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/apparmorfs.c

@@ -32,6 +32,7 @@
 #include "include/audit.h"
 #include "include/context.h"
 #include "include/crypto.h"
+#include "include/ipc.h"
 #include "include/policy_ns.h"
 #include "include/label.h"
 #include "include/policy.h"
@@ -2129,6 +2130,11 @@ static struct aa_sfs_entry aa_sfs_entry_ptrace[] = {
 	{ }
 };
 
+static struct aa_sfs_entry aa_sfs_entry_signal[] = {
+	AA_SFS_FILE_STRING("mask", AA_SFS_SIG_MASK),
+	{ }
+};
+
 static struct aa_sfs_entry aa_sfs_entry_domain[] = {
 	AA_SFS_FILE_BOOLEAN("change_hat",	1),
 	AA_SFS_FILE_BOOLEAN("change_hatv",	1),
@@ -2179,6 +2185,7 @@ static struct aa_sfs_entry aa_sfs_entry_features[] = {
 	AA_SFS_DIR("rlimit",			aa_sfs_entry_rlimit),
 	AA_SFS_DIR("caps",			aa_sfs_entry_caps),
 	AA_SFS_DIR("ptrace",			aa_sfs_entry_ptrace),
+	AA_SFS_DIR("signal",			aa_sfs_entry_signal),
 	AA_SFS_DIR("query",			aa_sfs_entry_query),
 	{ }
 };

security/apparmor/include/apparmor.h

@@ -28,6 +28,7 @@
 #define AA_CLASS_RLIMITS	5
 #define AA_CLASS_DOMAIN		6
 #define AA_CLASS_PTRACE		9
+#define AA_CLASS_SIGNAL		10
 #define AA_CLASS_LABEL		16
 
 #define AA_CLASS_LAST		AA_CLASS_LABEL

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;

security/apparmor/include/ipc.h

@@ -27,8 +27,14 @@ struct aa_profile;
 
 #define AA_PTRACE_PERM_MASK (AA_PTRACE_READ | AA_PTRACE_TRACE | \
 			     AA_MAY_BE_READ | AA_MAY_BE_TRACED)
+#define AA_SIGNAL_PERM_MASK (MAY_READ | MAY_WRITE)
+
+#define AA_SFS_SIG_MASK "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 lost"
 
 int aa_may_ptrace(struct aa_label *tracer, struct aa_label *tracee,
 		  u32 request);
+int aa_may_signal(struct aa_label *sender, struct aa_label *target, int sig);
 
