libc/sys/procctl.2

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.Dd September 2, 2021
.Dt PROCCTL 2
.Os
.Sh NAME
.Nm procctl
.Nd control processes
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/procctl.h
.Ft int
.Fn procctl "idtype_t idtype" "id_t id" "int cmd" "void *data"
.Sh DESCRIPTION
The
.Fn procctl
system call provides for control over processes.
The
.Fa idtype
and
.Fa id
arguments specify the set of processes to control.
If multiple processes match the identifier,
.Nm
will make a
.Dq best effort
to control as many of the selected processes as possible.
An error is only returned if no selected processes successfully complete
the request.
The following identifier types are supported:
.Bl -tag -width P_PGID
.It Dv P_PID
Control the process with the process ID
.Fa id .
.Fa id
zero is a shortcut for the calling process ID.
.It Dv P_PGID
Control processes belonging to the process group with the ID
.Fa id .
.El
.Pp
The control request to perform is specified by the
.Fa cmd
argument.
.Pp
All status changing requests
.Dv *_CTL
require the caller to have the right to debug the target.
All status query requests
.DV *_STATUS
require the caller to have the right to observe the target.
.Pp
The following commands are supported:
.Bl -tag -width PROC_TRAPCAP_STATUS
.It Dv PROC_ASLR_CTL
Controls the Address Space Layout Randomization (ASLR) in the program
images created
by
.Xr execve 2
in the specified process or its descendants that did not changed
the control nor modified it by other means.
The
.Fa data
parameter must point to the integer variable holding one of the following
values:
.Bl -tag -width PROC_ASLR_FORCE_DISABLE
.It Dv PROC_ASLR_FORCE_ENABLE
Request that ASLR is enabled after execution, even if it is disabled
system-wide.
The image flag and set-uid might prevent ASLR enablement still.
.It Dv PROC_ASLR_FORCE_DISABLE
Request that ASLR is disabled after execution.
Same notes as for
.Dv PROC_ASLR_FORCE_ENABLE
apply.
.It Dv PROC_ASLR_NOFORCE
Use the system-wide configured policy for ASLR.
.El
.It Dv PROC_ASLR_STATUS
Returns the current status of ASLR enablement for the target process.
The
.Fa data
parameter must point to the integer variable, where one of the
following values is written:
.Bl -tag -width PROC_ASLR_FORCE_DISABLE
.It Dv PROC_ASLR_FORCE_ENABLE
.It Dv PROC_ASLR_FORCE_DISABLE
.It Dv PROC_ASLR_NOFORCE
.El
.Pp
If the currently executed image in the process itself has ASLR enabled,
the
.Dv PROC_ASLR_ACTIVE
flag is or-ed with the value listed above.
.It Dv PROC_PROTMAX_CTL
Controls implicit application of PROT_MAX protection equal to the
.Fa prot
argument of the
.Xr mmap 2
syscall, in the target process.
The
.Fa data
parameter must point to the integer variable holding one of the following
values:
.Bl -tag -width PROC_PROTMAX_FORCE_DISABLE
.It Dv PROC_PROTMAX_FORCE_ENABLE
Enables implicit PROT_MAX application,
even if it is disabled system-wide by the sysctl
.Va vm.imply_prot_max .
The image flag might still prevent the enablement.
.It Dv PROC_PROTMAX_FORCE_DISABLE
Request that implicit application of PROT_MAX be disabled.
Same notes as for
.Dv PROC_PROTMAX_FORCE_ENABLE
apply.
.It Dv PROC_PROTMAX_NOFORCE
Use the system-wide configured policy for PROT_MAX.
.El
.It Dv PROC_PROTMAX_STATUS
Returns the current status of implicit PROT_MAX enablement for the
target process.
The
.Fa data
parameter must point to the integer variable, where one of the
following values is written:
.Bl -tag -width PROC_PROTMAX_FORCE_DISABLE
.It Dv PROC_PROTMAX_FORCE_ENABLE
.It Dv PROC_PROTMAX_FORCE_DISABLE
.It Dv PROC_PROTMAX_NOFORCE
.El
.Pp
If the currently executed image in the process itself has implicit PROT_MAX
application enabled, the
.Dv PROC_PROTMAX_ACTIVE
flag is or-ed with the value listed above.
