pdptrace(2): allow debugging in capability mode

The pdopenpid(2) syscall is allowed in capability mode. Add the chicken
switch security.bsd.ptrace_in_cap_mode, which disables it without
reboot, if needed.

The descriptor passed to pdptrace(2) must have the CAP_PTRACE
capability enabled. This capability is not enabled by default by
pdfork()/pdopenpid(), and the calls do not return a procdesc suitable
for debugging. The opening code must prepare for debugging in advance by
passing the PD_PTRACE_CAP flag to pdfork()/pdopenpid().

For ptrace(2), allow PT_CLEARSTEP and PT_GET_CHILDREN for the current
thread and process in cap mode as well.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D58989

sys/compat/freebsd32/freebsd32_misc.c

@@ -1050,9 +1050,6 @@ freebsd32_ptrace_useraction(struct thread *td, int req, bool pd_mode, pid_t pid,
 	void *addr;
 	int data, error, i;
 
-	if (!allow_ptrace)
-		return (ENOSYS);
-
 	error = 0;
 	addr = &r;
 	data = udata;

sys/kern/sys_process.c

@@ -51,6 +51,7 @@
 #include <sys/sleepqueue.h>
 #include <sys/sx.h>
 #include <sys/syscallsubr.h>
+#include <sys/sysctl.h>
 #include <sys/sysent.h>
 #include <sys/sysproto.h>
 #include <sys/vnode.h>
@@ -708,6 +709,24 @@ ptrace_vm_entry(struct thread *td, struct proc *p, struct ptrace_vm_entry *pve)
 	return (error);
 }
 
+static int
+ptrace_check_allowed(struct thread *td, int req, bool pd_mode, pid_t pid)
+{
+	if (!allow_ptrace)
+		return (ENOSYS);
+	if (!IN_CAPABILITY_MODE(td))
+		return (0);
+	if (!allow_ptrace_in_cap_mode)
+		return (ECAPMODE);
+	if (pd_mode)
+		return (0);
+	if (req == PT_GET_CHILDREN && pid == td->td_proc->p_pid)
+		return (0);
+	if (req == PT_CLEARSTEP && pid == td->td_tid)
+		return (0);
+	return (ECAPMODE);
+}
+
 /*
  * Process debugging system call.
  */
@@ -748,12 +767,11 @@ ptrace_useraction(struct thread *td, int req, bool pd_mode, pid_t pid, int pfd,
 	void *addr;
 	int error, data;
 
-	if (!allow_ptrace)
-		return (ENOSYS);
+	error = ptrace_check_allowed(td, req, pd_mode, pid);
+	if (error != 0)
+		return (error);
 
-	error = 0;
 	addr = &r;
-
 	switch (req) {
 	case PT_GET_EVENT_MASK:
 	case PT_LWPINFO:

sys/kern/syscalls.master

@@ -264,7 +264,7 @@
 25	AUE_GETEUID	STD|CAPENABLED {
 		uid_t geteuid(void);
 	}
-26	AUE_PTRACE	STD {
+26	AUE_PTRACE	STD|CAPENABLED {
 		int ptrace(
 		    int req,
 		    pid_t pid,
@@ -3442,7 +3442,7 @@
 			int flags
 		);
 	}
-605	AUE_PDPTRACE	STD {
+605	AUE_PDPTRACE	STD|CAPENABLED {
 		int pdptrace(
 			int req,
 			int pfd,