Re: kernel module for chmod restrictions while in securelevel one or higher


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
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