.It Dv PROC_SPROTECT
Set process protection state.
This is used to mark a process as protected from being killed if the system
exhausts the available memory and swap.
The
.Fa data
parameter must point to an integer containing an operation and zero or more
optional flags.
The following operations are supported:
.Bl -tag -width PPROT_CLEAR
.It Dv PPROT_SET
Mark the selected processes as protected.
.It Dv PPROT_CLEAR
Clear the protected state of selected processes.
.El
.Pp
The following optional flags are supported:
.Bl -tag -width PPROT_DESCEND
.It Dv PPROT_DESCEND
Apply the requested operation to all child processes of each selected process
in addition to each selected process.
.It Dv PPROT_INHERIT
When used with
.Dv PPROT_SET ,
mark all future child processes of each selected process as protected.
Future child processes will also mark all of their future child processes.
.El
.It Dv PROC_REAP_ACQUIRE
Acquires the reaper status for the current process.
Reaper status means that children orphaned by the reaper's descendants
that were forked after the acquisition of reaper status are reparented to the
reaper process.
After system initialization,
.Xr init 8
is the default reaper.
.It Dv PROC_REAP_RELEASE
Release the reaper state for the current process.
The reaper of the current process becomes the new reaper of the
current process's descendants.
.It Dv PROC_REAP_STATUS
Provides information about the reaper of the specified process,
or the process itself when it is a reaper.
The
.Fa data
argument must point to a
.Vt procctl_reaper_status
structure which is filled in by the syscall on successful return.
.Bd -literal
struct procctl_reaper_status {
	u_int	rs_flags;
	u_int	rs_children;
	u_int	rs_descendants;
	pid_t	rs_reaper;
	pid_t	rs_pid;
};
.Ed
The
.Fa rs_flags
may have the following flags returned:
.Bl -tag -width REAPER_STATUS_REALINIT
.It Dv REAPER_STATUS_OWNED
The specified process has acquired reaper status and has not
released it.
When the flag is returned, the specified process
.Fa id ,
pid, identifies the reaper, otherwise the
.Fa rs_reaper
field of the structure is set to the pid of the reaper
for the specified process id.
.It Dv REAPER_STATUS_REALINIT
The specified process is the root of the reaper tree, i.e.,
.Xr init 8 .
.El
.Pp
The
.Fa rs_children
field returns the number of children of the reaper among the descendants.
It is possible to have a child whose reaper is not the specified process,
since the reaper for any existing children is not reset on the
.Dv PROC_REAP_ACQUIRE
operation.
The
.Fa rs_descendants
field returns the total number of descendants of the reaper(s),
not counting descendants of the reaper in the subtree.
The
.Fa rs_reaper
field returns the reaper pid.
The
.Fa rs_pid
returns the pid of one reaper child if there are any descendants.
.It Dv PROC_REAP_GETPIDS
Queries the list of descendants of the reaper of the specified process.
The request takes a pointer to a
.Vt procctl_reaper_pids
structure in the
.Fa data
parameter.
.Bd -literal
struct procctl_reaper_pids {
	u_int	rp_count;
	struct procctl_reaper_pidinfo *rp_pids;
};
.Ed
When called, the
.Fa rp_pids
field must point to an array of
.Vt procctl_reaper_pidinfo
structures, to be filled in on return,
and the
.Fa rp_count
field must specify the size of the array,
into which no more than
.Fa rp_count
elements will be filled in by the kernel.
.Pp
The
.Vt "struct procctl_reaper_pidinfo"
structure provides some information about one of the reaper's descendants.
Note that for a descendant that is not a child, it may be incorrectly
identified because of a race in which the original child process exited
and the exited process's pid was reused for an unrelated process.
.Bd -literal
struct procctl_reaper_pidinfo {
	pid_t	pi_pid;
	pid_t	pi_subtree;
	u_int	pi_flags;
};
.Ed
The
.Fa pi_pid
field is the process id of the descendant.
The
.Fa pi_subtree
field provides the pid of the child of the reaper, which is the (grand-)parent
of the process.
The
.Fa pi_flags
field returns the following flags, further describing the descendant:
.Bl -tag -width REAPER_PIDINFO_REAPER
.It Dv REAPER_PIDINFO_VALID
Set to indicate that the
.Vt procctl_reaper_pidinfo
structure was filled in by the kernel.
