doc/cap_launch.3

.TH CAP_LAUNCH 3 "2021-08-01" "" "Linux Programmer's Manual"
.SH NAME
.nf
#include <sys/capability.h>

cap_launch_t cap_new_launcher(const char *arg0, const char * const *argv,
    const char * const *envp);

cap_launch_t cap_func_launcher(int (callback_fn)(void *detail));

void cap_launcher_callback(cap_launch_t attr,
    int (callback_fn)(void *detail));
void cap_launcher_set_mode(cap_launch_t attr, cap_mode_t flavor);
cap_iab_t cap_launcher_set_iab(cap_launch_t attr, cap_iab_t iab);
void cap_launcher_set_chroot(cap_launch_t attr, const char *chroot);

#include <sys/types.h>

pid_t cap_launch(cap_launch_t attr, void *detail);
void cap_launcher_setuid(cap_launch_t attr, uid_t uid);
void cap_launcher_setgroups(cap_launch_t attr, gid_t gid,
.fi
.sp
Link with \fI\-lcap\fP.
.SH DESCRIPTION
A launcher provides a mechanism for code to execute a callback
function and/or a program executable in an environment with a modified
security context. Essentially it provides a mechanism for a program to
.BR fork (2)
a new context from that of the main program manipulate capability and other privileged state in that
.BR fork (2)d
process before (optionally)
.BR execve (2)ing
a new program. When the application links to \fI\-lpsx\fP this support
is needed to robustly execute the launched code without modifying the
privilge of the whole (POSIX semantics honoring) main application.
.PP
A launcher is defined by one of these two functions:
.BR cap_new_launcher ()
or
.BR cap_func_launcher ().
The return value being of opaque type
.B cap_launch_t
a return value of NULL implies an error has occurred.
.PP
Once defined, a
.B cap_launch_t
value can be used with
.BR cap_launch ()
to execute that \fIlauncher\fP. In such cases, a non-negative return
value indicates success: zero meaning success without a program being
invoked; non-zero being equal to the process ID
.RB ( pid_t )
of the newly launched program.
.PP
A
.B cap_launch_t
occupies allocated memory and should be freed with
.BR cap_free (3).
Before being
.BR cap_free (3)d
a
.B cap_value_t
value may be reused for multiple independent launches. The
.I detail
argument to
.BR cap_launch (),
in conjunction with the launcher's callback function, can be used to
customize the invocation of the launch. Care must be taken to leverage
custom shared memory (see
.BR mmap (2))
or some other IPC to return values to the main program via
.I detail
since the callback and any subsequent program execution will occur
outside the main process of the calling application. An example of
this would be to allocate detail as follows:
.nf

   const *char[] args = { "echo", "hello", NULL };
   cap_launch_t cmd = cap_new_launcher("/usr/bin/echo", args, NULL);
   int *detail = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, 
                      MAP_SHARED | MAP_ANONYMOUS, -1, 0);
   cap_launcher_callback(cmd, &answer_detail_fn);
   *detail = 41;
   pid_t pid = cap_launch(cmd, detail);
   printf("launcher callback set detail to %d\\n", *detail);
   munmap(detail, sizeof(int));

.if
.PP
Unless modified by the callback function, the launched code will
execute with the capability and other security context of the
application.

If the callback function returns anything other than zero, a
.BR cap_launch ()
will be aborted. If detail of the failure is important to the caller,
it should be communicated via the
.I detail
argument.

The following functions can be used to instruct the launcher to modify
the security state of the invoked program without altering the state
of the calling program. Such modifications must be performed prior to
calling \fBcap_launch\fP() if they are to have the desired
effect. Further, they are only invoked after any installed callback
has completed. For example, one can drop or modify capabilities,
\fIjust\fP for executing a file.
.PP
The following functions instruct the launcher to do some common tasks
of this sort (note some require permitted capability bits to succeed):
.sp
.BR cap_launcher_callback ()
can be used to install or replace the currently installed callback
function of the launcher.
.sp
.BR cap_launcher_set_mode ()
can be used to ensure that the libcap-mode of the launched program is
the desired one.
.sp
.BR cap_launcher_set_iab ()
This function returns the \fBcap_iab_t\fP previously associated with
the launcher. Calling this function with an IAB value of NULL will
configure the launcher to not set an IAB value (the default).  See
\fBcap_iab\fP(3) for details on the IAB set. Note, the launcher is
associated directly with the supplied \fIiab\fP value, and does not
make a copy of it. Set with NULL to regain control over the memory
associated with that IAB value, otherwise the IAB value will be
\fBcap_free\fI()\fP'd when the launcher is.
.sp
.BR cap_launcher_set_chroot ()
This function causes the launched program executable to be invoked
inside a chroot \fIroot\fP directory.
.sp
.BR cap_launcher_setuid ()
This function causes the launched program executable to be invoked
with the specified user identifier (\fIuid_t\fP).
.sp
.BR cap_launcher_setgroups ()
This function causes the launched program executable to be invoked
with the specified primary and supplementary group IDs.
.sp
.PP
Note, if any of the launcher enhancements made by the above functions
should fail to take effect (typically for a lack of sufficient
privilege), the launch will fail and return -1.

.SH "ERRORS"
A return of NULL for a
.B cap_launch_t
should be considered an error.
.PP
.BR cap_launch ()
returns -1 in the case of an error.
.PP
In all such cases consult
.BR errno (3)
for further details.
.SH "HISTORY"
The \fBcap_launch\fP() family of functions were introduced in libcap
2.33. It primarily addresses a complexity with \fI-lpsx\fP linked
pthreads(7) applications that use capabilities but also honor POSIX
semantics.

Using \fI\-lcap\fP and \fI\-lpthread\fP together without the POSIX
semantics support from \fI\-lpsx\fP introduces a subtle class of
exposure to privilege escalation. (See
https://sites.google.com/site/fullycapable/who-ordered-libpsx for an
explanation.)
.SH "SEE ALSO"
.BR libpsx (3),
.BR psx_syscall (3),
.BR libcap (3),
.BR cap_mode (3),
.BR cap_iab (3),
.BR capabilities (7),
.BR errno (3),
.BR fork (2),
.BR mmap (2),
.BR chroot (2),
and
.BR munmap (2).