Re: hard links to symlinks

On Fri, Jul 17, 2009 at 12:34:38PM +0200, Jilles Tjoelker wrote:
> In compliance with POSIX.1-2008, linkat() allows creating hard links to
> symlinks. This means that it is a much worse idea to trust a symlink
> just because it is owned by a trusted user (if it is in a directory
> writable by other users). Security issues like
> http://archives.neohapsis.com/archives/postfix/2008-08/0391.html now
> affect FreeBSD 8 local filesystems as well.
> 
> Given that Linux and Solaris have allowed this for a long time, I think
> this functionality should remain, but it should probably be mentioned in
> the release notes. The security.bsd.hardlink_check_uid sysctl can be
> used to avoid vulnerabilities.
> 
> By the way, the man page erroneously says the AT_SYMLINK_NOFOLLOW flag
> should be set to have linkat() follow symlinks. The standard and the
> implementation are correct, AT_SYMLINK_FOLLOW; the AT_SYMLINK_NOFOLLOW
> flag is not valid for this function.
Please commit the fix.

> 
> Note that the link command and the link() function always follow
> symlinks (this is POSIX.1-2001 and POSIX.1-2008 compliant), and the ln
> command will not create hard links to symlinks either. Hence, people may
> think it is not possible (the check in the Postfix advisory will not
> detect FreeBSD 8's capability).
> 
> I have a patch to add the POSIX.1-2008 -L and -P options to ln, making
> it possible to choose the desired behaviour (follow/don't follow
> symlinks). I think this is too late for 8.0, however.

You can make it in before BETA3, in my opinion.