Zero-filling the
.Fa rp_pids
array and testing the
.Dv REAPER_PIDINFO_VALID
flag allows the caller to detect the end
of the returned array.
.It Dv REAPER_PIDINFO_CHILD
The
.Fa pi_pid
field identifies the direct child of the reaper.
.It Dv REAPER_PIDINFO_REAPER
The reported process is itself a reaper.
The descendants of the subordinate reaper are not reported.
.El
.It Dv PROC_REAP_KILL
Request to deliver a signal to some subset of the descendants of the reaper.
The
.Fa data
parameter must point to a
.Vt procctl_reaper_kill
structure, which is used both for parameters and status return.
.Bd -literal
struct procctl_reaper_kill {
	int	rk_sig;
	u_int	rk_flags;
	pid_t	rk_subtree;
	u_int	rk_killed;
	pid_t	rk_fpid;
};
.Ed
The
.Fa rk_sig
field specifies the signal to be delivered.
Zero is not a valid signal number, unlike for
.Xr kill 2 .
The
.Fa rk_flags
field further directs the operation.
It is or-ed from the following flags:
.Bl -tag -width REAPER_KILL_CHILDREN
.It Dv REAPER_KILL_CHILDREN
Deliver the specified signal only to direct children of the reaper.
.It Dv REAPER_KILL_SUBTREE
Deliver the specified signal only to descendants that were forked by
the direct child with pid specified in the
.Fa rk_subtree
field.
.El
If neither the
.Dv REAPER_KILL_CHILDREN
nor the
.Dv REAPER_KILL_SUBTREE
flags are specified, all current descendants of the reaper are signalled.
.Pp
If a signal was delivered to any process, the return value from the request
is zero.
In this case, the
.Fa rk_killed
field identifies the number of processes signalled.
The
.Fa rk_fpid
field is set to the pid of the first process for which signal
delivery failed, e.g., due to permission problems.
If no such process exists, the
.Fa rk_fpid
field is set to -1.
.It Dv PROC_TRACE_CTL
Enable or disable tracing of the specified process(es), according to the
value of the integer argument.
Tracing includes attachment to the process using the
.Xr ptrace 2
and
.Xr ktrace 2 ,
debugging sysctls,
.Xr hwpmc 4 ,
.Xr dtrace 1 ,
and core dumping.
Possible values for the
.Fa data
argument are:
.Bl -tag -width PROC_TRACE_CTL_DISABLE_EXEC
.It Dv PROC_TRACE_CTL_ENABLE
Enable tracing, after it was disabled by
.Dv PROC_TRACE_CTL_DISABLE .
Only allowed for self.
.It Dv PROC_TRACE_CTL_DISABLE
Disable tracing for the specified process.
Tracing is re-enabled when the process changes the executing
program with the
.Xr execve 2
syscall.
A child inherits the trace settings from the parent on
.Xr fork 2 .
.It Dv PROC_TRACE_CTL_DISABLE_EXEC
Same as
.Dv PROC_TRACE_CTL_DISABLE ,
but the setting persists for the process even after
.Xr execve 2 .
.El
.It Dv PROC_TRACE_STATUS
Returns the current tracing status for the specified process in
the integer variable pointed to by
.Fa data .
If tracing is disabled,
.Fa data
is set to -1.
If tracing is enabled, but no debugger is attached by the
.Xr ptrace 2
syscall,
.Fa data
is set to 0.
If a debugger is attached,
.Fa data
is set to the pid of the debugger process.
.It Dv PROC_TRAPCAP_CTL
Controls the capability mode sandbox actions for the specified
sandboxed processes,
on a return from any syscall which gives either a
.Er ENOTCAPABLE
or
.Er ECAPMODE
error.
If the control is enabled, such errors from the syscalls cause
delivery of the synchronous
.Dv SIGTRAP
signal to the thread immediately before returning from the syscalls.
.Pp
Possible values for the
.Fa data
argument are:
.Bl -tag -width PROC_TRAPCAP_CTL_DISABLE
.It Dv PROC_TRAPCAP_CTL_ENABLE
Enable the
.Dv SIGTRAP
signal delivery on capability mode access violations.
