kernel module for chmod restrictions while in securelevel one or higher

[Robert Watson]

On Fri, 30 Jul 2010, Selphie Keller wrote:

> Kernel module for chmod restrictions while in securelevel one or higher: 
> http://gist.github.com/501800 (fbsd 8.x)
>
> Was looking at the new recent sendfile/mbuf exploit and it was using a 
> shellcode that calls chmod syscall to make a setuid/setgid binary. However 
> was thinking of ways to block the creation of suid/sgid binaries if the 
> machine is in a securelevel, beyond the normal things like nosuid/noexec 
> mount flags for /tmp.
>
> So came up with this quick module to handle it, but the concept of 
> restricting the creation of suid/sgid binaries while in securelevel seems 
> like a good idea to be part of the base.

While I'm not convinced this approach is a good idea (the remainder of the 
thread suggests some reasons why), a better way to implement the same policy 
would be to use the MAC Framework's mpo_vnode_check_setmode entry point.  This 
would catch all the chmod variations, including ones in compatibility ABIs 
such as the Linux ABI.

Robert