apparmor: add the ability to mediate signals

Add signal mediation where the signal can be mediated based on the
signal, direction, or the label or the peer/target. The signal perms
are verified on a cross check to ensure policy consistency in the case
of incremental policy load/replacement.

The optimization of skipping the cross check when policy is guaranteed
to be consistent (single compile unit) remains to be done.

policy rules have the form of
  SIGNAL_RULE = [ QUALIFIERS ] 'signal' [ SIGNAL ACCESS PERMISSIONS ]
                [ SIGNAL SET ] [ SIGNAL PEER ]

  SIGNAL ACCESS PERMISSIONS = SIGNAL ACCESS | SIGNAL ACCESS LIST

  SIGNAL ACCESS LIST = '(' Comma or space separated list of SIGNAL
                           ACCESS ')'

  SIGNAL ACCESS = ( 'r' | 'w' | 'rw' | 'read' | 'write' | 'send' |
                    'receive' )

  SIGNAL SET = 'set' '=' '(' SIGNAL LIST ')'

  SIGNAL LIST = Comma or space separated list of SIGNALS

  SIGNALS = ( 'hup' | 'int' | 'quit' | 'ill' | 'trap' | 'abrt' |
              'bus' | 'fpe' | 'kill' | 'usr1' | 'segv' | 'usr2' |
	      'pipe' | 'alrm' | 'term' | 'stkflt' | 'chld' | 'cont' |
	      'stop' | 'stp' | 'ttin' | 'ttou' | 'urg' | 'xcpu' |
	      'xfsz' | 'vtalrm' | 'prof' | 'winch' | 'io' | 'pwr' |
	      'sys' | 'emt' | 'exists' | 'rtmin+0' ... 'rtmin+32'
            )

  SIGNAL PEER = 'peer' '=' AARE

eg.
  signal,                                 # allow all signals
  signal send set=(hup, kill) peer=foo,

Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Seth Arnold <seth.arnold@canonical.com>

security/apparmor/include/audit.h

@@ -86,6 +86,7 @@ enum audit_type {
 #define OP_SHUTDOWN "socket_shutdown"
 
 #define OP_PTRACE "ptrace"
+#define OP_SIGNAL "signal"
 
 #define OP_EXEC "exec"
 
@@ -126,6 +127,7 @@ struct apparmor_audit_data {
 			long pos;
 			const char *ns;
 		} iface;
+		int signal;
 		struct {
 			int rlim;
 			unsigned long max;