The enabled mode is inherited by the children of the process,
and is kept after
.Xr fexecve 2
calls.
.It Dv PROC_TRAPCAP_CTL_DISABLE
Disable the signal delivery on capability mode access violations.
Note that the global sysctl
.Dv kern.trap_enotcap
might still cause the signal to be delivered.
See
.Xr capsicum 4 .
.El
.Pp
On signal delivery, the
.Va si_errno
member of the
.Fa siginfo
signal handler parameter is set to the syscall error value,
and the
.Va si_code
member is set to
.Dv TRAP_CAP .
.Pp
See
.Xr capsicum 4
for more information about the capability mode.
.It Dv PROC_TRAPCAP_STATUS
Return the current status of signalling capability mode access
violations for the specified process.
The integer value pointed to by the
.Fa data
argument is set to the
.Dv PROC_TRAPCAP_CTL_ENABLE
value if the process control enables signal delivery, and to
.Dv PROC_TRAPCAP_CTL_DISABLE
otherwise.
.Pp
See the note about sysctl
.Dv kern.trap_enotcap
above, which gives independent global control of signal delivery.
.It Dv PROC_PDEATHSIG_CTL
Request the delivery of a signal when the parent of the calling
process exits.
.Fa idtype
must be
.Dv P_PID
and
.Fa id
must be the either caller's pid or zero, with no difference in effect.
The value is cleared for child processes
and when executing set-user-ID or set-group-ID binaries.
.Fa data
must point to a value of type
.Vt int
indicating the signal
that should be delivered to the caller.
Use zero to cancel a previously requested signal delivery.
.It Dv PROC_PDEATHSIG_STATUS
Query the current signal number that will be delivered when the parent
of the calling process exits.
.Fa idtype
must be
.Dv P_PID
and
.Fa id
must be the either caller's pid or zero, with no difference in effect.
.Fa data
must point to a memory location that can hold a value of type
.Vt int .
If signal delivery has not been requested, it will contain zero
on return.
.It Dv PROC_STACKGAP_CTL
Controls the stack gaps in the specified process.
A stack gap is the part of the growth area for a
.Dv MAP_STACK
mapped region that is reserved and never filled by memory.
Instead, the process is guaranteed to receive a
.Dv SIGSEGV
signal on accessing pages in the gap.
Gaps protect against stack overflow corrupting memory adjacent
to the stack.
.Pp
The
.Fa data
argument must point to an integer variable containing flags.
The following flags are allowed:
.Bl -tag -width PROC_STACKGAP_DISABLE_EXEC
.It Dv PROC_STACKGAP_ENABLE
This flag is only accepted for consistency with
.Dv PROC_STACKGAP_STATUS .
If stack gaps are enabled, the flag is ignored.
If disabled, the flag causes an
.Ev EINVAL
error to be returned.
After gaps are disabled in a process, they can only be re-enabled when an
.Xr execve 2
is performed.
.It Dv PROC_STACKGAP_DISABLE
Disable stack gaps for the process.
For existing stacks, the gap is no longer a reserved part of the growth
area and can be filled by memory on access.
.It Dv PROC_STACKGAP_ENABLE_EXEC
Enable stack gaps for programs started after an
.Xr execve 2
by the specified process.
.It Dv PROC_STACKGAP_DISABLE_EXEC
Inherit disabled stack gaps state after
.Xr execve 2 .
In other words, if the currently executing program has stack gaps disabled,
they are kept disabled on exec.
If gaps were enabled, they are kept enabled after exec.
.El
.Pp
The stack gap state is inherited from the parent on
.Xr fork 2 .
.It Dv PROC_STACKGAP_STATUS
Returns the current stack gap state for the specified process.
.Fa data
must point to an integer variable, which is used to return a bitmask
consisting of the following flags:
.Bl -tag -width PROC_STACKGAP_DISABLE_EXEC
.It Dv PROC_STACKGAP_ENABLE
Stack gaps are enabled.
.It Dv PROC_STACKGAP_DISABLE
Stack gaps are disabled.
.It Dv PROC_STACKGAP_ENABLE_EXEC
Stack gaps are enabled in the process after
.Xr execve 2 .
