Yama: add additional ptrace scopes

This expands the available Yama ptrace restrictions to include two more
modes. Mode 2 requires CAP_SYS_PTRACE for PTRACE_ATTACH, and mode 3
completely disables PTRACE_ATTACH (and locks the sysctl).

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>

Documentation/security/Yama.txt

@@ -34,7 +34,7 @@ parent to a child process (i.e. direct "gdb EXE" and "strace EXE" still
 work), or with CAP_SYS_PTRACE (i.e. "gdb --pid=PID", and "strace -p PID"
 still work as root).
 
-For software that has defined application-specific relationships
+In mode 1, software that has defined application-specific relationships
 between a debugging process and its inferior (crash handlers, etc),
 prctl(PR_SET_PTRACER, pid, ...) can be used. An inferior can declare which
 other process (and its descendents) are allowed to call PTRACE_ATTACH
@@ -46,6 +46,8 @@ restrictions, it can call prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, ...)
 so that any otherwise allowed process (even those in external pid namespaces)
 may attach.
 
+These restrictions do not change how ptrace via PTRACE_TRACEME operates.
+
 The sysctl settings are:
 
 0 - classic ptrace permissions: a process can PTRACE_ATTACH to any other
@@ -60,6 +62,12 @@ The sysctl settings are:
     inferior can call prctl(PR_SET_PTRACER, debugger, ...) to declare
     an allowed debugger PID to call PTRACE_ATTACH on the inferior.
 
+2 - admin-only attach: only processes with CAP_SYS_PTRACE may use ptrace
+    with PTRACE_ATTACH.
+
+3 - no attach: no processes may use ptrace with PTRACE_ATTACH. Once set,
+    this sysctl cannot be changed to a lower value.
+
 The original children-only logic was based on the restrictions in grsecurity.
 
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