries
    > (e.g. by creating hash tables indexed by lower and upper dentries).
    >
    > Is it worth the trouble?

    The only real problem here is that an unprivileged user with direct access
    to 'upper' could consume unlimited kernel memory without using a
    corresponding mount of storage space (so quotas etc wouldn't stop them).

    Maybe the easiest counter-measure to that is to keep track of a 'depth' of
    each dentry and disallow lookups that go much beyond PATH_MAX???

    Either that or rely in the sysadmin to hide the upper/lower filesystems from
    untrusted users. I particularly see value in sysadmins being able to modify
    these filesystem - not so much for 'regular users'.

    NeilBrown

    >
    > Any other ideas?
    >
    > Thanks,
    > Miklos
    >
    >


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