.It Dv PROC_STACKGAP_DISABLE_EXEC
Stack gaps are disabled in the process after
.Xr execve 2 .
.El
.It Dv PROC_NO_NEW_PRIVS_CTL
Allows one to ignore the SUID and SGID bits on the program
images activated by
.Xr execve 2
in the specified process and its future descendants.
The
.Fa data
parameter must point to the integer variable holding the following
value:
.Bl -tag -width PROC_NO_NEW_PRIVS_ENABLE
.It Dv PROC_NO_NEW_PRIVS_ENABLE
Request SUID and SGID bits to be ignored.
.El
.Pp
It is not possible to disable it once it has been enabled.
.It Dv PROC_NO_NEW_PRIVS_STATUS
Returns the current status of SUID/SGID enablement for the target process.
The
.Fa data
parameter must point to the integer variable, where one of the
following values is written:
.Bl -tag -width PROC_NO_NEW_PRIVS_DISABLE
.It Dv PROC_NO_NEW_PRIVS_ENABLE
.It Dv PROC_NO_NEW_PRIVS_DISABLE
.El
.It Dv PROC_WXMAP_CTL
Controls the 'write exclusive against execution' permissions for the
mappings in the process address space.
It overrides the global settings established by the
.Dv kern.elf{32/64}.allow_wx
sysctl,
and the corresponding bit in the ELF control note, see
.Xr elfctl 1 .
.Pp
The
.Fa data
parameter must point to the integer variable holding one of the
following values:
.Bl -tag -width PROC_WX_MAPPINGS_DISALLOW_EXEC
.It Dv PROC_WX_MAPPINGS_PERMIT
Enable creation of mappings that have both write and execute
protection attributes, in the specified process' address space.
.It Dv PROC_WX_MAPPINGS_DISALLOW_EXEC
In the new address space created by
.Xr execve 2 ,
disallow creation of mappings that have both write and execute
permissions.
.El
.Pp
Once creation of writeable and executable mappings is allowed,
it is impossible (and pointless) to disallow it.
The only way to ensure the absence of such mappings after they
were enabled in a given process, is to set the
.Dv PROC_WX_MAPPINGS_DISALLOW_EXEC
flag and
.Xr execve 2
an image.
.It Dv PROC_WXMAP_STATUS
Returns the current status of the 'write exclusive against execution'
enforcement for the specified process.
The
.Dv data
parameter must point to the integer variable, where one of the
following values is written:
.Bl -tag -width PROC_WX_MAPPINGS_DISALLOW_EXEC
.It Dv PROC_WX_MAPPINGS_PERMIT
Creation of simultaneously writable and executable mapping is permitted,
otherwise the process cannot create such mappings.
.It Dv PROC_WX_MAPPINGS_DISALLOW_EXEC
After
.Xr execve 2 ,
the new address space should disallow creation of simultaneously
writable and executable mappings.
.El
.Pp
Additionally, if the address space of the process disallows
creation of simultaneously writable and executable mappings and
it is guaranteed that no such mapping was created since address space
creation, the
.Dv PROC_WXORX_ENFORCE
flag is set in the returned value.
.El
.Sh x86 MACHINE-SPECIFIC REQUESTS
.Bl -tag -width PROC_KPTI_STATUS
.It Dv PROC_KPTI_CTL
AMD64 only.
Controls the Kernel Page Table Isolation (KPTI) option for the children
of the specified process.
For the command to work, the
.Va vm.pmap.kpti
tunable must be enabled on boot.
It is not possible to change the KPTI setting for a running process,
except at the
.Xr execve 2 ,
where the address space is reinitialized.
.Pp
The
.Fa data
parameter must point to an integer variable containing one of the
following commands:
.Bl -tag -width PROC_KPTI_CTL_DISABLE_ON_EXEC
.It Dv PROC_KPTI_CTL_ENABLE_ON_EXEC
Enable KPTI after
.Xr execve 2 .
.It Dv PROC_KPTI_CTL_DISABLE_ON_EXEC
Disable KPTI after
.Xr execve 2 .
Only root or a process having the
.Va PRIV_IO
privilege might use this option.
.El
.It Dv PROC_KPTI_STATUS
Returns the current KPTI status for the specified process.
