Race condition in "rm -r"

[terra () DIKU DK (Morten Welinder)]

Synopsis:

  If root ever does "rm -rf /tmp/foo" for a directory structure
  not completely owned by root, a local user can delete all files
  that root can.

  Such deletions are common for (a) /tmp cleanup and (b) before
  creating a specific directory in /tmp/.

Details:

  "rm -r" implementations (Solaris 7, Gnu 4.0 checked) walk the
  directories, lstats files, and chdirs into directories (and
  recurs).  Oh, and deletes stuff, of course.

  It is possible to replace a directory by a symlink, say "/",
  and have rm cheerfully follow it.

  The race window is small -- it will take thousands upon thousands
  of tries.  Moreover, since the racing involves rmdir -- which
  typically is not very fast -- you'll need patience and "nice"
  and any other help you can get.

Solutions:

  Maybe stat "." after chdir to verify that we ended up the
  expected place?

Also affected:

  chmod, chown, chgrp.  (Probably; this is guesswork.)