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

[Alex Riesen]

On Mon, Apr 22, 2002 at 12:18:19PM -0700, Chris Wright wrote:
> * Florian Weimer ([e-mail]) 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

that is close-on-exec. Different semantics.

       F_GETFD  Read  the  close-on-exec flag.  If the FD_CLOEXEC
                bit is 0, the file will remain open across  exec,
                otherwise it will be closed.

> 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)