Re: [PATCH v2] vfs: add NOFOLLOW flag to umount(2)

[Miklos Szeredi]

On Sun, 21 Feb 2010, Al Viro wrote:
> On Thu, Feb 11, 2010 at 12:21:00PM -0500, Christoph Hellwig wrote:
> > On Wed, Feb 10, 2010 at 12:15:53PM +0100, Miklos Szeredi wrote:
> > >  - renamed flag to UMOUNT_NOFOLLOW
> > >  - added UMOUNT_UNUSED for feature detection
> > 
> > Umm, why?  MNT_ certainly isn't the best naming for unmount flags,
> > but switching convention after we had a few doesn't make any sense.
> 
> Actually, I've got more interesting question: what's being attempted
> there?  Is that just a "let's protect ourselves against somebody feeding
> us an untrusted symlink"?  I'm not sure if it makes much sense; if we
> are dealing with pathnames on untrusted fs, there's nothing to stop the
> attacker from having /mnt/foo/dir (originally containing a mountpoint
> at /mnt/foo/dir/usr) killed and replaced with a symlink to /, making any
> code that does umount() on such pathnames vulnerable as hell anyway.

It is trivial to check the path up to the mountpoint (chdir + getcwd).
But doing that on the mountpoint will make it busy, so NOFOLLOW is
really needed there.

Thanks,
Miklos
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