CAPSH(1) User Commands CAPSH(1)
NAME
capsh - capability shell wrapper
SYNOPSIS
capsh [OPTION]...
DESCRIPTION
Linux capability support and use can be explored and constrained with
this tool. This tool provides a handy wrapper for certain types of ca-
pability testing and environment creation. It also provides some debug-
ging features useful for summarizing capability state.
OPTIONS
capsh takes a number of optional arguments, acting on them in the order
they are provided. They are as follows:
--help Display the list of commands supported by capsh.
--print
Display prevailing capability and related state.
--current
Display prevailing capability state, 1e capabilities and IAB
vector.
-- [args]
Execute /bin/bash with trailing arguments. Note, you can use -c
'command to execute' for specific commands.
-+ [args]
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.
== [args]
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 oc-
curs after a --chroot=/some/path argument the PATH located bi-
nary may not be resolve to the same binary as that running ini-
tially. This behavior is an intended feature as it can complete
the chroot transition.
=+ [args]
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.
--caps=cap-set
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).
--drop=cap-list
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 effec-
tive set.
--inh=cap-list
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 inherita-
ble and permitted capability sets, or capsh should be operating
with CAP_SETPCAP in its effective set.
--user=username
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 capa-
bilities will be cleared, but the permitted set will not be, so
the running program is still privileged.
--mode Display the prevailing libcap mode as guessed by the
cap_get_mode(3) function.
--mode=<mode>
Force the program into a cap_set_mode(3) security mode. This is
a set of securebits and prevailing capability arrangement recom-
mended for its pre-determined security stance.
--modes
Lists all of the libcap modes supported by --mode=<mode>.
--inmode=<mode>
Confirm that the prevailing mode is that specified in <mode>, or
exit with a status 1.
--uid=id
Force all UID values to equal id using the setuid(2) system
call. This argument may require explicit preparation of the ef-
fective set.
--cap-uid=<uid>
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.
--is-uid=<id>
Exit with status 1 unless the current UID equals <id>.
--gid=<id>
Force all GID values to equal id using the setgid(2) system
call.
--is-gid=<id>
Exit with status 1 unless the current GIQ equals <id>.
--groups=<gid-list>
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.
--keep=<0|1>
In a non-pure capability mode, the kernel provides liberal priv-
ilege to the super-user. However, it is normally the case that
when the super-user changes UID to some lesser user, then capa-
bilities are dropped. For these situations, the kernel can per-
mit 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.
--secbits=N
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.
--chroot=/some/path
Execute the chroot(2) system call with the new root-directory
(/) equal to path. This operation requires CAP_SYS_CHROOT to be
in effect.
--forkfor=sec
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.
--killit=sig
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.
--explain=cap_xxx
Give a brief textual description of what privileges the speci-
fied capability makes available to a running program. Note, in-
stead of cap_xxx, one can provide a decimal number and capsh
will look up the corresponding capability's description.
--shell=/full/path
This option changes the shell that is invoked when the argument
== is encountered.
--strict
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 In-
heritable 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 be-
havior.
--suggest=phrase
Scan each of the textual descriptions of capabilities, known to
capsh, and display all descriptions that include phrase.
--decode=N
This is a convenience feature. If you look at /proc/1/status
there are some capability related fields of the following form:
CapInh: 0000000000000000
CapPrm: 0000003fffffffff
CapEff: 0000003fffffffff
CapBnd: 0000003fffffffff
CapAmb: 0000000000000000
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:
$ capsh --decode=3
0x0000000000000003=cap_chown,cap_dac_override
--supports=xxx
As the kernel evolves, more capabilities are added. This option
can be used to verify the existence of a capability on the sys-
tem. 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.
--has-p=xxx
Exit with status 1 unless the permitted vector has capability
xxx raised.
--has-ambient
Performs a check to see if the running kernel supports ambient
capabilities. If not, capsh exits with status 1.
--has-a=xxx
Exit with status 1 unless the ambient vector has capability xxx
raised.
--has-b=xxx
Exit with status 1 unless the bounding vector has capability xxx
in its (default) non-blocked state.
--iab=xxx
Attempts to set the IAB tuple of inheritable capability vectors.
The text conventions used for xxx are those of
cap_iab_from_text(3).
--addamb=xxx
Adds the specified ambient capability to the running process.
--delamb=xxx
Removes the specified ambient capability from the running
process.
--noamb
Drops all ambient capabilities from the running process.
--noenv
Suppresses overriding of the HOME and USER environment variables
when a subsequent --user argument is processed.
--quiet
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 sup-
ported capability values.
EXIT STATUS
Following successful execution, capsh exits with status 0. Following an
error, capsh immediately exits with status 1.
AUTHOR
Written by Andrew G. Morgan <morgan@kernel.org>.
REPORTING BUGS
Please report bugs via:
https://bugzilla.kernel.org/buglist.cgi?component=lib-
cap&list_id=1090757
SEE ALSO
libcap(3), cap_from_text(3), cap_iab(3) capabilities(7), captree(8),
getcap(8), getpcaps(8), and setcap(8).
libcap 2021-10-22 CAPSH(1)
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