Inmutable bit in some binaries

[Jeremy Chadwick]

On Wed, Feb 03, 2010 at 01:33:15PM +0100, Jordi Espasa Clofent wrote:
> HI all,
> 
> I'm hardening one test box and at present I'm planning to do:
> 
> # chflags -R schg <file>
> 
> where <file> will be some binaries that seems to be common targets
> for rootkits and lammers:
> 
> ls
> du
> ps
> find
> top
> locate
> strings
> ifconfig
> netstat login
> 
> I wonder if changing these files permissions as I've shown above
> will be cause some troubles in future upgrade (recompilation, the
> classic way, not the binary upgrade one) process. ¿It will?

It's possible installworld will break (fail/exit) when trying to
overwrite some of these binaries.  However...

install(1) supports the -f flags to specify what the destination file
should have its file flags (chflags) set to, and from looking at the
code (src/usr.bin/xinstall/xinstall.c), there are some places which
indicate before copying/installing a file the software may attempt to
unset file flags first.  I'm not 100% familiar with this code, but it
appears as if use of the -S flag to install(1) would cause it to behave
like described.  It should be easy enough for you to test.

Otherwise, my recommendation is to build a test system / virtual box of
some sort (VMware, etc.) and try it.  See what happens.

Opinion below:

What you're attempting to solve, ultimately, is security through
obscurity and gets you a very large headache.  :-)  Your schg idea would
only work if you planned on using kern.securelevel=1 or higher.  This
sounds great in concept, but many a time on this list have people posted
problems with system administrative tasks (re: upgrading) when this
sysctl is set to non-default (-1).

If you plan on using this, I would advocate *all* installation/work be
done in single-user mode.  I know quite a few people who completely
ignore the "boot into single user" step of src/Makefile and instead do
it in multi-user mode -- only to be found later screaming/crying when
binaries don't work or behave oddly because, say, /libexec/ld-elf.so was
supposed to be updated yet wasn't due to their choosing to not follow
the proper procedure.  If your system doesn't have an out-of-band
method of getting to it (ex. serial console), then I wouldn't bother
trying any of the above.

Otherwise, I'm pretty sure others here have made use of read-only
environments, such as read-only NFS root filesystems (sometimes
accomplished via PXE) and/or /usr, or CD-based OS (good luck changing
any files there).  I can't help in that regard.

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |