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Sheharyaar's Notes

Capabilities

  • Traditionally, Linux supported two types of processes: privileged and unprivileged. This had various security concerns like — a program that needs only the privilege to create a raw socket can also be used to mount a file since it is executing as root. This made the attack surface very large.
  • The developers decided to break the privileges into multiple capabilities, to reduce the attack surface. For a complete list of privileges, see capabilities(7).

Process capabilities

A process can obtain capabilities in 2 ways:

  • Inherited Capabilities: A process can inherit a subset of parent’s capabilities set. This can be checked using /proc/PID/status.
$ cat /proc/$$/status
...
CapInh:	0000000000000000
CapPrm:	0000000000000000
CapEff:	0000000000000000
CapBnd:	000001ffffffffff
CapAmb:	0000000000000000
...
  • File Capabilities: If the process is capability-aware, then the process binary can be granted capabilities using setcap.

Types of Capabilities

As you can see in the previous code snippet, there are five different sets of capabilities:

  • Effective (CapEff) — effective capabilities are the one that are currently in use by the process.
  • Permitted (CapPrm) — these are the capabilities that the process is allowed to use.
  • Inherited (CapInh) — capabilities that are allowed to be inherited from the parent during exec family of system calls.
  • Bounded (CapBnd) — can be used to restrict capabilities that a process will ever be able to use. Only these capabilities will be allowed in inherited and permitted sets.
  • Ambient (CapAmb) — applies to all non-SUID binaries without file capabilities. It preserves capabilities during exec family calls. Some capabilities can be dropped if they are nnot present in inherited or permitted sets.

References