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We create temporary files in /tmp/ with predictable names. These
could be used by an attacker to DoS a box by setting a symlink to
some other file (say, /etc/shadow) and waiting for us to overwrite
it.
The minimalistic solution employed by this patch is to wrap all such
file writing with a paranoid wrapper that:
1) Check to see if the target exists
2) Issues a warning if it was a symlink
3) Deletes it
4) Waits (0.1 seconds if it was a file, 5 seconds if it was a symlink)
5) Opens the file with EXCL, which will fail if the file has come back.
If this succeeds (as it normally will) it has exactly the same semantics
as the original code (a must, as we are right at a release boundary).
However, under no circumstances will it follow a preexisting symlink (the
operating system guarantees this with EXCL) so the danger of an exploit
has been converted into the possibility of a failure, with an appropriate
warning.
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