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|
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 that have listening socket 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
```
After checking the networking apps, you should check all running apps with
pscap. If you are a developer and have to give your application
CAP_DAC_OVERRIDE, you must be accessing files for which you have no permission
to access. This typically can be resolved by having membership in the correct
groups. Try to avoid needing CAP_DAC_OVERRIDE...you may as well be root if you
need it.
Some application developers have chosen to use file system base capabilities
rather than be setuid root and have to drop capabilities. Libcap-ng provides
filecap to recursively search directories and show you which ones have
capabilities and exactly what those are.
C Examples
----------
As an application developer, there are probably 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
```c
capng_clear(CAPNG_SELECT_BOTH);
capng_apply(CAPNG_SELECT_BOTH);
```
2) Keep one capability
```c
capng_clear(CAPNG_SELECT_BOTH);
capng_update(CAPNG_ADD, CAPNG_EFFECTIVE|CAPNG_PERMITTED, CAP_CHOWN);
capng_apply(CAPNG_SELECT_BOTH);
```
3) Keep several capabilities
```c
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
```c
if (capng_have_capabilities(CAPNG_SELECT_CAPS) > CAPNG_NONE)
do_something();
```
5) Check for a specific capability
```c
if (capng_have_capability(CAPNG_EFFECTIVE, CAP_CHOWN))
do_something();
```
6) Retain capabilities across a uid change
```c
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. (Only python3 is supported from 0.8.4 onward.) You simply add 'import capng' in your script. Here are the same examples as above in python:
1) Drop all capabilities
```python
capng.capng_clear(capng.CAPNG_SELECT_BOTH)
capng.capng_apply(capng.CAPNG_SELECT_BOTH)
```
2) Keep one capability
```python
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
```python
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
```python
if capng.capng_have_capabilities(capng.CAPNG_SELECT_CAPS) > capng.CAPNG_NONE:
do_something()
```
5) Check for a specific capability
```python
if capng.capng_have_capability(capng.CAPNG_EFFECTIVE, capng.CAP_CHOWN):
do_something()
```
6) Retain capabilities across a uid change
```python
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.
Ambient Capabilities
--------------------
Ambient capabilities arrived in the 4.3 Linux kernel. Ambient capabilities
allow a privileged process to bestow capabilities to a child process. This
is how systemd grants capabilities to a daemon running in a service account.
The problem with ambient capabilities is they are inherited forever. Every
process exec'ed from the original service also has the capabilities. This is
a security issue.
To find and fix this, you can run the pscap program and grep for '@'. The '@'
symbol denotes processes that have ambient capabilities. For example:
```
# pscap | grep @
1 1655 systemd-oom systemd-oomd dac_override, kill @ +
1 1656 systemd-resolve systemd-resolve net_raw @ +
```
To fix this, libcap-ng 0.8.3 and later ships libdrop_ambient.so.0. It is
designed to be used with LD_PRELOAD. It has a constructor function that forces
the dropping of ambient capabilities. By the time the application starts, it
has both effective and ambient capabilities - meaning is safe to drop ambient
capabilities very early. You can either link it to an application run as a
systemd service (using ld), or create a wrapper script that then starts the
daemon.
Building
--------
After cloning libcap-ng, run:
```
cd libcap-ng
./autogen.sh
./configure
make
make install
```
If you want python bindings, add that option to the configure command. There is also a spec file to use if you are on a rpm based distribution. To do that, run "make dist" instead of make in the above instructions. Then use the resulting tar file with the spec file.
NOTE: to distributions
----------------------
There is a "make check" target. It only works if the available kernel headers
roughly match the build root kernel. Iow, if you have a chroot build system
that is using a much older kernel, the macros in the kernel header files will
describe functionality that does not exist in the build root. The capng_init
function will probe the kernel and decide we can only do v1 rather than v3
capabilities instead of what the kernel headers said was possible. If that is
your case, just don't do the "make check" as part of the build process. This
problem should go away as build roots eventually switch to the 5.0 or later
kernels.
Reporting
---------
Report any bugs in this package to:
https://github.com/stevegrubb/libcap-ng/issue
|
