Re: [SECURITY] FDs 0, 1, 2 for SUID/SGID programs

[Chris Wright <chris@wirex.com>]

* Florian Weimer (Weimer@CERT.Uni-Stuttgart.DE) wrote:
> http://www.pine.nl/advisories/pine-cert-20020401.html probably affects
> Linux, too (if a SUID/SGID program is invoked with FD 2 closed, error
> messages might be written to a file opened by the program ).

AFAIK, the standards clearly specify behaviour wrt. open file descriptors
and clone-on-exec file descriptors across execve().  However, there
is nothing specified when it comes to closed file descpriptors across
execve(), notably FD's 0, 1 and 2 are certainly not required to be open
across an execve() of a SUID/SGID applictaion.  One could argue that
SUID/SGID apps that trust the file descriptors they inherit across exec()
are buggy.

Having said that, there are a number of implementations of this type
of protection for the linux kernel stemming from the Openwall project.
If you are interested, see:

	http://www.openwall.com	(CONFIG_SECURE_FD_0_1_2)
	http://lsm.immunix.org	(CONFIG_OWLSM_FD)
	http://grsecurity.net	(CONFIG_GRKERNSEC_FD)

cheers,
-chris