.Fa data
must point to the integer variable, which returns the
following statuses:
.Bl -tag -width PROC_KPTI_CTL_DISABLE_ON_EXEC
.It Dv PROC_KPTI_CTL_ENABLE_ON_EXEC
.It Dv PROC_KPTI_CTL_DISABLE_ON_EXEC
.El
.Pp
The status is or-ed with the
.Va PROC_KPTI_STATUS_ACTIVE
in case KPTI is active for the current address space of the process.
.Sh NOTES
Disabling tracing on a process should not be considered a security
feature, as it is bypassable both by the kernel and privileged processes,
and via other system mechanisms.
As such, it should not be utilized to reliably protect cryptographic
keying material or other confidential data.
.Pp
Note that processes can trivially bypass the 'no simultaneously
writable and executable mappings' policy by first marking some mapping
as writeable and write code to it, then removing write and adding
execute permission.
This may be legitimately required by some programs, such as JIT compilers.
.Sh RETURN VALUES
If an error occurs, a value of -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn procctl
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa data
parameter points outside the process's allocated address space.
.It Bq Er EINVAL
The
.Fa cmd
argument specifies an unsupported command.
.Pp
The
.Fa idtype
argument specifies an unsupported identifier type.
.It Bq Er EPERM
The calling process does not have permission to perform the requested
operation on any of the selected processes.
.It Bq Er ESRCH
No processes matched the requested
.Fa idtype
and
.Fa id .
.It Bq Er EINVAL
An invalid operation or flag was passed in
.Fa data
for a
.Dv PROC_SPROTECT
command.
.It Bq Er EPERM
The
.Fa idtype
argument is not equal to
.Dv P_PID ,
or
.Fa id
is not equal to the pid of the calling process, for
.Dv PROC_REAP_ACQUIRE
or
.Dv PROC_REAP_RELEASE
requests.
.It Bq Er EINVAL
Invalid or undefined flags were passed to a
.Dv PROC_REAP_KILL
request.
.It Bq Er EINVAL
An invalid or zero signal number was requested for a
.Dv PROC_REAP_KILL
request.
.It Bq Er EINVAL
The
.Dv PROC_REAP_RELEASE
request was issued by the
.Xr init 8
process.
.It Bq Er EBUSY
The
.Dv PROC_REAP_ACQUIRE
request was issued by a process that had already acquired reaper status
and has not yet released it.
.It Bq Er EBUSY
The
.Dv PROC_TRACE_CTL
request was issued for a process already being traced.
.It Bq Er EPERM
The
.Dv PROC_TRACE_CTL
request to re-enable tracing of the process
.Po Dv PROC_TRACE_CTL_ENABLE Pc ,
or to disable persistence of
.Dv PROC_TRACE_CTL_DISABLE
on
.Xr execve 2
was issued for a non-current process.
.It Bq Er EINVAL
The value of the integer
.Fa data
parameter for the
.Dv PROC_TRACE_CTL
or
.Dv PROC_TRAPCAP_CTL
request is invalid.
.It Bq Er EINVAL
The
.Dv PROC_PDEATHSIG_CTL
or
.Dv PROC_PDEATHSIG_STATUS
request referenced an unsupported
.Fa id ,
.Fa idtype
or invalid signal number.
.El
.Sh SEE ALSO
.Xr dtrace 1 ,
.Xr proccontrol 1 ,
.Xr protect 1 ,
.Xr cap_enter 2 ,
.Xr kill 2 ,
.Xr ktrace 2 ,
.Xr mmap 2 ,
.Xr mprotect 2 ,
.Xr ptrace 2 ,
.Xr wait 2 ,
.Xr capsicum 4 ,
.Xr hwpmc 4 ,
.Xr init 8
.Sh HISTORY
The
.Fn procctl
function appeared in
.Fx 10.0 .
.Pp
The reaper facility is based on a similar feature of Linux and
DragonflyBSD, and first appeared in
.Fx 10.2 .
.Pp
The
.Dv PROC_PDEATHSIG_CTL
facility is based on the prctl(PR_SET_PDEATHSIG, ...) feature of Linux,
and first appeared in
.Fx 11.2 .
.Pp
The ASLR support was added to system for the checklists compliance in
.Fx 13.0 .