Taking Advantage of Linux Capabilities

Concentrating on user privileges to appease the security paranoid.

A common topic of discussion nowadays is security, and for good reason. Security is becoming more important as the world becomes further networked. Like all good systems, Linux is evolving in order to address increasingly important security concerns.

One aspect of security is user privileges. UNIX-style user privileges come in two varieties, user and root. Regular users are absolutely powerless; they cannot modify any processes or files but their own. Access to hardware and most network specifications also are denied. Root, on the other hand, can do anything from modifying all processes and files to having unrestricted network and hardware access. In some cases root can even physically damage hardware.

Sometimes a middle ground is desired. A utility needs special privileges to perform its function, but unquestionable god-like root access is overkill. The ping utility is setuid root simply so it can send and receive ICMP messages. The danger lies in the fact that ping can be exploited before it has dropped its root privileges, giving the attacker root access to your server.

Fortunately, such a middle ground now exists, and it's called POSIX capabilities. Capabilities divide system access into logical groups that may be individually granted to, or removed from, different processes. Capabilities allow system administrators to fine-tune what a process is allowed to do, which may help them significantly reduce security risks to their system. The best part is that your system already supports it. If you're lucky, no patching should be necessary.

A list of all the capabilities that your system is, well, capable of, is available in /usr/include/linux/capability.h, starting with CAP_CHOWN. They're pretty self-explanatory and well commented. Capability checks are sprinkled throughout the kernel source, and grepping for them can make for some fun midnight reading.

Each capability is nothing more than a bit in a bitmap. With 32 bits in a capability set, and 28 sets currently defined, there are currently discussions as to how to expand this number. Some purists believe that additional capabilities would be too confusing, while others argue that there should be many more, even a capability for each system call. Time and Linus will ultimately decide how this exciting feature develops.

The Proc Interface

As of kernel 2.4.17, the file /proc/sys/kernel/cap-bound contains a single 32-bit integer that defines the current global capability set. The global capability set determines what every process on the system is allowed to do. If a capability is stripped from the system, it is impossible for any process, even root processes, to regain them.

For example, many crackers' rootkits (a set of tools that cover up their activities and install backdoors into the system) will load kernel modules that hide illicit processes and files from the system administrator. To counter this, the administrator could simply remove the CAP_SYS_MODULE capability from the system as the last step in the system startup process. This step would prevent any kernel modules from being loaded or unloaded. Once a capability has been removed, it cannot be re-added. The system must be restarted (which means you might have to use the power button if you've removed the CAP_SYS_BOOT capability) to regain the full-capability set.

Okay, I lied. There are two ways to add back a capability:

  1. init can re-add capabilities, in theory; there's no actual implementation to my knowledge. This is to facilitate capability-aware systems in the event that init needs to change runlevels.

  2. If a process is capable of CAP_SYS_RAWIO, it can modify kernel memory through /dev/mem. Among other things, it can modify kernel memory to grant itself whatever access it desires. Remove CAP_SYS_RAWIO, but be careful: by removing CAP_SYS_RAWIO, programs such as X most likely will fail to run.

Editing cap-bound by hand is kind of tedious. Fortunately for you, there's a utility called lcap that provides a friendlier interface to cap-bound. Here's how one would remove CAP_SYS_CHOWN:

lcap CAP_SYS_CHOWN
Once done, it becomes impossible to change a file's owner:
chown nobody test.txt
chown: changing ownership of `test.txt':
       Operation not permitted
Here's how you would remove all capabilities except CAP_SYS_BOOT, CAP_SYS_KILL and CAP_SYS_NICE:
lcap -z CAP_SYS_BOOT CAP_SYS_KILL CAP_SYS_NICE
One thing to note: modifying cap-bound restricts the capabilities of future processes only. Okay, not exactly future processes but any process that calls exec(2) (see the function compute_creds in the kernel source file fs/exec.c). Currently running processes keep the capabilities with which they started.

Modifying the capabilities of an existing process leads us into the next section, and here's the catch I spoke about above. Running lcap with no arguments lists what your system is capable of. If you see that CAP_SETPCAP is disabled, you need to make a change to your kernel. It's simple enough to describe here. In the kernel source tree, edit include/linux/capability.h. You're changing the lines:

#define CAP_INIT_EFF_SET
to_cap_t(~0 & ~CAP_TO_MASK(CAP_SETPCAP))
#define CAP_INIT_INH_SET  to_cap_t(0)

so that they read:

#define CAP_INIT_EFF_SET  to_cap_t(~0)
#define CAP_INIT_INH_SET  to_cap_t(~0)
and then recompile.

There's actually a reason that CAP_SETPCAP is disabled by default: it's deemed a security risk to leave it enabled on a production system (a patch exists for this condition but has yet to be applied as of this writing). To be on the safe side, make sure to remove this capability when you're done playing.

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Doesn't work for me.

Anonymous's picture

# execcap 'cap_net_raw=ep' /sbin/sucap nobody nobody /bin/ping www.yahoo.com
Caps: =ep cap_setpcap-ep
Caps: =
[debug] uid:65534, real uid:65534
sucaps: capsetp: Operation not permitted
sucap: child did not exit cleanly.

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