tools/capable_example.txt

Demonstrations of capable, the Linux eBPF/bcc version.


capable traces calls to the kernel cap_capable() function, which does security
capability checks, and prints details for each call. For example:

# ./capable.py 
TIME      UID    PID    COMM             CAP  NAME                 AUDIT  INSETID
22:11:23  114    2676   snmpd            12   CAP_NET_ADMIN        1      N/A
22:11:23  0      6990   run              24   CAP_SYS_RESOURCE     1      N/A
22:11:23  0      7003   chmod            3    CAP_FOWNER           1      N/A
22:11:23  0      7003   chmod            4    CAP_FSETID           1      N/A
22:11:23  0      7005   chmod            4    CAP_FSETID           1      N/A
22:11:23  0      7005   chmod            4    CAP_FSETID           1      N/A
22:11:23  0      7006   chown            4    CAP_FSETID           1      N/A
22:11:23  0      7006   chown            4    CAP_FSETID           1      N/A
22:11:23  0      6990   setuidgid        6    CAP_SETGID           1      N/A
22:11:23  0      6990   setuidgid        6    CAP_SETGID           1      N/A
22:11:23  0      6990   setuidgid        7    CAP_SETUID           1      N/A
22:11:24  0      7013   run              24   CAP_SYS_RESOURCE     1      N/A
22:11:24  0      7026   chmod            3    CAP_FOWNER           1      N/A
22:11:24  0      7026   chmod            4    CAP_FSETID           1      N/A
22:11:24  0      7028   chmod            4    CAP_FSETID           1      N/A
22:11:24  0      7028   chmod            4    CAP_FSETID           1      N/A
22:11:24  0      7029   chown            4    CAP_FSETID           1      N/A
22:11:24  0      7029   chown            4    CAP_FSETID           1      N/A
22:11:24  0      7013   setuidgid        6    CAP_SETGID           1      N/A
22:11:24  0      7013   setuidgid        6    CAP_SETGID           1      N/A
22:11:24  0      7013   setuidgid        7    CAP_SETUID           1      N/A
22:11:25  0      7036   run              24   CAP_SYS_RESOURCE     1      N/A
22:11:25  0      7049   chmod            3    CAP_FOWNER           1      N/A
22:11:25  0      7049   chmod            4    CAP_FSETID           1      N/A
22:11:25  0      7051   chmod            4    CAP_FSETID           1      N/A
22:11:25  0      7051   chmod            4    CAP_FSETID           1      N/A

A recent kernel version >= 5.1 also reports the INSETID bit to cap_capable():

# ./capable.py
TIME      UID    PID    TID    COMM             CAP  NAME                 AUDIT  INSETID
08:22:36  0      12869  12869  chown            0    CAP_CHOWN            1      0
08:22:36  0      12869  12869  chown            0    CAP_CHOWN            1      0
08:22:36  0      12869  12869  chown            0    CAP_CHOWN            1      0
08:23:02  0      13036  13036  setuidgid        6    CAP_SETGID           1      0
08:23:02  0      13036  13036  setuidgid        6    CAP_SETGID           1      0
08:23:02  0      13036  13036  setuidgid        7    CAP_SETUID           1      1
08:23:13  0      13085  13085  chmod            3    CAP_FOWNER           1      0
08:23:13  0      13085  13085  chmod            4    CAP_FSETID           1      0
08:23:13  0      13085  13085  chmod            3    CAP_FOWNER           1      0
08:23:13  0      13085  13085  chmod            4    CAP_FSETID           1      0
08:23:13  0      13085  13085  chmod            4    CAP_FSETID           1      0
08:24:27  0      13522  13522  ping             13   CAP_NET_RAW          1      0
[...]

This can be useful for general debugging, and also security enforcement:
determining a whitelist of capabilities an application needs.

The output above includes various capability checks: snmpd checking
CAP_NET_ADMIN, run checking CAP_SYS_RESOURCES, then some short-lived processes
checking CAP_FOWNER, CAP_FSETID, etc.

To see what each of these capabilities does, check the capabilities(7) man
page and the kernel source.

It is possible to include a kernel stack trace to the capable events by passing
-K to the command:

# ./capable.py -K
TIME      UID    PID    COMM             CAP  NAME                 AUDIT
15:32:21  1000   10708  fetchmail        7    CAP_SETUID           1
        cap_capable+0x1 [kernel]
        ns_capable_common+0x7a [kernel]
        __sys_setresuid+0xc8 [kernel]
        do_syscall_64+0x56 [kernel]
        entry_SYSCALL_64_after_hwframe+0x49 [kernel]
15:32:21  1000   30047  procmail         6    CAP_SETGID           1
        cap_capable+0x1 [kernel]
        ns_capable_common+0x7a [kernel]
        may_setgroups+0x2f [kernel]
        __x64_sys_setgroups+0x18 [kernel]
        do_syscall_64+0x56 [kernel]
        entry_SYSCALL_64_after_hwframe+0x49 [kernel]

Similarly, it is possible to include user-space stack with -U (or they can be
used both at the same time to include user and kernel stack).

USAGE:

# ./capable.py -h
usage: capable.py [-h] [-v] [-p PID] [-K] [-U]

Trace security capability checks

optional arguments:
  -h, --help          show this help message and exit
  -v, --verbose       include non-audit checks
  -p PID, --pid PID   trace this PID only
  -K, --kernel-stack  output kernel stack trace
  -U, --user-stack    output user stack trace

examples:
    ./capable             # trace capability checks
    ./capable -v          # verbose: include non-audit checks
    ./capable -p 181      # only trace PID 181
    ./capable -K          # add kernel stacks to trace
    ./capable -U          # add user-space stacks to trace