 #endif /* __AA_IPC_H */

security/apparmor/include/sig_names.h

+#include <linux/signal.h>
+
+#define SIGUNKNOWN 0
+#define MAXMAPPED_SIG 35
+/* provide a mapping of arch signal to internal signal # for mediation
+ * those that are always an alias SIGCLD for SIGCLHD and SIGPOLL for SIGIO
+ * map to the same entry those that may/or may not get a separate entry
+ */
+static const int sig_map[MAXMAPPED_SIG] = {
+	[0] = MAXMAPPED_SIG,	/* existence test */
+	[SIGHUP] = 1,
+	[SIGINT] = 2,
+	[SIGQUIT] = 3,
+	[SIGILL] = 4,
+	[SIGTRAP] = 5,		/* -, 5, - */
+	[SIGABRT] = 6,		/*  SIGIOT: -, 6, - */
+	[SIGBUS] = 7,		/* 10, 7, 10 */
+	[SIGFPE] = 8,
+	[SIGKILL] = 9,
+	[SIGUSR1] = 10,		/* 30, 10, 16 */
+	[SIGSEGV] = 11,
+	[SIGUSR2] = 12,		/* 31, 12, 17 */
+	[SIGPIPE] = 13,
+	[SIGALRM] = 14,
+	[SIGTERM] = 15,
+	[SIGSTKFLT] = 16,	/* -, 16, - */
+	[SIGCHLD] = 17,		/* 20, 17, 18.  SIGCHLD -, -, 18 */
+	[SIGCONT] = 18,		/* 19, 18, 25 */
+	[SIGSTOP] = 19,		/* 17, 19, 23 */
+	[SIGTSTP] = 20,		/* 18, 20, 24 */
+	[SIGTTIN] = 21,		/* 21, 21, 26 */
+	[SIGTTOU] = 22,		/* 22, 22, 27 */
+	[SIGURG] = 23,		/* 16, 23, 21 */
+	[SIGXCPU] = 24,		/* 24, 24, 30 */
+	[SIGXFSZ] = 25,		/* 25, 25, 31 */
+	[SIGVTALRM] = 26,	/* 26, 26, 28 */
+	[SIGPROF] = 27,		/* 27, 27, 29 */
+	[SIGWINCH] = 28,	/* 28, 28, 20 */
+	[SIGIO] = 29,		/* SIGPOLL: 23, 29, 22 */
+	[SIGPWR] = 30,		/* 29, 30, 19.  SIGINFO 29, -, - */
+#ifdef SIGSYS
+	[SIGSYS] = 31,		/* 12, 31, 12. often SIG LOST/UNUSED */
+#endif
+#ifdef SIGEMT
+	[SIGEMT] = 32,		/* 7, - , 7 */
+#endif
+#if defined(SIGLOST) && SIGPWR != SIGLOST		/* sparc */
+	[SIGLOST] = 33,		/* unused on Linux */
+#endif
+#if defined(SIGLOST) && defined(SIGSYS) && SIGLOST != SIGSYS
+	[SIGUNUSED] = 34,	/* -, 31, - */
+#endif
+};
+
+/* this table is ordered post sig_map[sig] mapping */
+static const char *const sig_names[MAXMAPPED_SIG + 1] = {
+	"unknown",
+	"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",
+	"lost",
+	"unused",
+
+	"exists",	/* always last existence test mapped to MAXMAPPED_SIG */
+};
+

security/apparmor/ipc.c

@@ -20,6 +20,7 @@
 #include "include/context.h"
 #include "include/policy.h"
 #include "include/ipc.h"
+#include "include/sig_names.h"
 
 /**
  * audit_ptrace_mask - convert mask to permission string
@@ -121,3 +122,101 @@ int aa_may_ptrace(struct aa_label *tracer, struct aa_label *tracee,
 }
 
