capsh - capability shell wrapper
Linux capability support and use can be explored and constrained with this tool. This tool provides a handy wrapper for certain types of capability testing and environment creation. It also provides some debugging features useful for summarizing capability state.
capsh takes a number of optional arguments, acting on them in the order they are provided. They are as follows:
Display the list of commands supported by capsh.
Display prevailing capability and related state.
Display prevailing capability state, 1e capabilities and IAB vector.
Execute /bin/bash with trailing arguments. Note, you can use -c ’command to execute’ for specific commands.
Uses cap_launch(3) to fork a child to execute the shell. When the child exits, capsh exits with the status of the child or 1 in the case that the child was terminated by a signal.
Execute capsh again with the remaining arguments. Useful for testing exec() behavior. Note, PATH is searched when the running capsh was found via the shell’s PATH searching. If the exec occurs after a --chroot=/some/path argument the PATH located binary may not be resolve to the same binary as that running initially. This behavior is an intended feature as it can complete the chroot transition.
Uses cap_launch(3) to fork a child to re-execute capsh. When this child exits, capsh exits with the status of the child or 1 in the case that the child was terminated by a signal.
Set the prevailing process capabilities to those specified by cap-set. Where cap-set is a text-representation of capability state as per cap_from_text(3).
Remove the listed capabilities from the prevailing bounding set. The capabilities are a comma-separated list of capabilities as recognized by the cap_from_name(3) function. Use of this feature requires that capsh is operating with CAP_SETPCAP in its effective set.
Set the inheritable set of capabilities for the current process to equal those provided in the comma separated list. For this action to succeed, the prevailing process should already have each of these capabilities in the union of the current inheritable and permitted capability sets, or capsh should be operating with CAP_SETPCAP in its effective set.
Assume the identity of the named user. That is, look up the user’s UID and GID with getpwuid(3) and their group memberships with getgrouplist(3) and set them all using cap_setuid(3) and cap_setgroups(3). Following this command, the effective capabilities will be cleared, but the permitted set will not be, so the running program is still privileged.
Display the prevailing libcap mode as guessed by the cap_get_mode(3) function.
Force the program into a cap_set_mode(3) security mode. This is a set of securebits and prevailing capability arrangement recommended for its pre-determined security stance.
Lists all of the libcap modes supported by --mode=<mode>.
Confirm that the prevailing mode is that specified in <mode>, or exit with a status 1.
Force all UID values to equal id using the setuid(2) system call. This argument may require explicit preparation of the effective set.
use the cap_setuid(3) function to set the UID of the current process. This performs all preparations for setting the UID without dropping capabilities in the process. Following this command the prevailing effective capabilities will be lowered.
Exit with status 1 unless the current UID equals <id>.
Force all GID values to equal id using the setgid(2) system call.
Exit with status 1 unless the current GIQ equals <id>.
Set the supplementary groups to the numerical list provided. The groups are set with the setgroups(2) system call. See --user for a more convenient way of doing this.
In a non-pure capability mode, the kernel provides liberal privilege to the super-user. However, it is normally the case that when the super-user changes UID to some lesser user, then capabilities are dropped. For these situations, the kernel can permit the process to retain its capabilities after a setuid(2) system call. This feature is known as keep-caps support. The way to activate it using this program is with this argument. Setting the value to 1 will cause keep-caps to be active. Setting it to 0 will cause keep-caps to deactivate for the current process. In all cases, keep-caps is deactivated when an exec() is performed. See --secbitsand--mode for ways to disable this feature.
Set the security-bits for the program. This is done using the prctl(2) PR_SET_SECUREBITS operation. The list of supported bits and their meaning can be found in the <sys/secbits.h> header file. The program will list these bits via the --print command. The argument is expressed as a numeric bitmask, in any of the formats permitted by strtoul(3). An alternative to this bit-twiddling is embedded in the --mode* commandline arguments.
Execute the chroot(2) system call with the new root-directory (/) equal to path. This operation requires CAP_SYS_CHROOT to be in effect.
This command causes the program to fork a child process for so many seconds. The child will sleep that long and then exit with status 0. The purpose of this command is to support exploring the way processes are killable in the face of capability changes. See the --killit command. Only one fork can be active at a time.
This commands causes a --forkfor child to be kill(2)d with the specified signal. The command then waits for the child to exit. If the exit status does not match the signal being used to kill it, the capsh program exits with status 1.
Give a brief textual description of what privileges the specified capability makes available to a running program. Note, instead of cap_xxx, one can provide a decimal number and capsh will look up the corresponding capability’s description.
This option changes the shell that is invoked when the argument == is encountered.
This option toggles the suppression of subsequent attempts to fixup --caps= and --inh= arguments. That is, when the prevailing Effective flag does not contain CAP_SETPCAP the to be raised Inheritable Flag values (in strict mode) are limited to those in the Permitted set. The strict mode defaults to off. Supplying this argument an even number of times restores this default behavior.
Scan each of the textual descriptions of capabilities, known to capsh, and display all descriptions that include phrase.
This is a convenience feature. If you look at /proc/1/status there are some capability related fields of the following form:
This option provides a quick way to decode a capability vector represented in this hexadecimal form. Here’s an example that decodes the two lowest capability bits:
As the kernel evolves, more capabilities are added. This option can be used to verify the existence of a capability on the system. For example, --supports=cap_syslog will cause capsh to promptly exit with a status of 1 when run on kernel 2.6.27. However, when run on kernel 2.6.38 it will silently succeed.
Exit with status 1 unless the permitted vector has capability xxx raised.
Performs a check to see if the running kernel supports ambient capabilities. If not, capsh exits with status 1.
Exit with status 1 unless the ambient vector has capability xxx raised.
Exit with status 1 unless the bounding vector has capability xxx in its (default) non-blocked state.
Attempts to set the IAB tuple of inheritable capability vectors. The text conventions used for xxx are those of cap_iab_from_text(3).
Adds the specified ambient capability to the running process.
Removes the specified ambient capability from the running process.
Drops all ambient capabilities from the running process.
Suppresses overriding of the HOME and USER environment variables when a subsequent --user argument is processed.
This argument is ignored unless it is the first one. If present, it suppresses the capsh runtime check to confirm the running libcap is recent enough that it can name all of the kernel supported capability values.
Following successful execution, capsh exits with status 0. Following an error, capsh immediately exits with status 1.
Written by Andrew G. Morgan <morgan [AT] kernel.org>.