libcap-ng
The libcap-ng library should make programming with posix capabilities easier. The library has some utilities to help you analyse a system for apps that may have too much privileges.
The included utilities are designed to let admins and developers spot apps from various ways that may be running with too much privilege. For example, any investigation should start with network facing apps since they would be prime targets for intrusion. The netcap program will check all running apps and display the results. Sample output from netcap:
ppid pid acct command type port capabilities
1 2295 root nasd tcp 8000 full
2323 2383 root dnsmasq tcp 53 net_admin, net_raw +
1 2286 root sshd tcp 22 full
1 2365 root cupsd tcp 631 full
1 2286 root sshd tcp6 22 full
1 2365 root cupsd tcp6 631 full
2323 2383 root dnsmasq udp 53 net_admin, net_raw +
2323 2383 root dnsmasq udp 67 net_admin, net_raw +
1 2365 root cupsd udp 631 full
C Examples
As an application developer, there are probabaly 6 use cases that you are interested in: drop all capabilities, keep one capability, keep several capabilities, check if you have any capabilities at all, check for certain capabilities, and retain capabilities across a uid change.
1) Drop all capabilities
capng_clear(CAPNG_SELECT_BOTH);
capng_apply(CAPNG_SELECT_BOTH);
2) Keep one capability
capng_clear(CAPNG_SELECT_BOTH);
capng_update(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED, CAP_CHOWN);
capng_apply(CAPNG_SELECT_BOTH);
3) Keep several capabilities
capng_clear(CAPNG_SELECT_BOTH);
capng_updatev(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED, CAP_SETUID, CAP_SETGID, -1);
capng_apply(CAPNG_SELECT_BOTH);
4) Check if you have any capabilities
if (capng_have_capabilities(CAPNG_SELECT_CAPS) > CAPNG_NONE)
do_something();
5) Check for certain capabilities
if (capng_have_capability(CAPNG_EFFECTIVE, CAP_CHOWN))
do_something();
6) Retain capabilities across a uid change
capng_clear(CAPNG_SELECT_BOTH);
capng_update(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED, CAP_CHOWN);
if (capng_change_id(99, 99, CAPNG_DROP_SUPP_GRP | CAPNG_CLEAR_BOUNDING))
error();
Now, isn't that a lot simpler? Note that the last example takes about 60 lines of code using the older capabilities library. As of the 0.6 release, there is a m4 macro file to help adding libcap-ng to your autotools config system. In configure.ac, add LIBCAP_NG_PATH. Then in Makefile.am locate the apps that link to libcap-ng, add $(CAPNG_LDADD) to their LDADD entries. And lastly, surround the optional capabilities code with #ifdef HAVE_LIBCAP_NG.
Python
Libcap-ng 0.6 and later has python bindings. You simply add import capng in your script. Here are the same examples as above in python:
1) Drop all capabilities
capng.capng_clear(capng.CAPNG_SELECT_BOTH)
capng.capng_apply(capng.CAPNG_SELECT_BOTH)
2) Keep one capability
capng.capng_clear(capng.CAPNG_SELECT_BOTH)
capng.capng_update(capng.CAPNG_ADD, capng.CAPNG_EFFECTIVE|capng.CAPNG_PERMITTED, capng.CAP_CHOWN)
capng.capng_apply(capng.CAPNG_SELECT_BOTH)
3) Keep several capabilities
capng.capng_clear(capng.CAPNG_SELECT_BOTH)
capng.capng_updatev(capng.CAPNG_ADD, capng.CAPNG_EFFECTIVE|capng.CAPNG_PERMITTED, capng.CAP_SETUID, capng.CAP_SETGID, -1)
capng.capng_apply(capng.CAPNG_SELECT_BOTH)
4) Check if you have any capabilities
if capng.capng_have_capabilities(capng.CAPNG_SELECT_CAPS) > capng.CAPNG_NONE:
do_something()
5) Check for certain capabilities
if capng.capng_have_capability(capng.CAPNG_EFFECTIVE, capng.CAP_CHOWN):
do_something()
6) Retain capabilities across a uid change
capng.capng_clear(capng.CAPNG_SELECT_BOTH)
capng.capng_update(capng.CAPNG_ADD, capng.CAPNG_EFFECTIVE|capng.CAPNG_PERMITTED, capng.CAP_CHOWN)
if capng.capng_change_id(99, 99, capng.CAPNG_DROP_SUPP_GRP | capng.CAPNG_CLEAR_BOUNDING) < 0:
error()
The one caveat is that printing capabilities from python does not work. But you can still manipulate capabilities, though.
NOTE: to distributions
There is a "make check" target. It only works if the headers match the kernel. IOW, if you have a chroot build system that is using a much older kernel, the macros in the kernel header files will do the wrong thing when the capng_init function probes the kernel and decides we are doing v1 rather than v3 protocol. If that is your case, just don't do the "make check" as part of the build process.
Reporting
Report any bugs in this package to: https://github.com/stevegrubb/libcap-ng/issue