 
+static inline int map_signal_num(int sig)
+{
+	if (sig > SIGRTMAX)
+		return SIGUNKNOWN;
+	else if (sig >= SIGRTMIN)
+		return sig - SIGRTMIN + 128;	/* rt sigs mapped to 128 */
+	else if (sig <= MAXMAPPED_SIG)
+		return sig_map[sig];
+	return SIGUNKNOWN;
+}
+
+/**
+ * audit_file_mask - convert mask to permission string
+ * @buffer: buffer to write string to (NOT NULL)
+ * @mask: permission mask to convert
+ */
+static void audit_signal_mask(struct audit_buffer *ab, u32 mask)
+{
+	if (mask & MAY_READ)
+		audit_log_string(ab, "receive");
+	if (mask & MAY_WRITE)
+		audit_log_string(ab, "send");
+}
+
+/**
+ * audit_cb - call back for signal specific audit fields
+ * @ab: audit_buffer  (NOT NULL)
+ * @va: audit struct to audit values of  (NOT NULL)
+ */
+static void audit_signal_cb(struct audit_buffer *ab, void *va)
+{
+	struct common_audit_data *sa = va;
+
+	if (aad(sa)->request & AA_SIGNAL_PERM_MASK) {
+		audit_log_format(ab, " requested_mask=");
+		audit_signal_mask(ab, aad(sa)->request);
+		if (aad(sa)->denied & AA_SIGNAL_PERM_MASK) {
+			audit_log_format(ab, " denied_mask=");
+			audit_signal_mask(ab, aad(sa)->denied);
+		}
+	}
+	if (aad(sa)->signal <= MAXMAPPED_SIG)
+		audit_log_format(ab, " signal=%s", sig_names[aad(sa)->signal]);
+	else
+		audit_log_format(ab, " signal=rtmin+%d",
+				 aad(sa)->signal - 128);
+	audit_log_format(ab, " peer=");
+	aa_label_xaudit(ab, labels_ns(aad(sa)->label), aad(sa)->peer,
+			FLAGS_NONE, GFP_ATOMIC);
+}
+
+/* TODO: update to handle compound name&name2, conditionals */
+static void profile_match_signal(struct aa_profile *profile, const char *label,
+				 int signal, struct aa_perms *perms)
+{
+	unsigned int state;
+
+	/* TODO: secondary cache check <profile, profile, perm> */
+	state = aa_dfa_next(profile->policy.dfa,
+			    profile->policy.start[AA_CLASS_SIGNAL],
+			    signal);
+	state = aa_dfa_match(profile->policy.dfa, state, label);
+	aa_compute_perms(profile->policy.dfa, state, perms);
+}
+
+static int profile_signal_perm(struct aa_profile *profile,
+			       struct aa_profile *peer, u32 request,
+			       struct common_audit_data *sa)
+{
+	struct aa_perms perms;
+
+	if (profile_unconfined(profile) ||
+	    !PROFILE_MEDIATES(profile, AA_CLASS_SIGNAL))
+		return 0;
+
+	aad(sa)->peer = &peer->label;
+	profile_match_signal(profile, peer->base.hname, aad(sa)->signal,
+			     &perms);
+	aa_apply_modes_to_perms(profile, &perms);
+	return aa_check_perms(profile, &perms, request, sa, audit_signal_cb);
+}
+
+static int aa_signal_cross_perm(struct aa_profile *sender,
+				struct aa_profile *target,
+				struct common_audit_data *sa)
+{
+	return xcheck(profile_signal_perm(sender, target, MAY_WRITE, sa),
+		      profile_signal_perm(target, sender, MAY_READ, sa));
+}
+
+int aa_may_signal(struct aa_label *sender, struct aa_label *target, int sig)
+{
+	DEFINE_AUDIT_DATA(sa, LSM_AUDIT_DATA_NONE, OP_SIGNAL);
+
+	aad(&sa)->signal = map_signal_num(sig);
+	return xcheck_labels_profiles(sender, target, aa_signal_cross_perm,
+				      &sa);
+}

security/apparmor/lsm.c

@@ -656,6 +656,26 @@ static int apparmor_task_setrlimit(struct task_struct *task,
 	return error;
 }
 
+static int apparmor_task_kill(struct task_struct *target, struct siginfo *info,
+			      int sig, u32 secid)
+{
+	struct aa_label *cl, *tl;
+	int error;
+
+	if (secid)
+		/* TODO: after secid to label mapping is done.
+		 *  Dealing with USB IO specific behavior
+		 */
+		return 0;
+	cl = __begin_current_label_crit_section();
+	tl = aa_get_task_label(target);
+	error = aa_may_signal(cl, tl, sig);
+	aa_put_label(tl);
+	__end_current_label_crit_section(cl);
+
+	return error;
+}
+
 static struct security_hook_list apparmor_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(ptrace_access_check, apparmor_ptrace_access_check),
 	LSM_HOOK_INIT(ptrace_traceme, apparmor_ptrace_traceme),
@@ -697,6 +717,7 @@ static struct security_hook_list apparmor_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(bprm_secureexec, apparmor_bprm_secureexec),
 
 	LSM_HOOK_INIT(task_setrlimit, apparmor_task_setrlimit),
+	LSM_HOOK_INIT(task_kill, apparmor_task_kill),
 };
 
 /*