Re: Filesystem Capabilities in 2.6?

[Theodore Ts'o]

On Sun, Nov 03, 2002 at 09:49:10PM -0500, Jan Harkes wrote:
> For several years, I have had only one suid root binary on my system.
> All other 'setuid' applications are simply symlinks to this binary.
> 
> $ ls -l /bin/ping*
> lrwxrwxrwx    1 root     root           14 Nov 18  2001 /bin/ping -> /usr/bin/super
> -rwxr-xr-x    1 root     root        15244 Nov 18  2001 /bin/ping.suid
> 
> There is a a nice configuration file that is used to decide whether to
> use suid or setgid, which parts of the environment to drop/keep. And all
> of this based on the user, the time and any other conditions I would
> like to enforce.
> 
> Now super does not (yet) support capabilities. But it shouldn't be too
> hard to modify it so that it forks, drops capabilities, (possibly change
> the euid to the original user?) and exec the actual binary.

This sounds like the right way to go.  I do hope the configuration
file includes an SHA checksum of the secutable.  And to avoid race
conditions, there really ought to be a new system call, fexecve(2)
which takes an open file descriptor instead of a pathname.
(Unfortunately, we're in feature freeze now, so that will have to wait
until 2.7.)

Failing that, though, /usr/bin/super should really check the
permissions starting from the root of the entire pathaname, and fail
the exec if any of the containing directories are writable by a
non-root user.  (And of course, the executable should not be writable
by a non-root users for the same reason.)

With these checks, though, adding support for capabilities in
/usr/bin/super sounds like the right approach.  It doesn't require any
kernel changes (well, fexecve(2) would be nice, but it's not strictly
required).  There is of course a slight performance penalty associated
with the use of the helper program, but the start time of most setuid
root programs probably isn't a performance critical concern.

						- Ted