[PATCH v2 09/15] tty: audit: Handle tty audit enable atomically

[Peter Hurley]

The audit_tty and audit_tty_log_passwd fields are actually bool
values, so merge into single memory location to access atomically.

NB: audit log operations may still occur after tty audit is disabled
which is consistent with the existing functionality

Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
 drivers/tty/tty_audit.c | 53 ++++++++++++++++++++-----------------------------
 include/linux/audit.h   |  4 ++++
 include/linux/sched.h   |  1 -
 kernel/audit.c          | 25 +++++++++++------------
 4 files changed, 38 insertions(+), 45 deletions(-)

diff --git a/drivers/tty/tty_audit.c b/drivers/tty/tty_audit.c
index 50380d8..3d90f88 100644
--- a/drivers/tty/tty_audit.c
+++ b/drivers/tty/tty_audit.c
@@ -131,7 +131,6 @@ void tty_audit_exit(void)
 void tty_audit_fork(struct signal_struct *sig)
 {
 	sig->audit_tty = current->signal->audit_tty;
-	sig->audit_tty_log_passwd = current->signal->audit_tty_log_passwd;
 }
 
 /**
@@ -141,11 +140,9 @@ void tty_audit_tiocsti(struct tty_struct *tty, char ch)
 {
 	struct tty_audit_buf *buf;
 	dev_t dev;
-	int should_audit;
 	unsigned long flags;
 
 	spin_lock_irqsave(&current->sighand->siglock, flags);
-	should_audit = current->signal->audit_tty;
 	buf = current->signal->tty_audit_buf;
 	if (buf)
 		atomic_inc(&buf->count);
@@ -160,7 +157,7 @@ void tty_audit_tiocsti(struct tty_struct *tty, char ch)
 		tty_audit_buf_put(buf);
 	}
 
-	if (should_audit && audit_enabled) {
+	if (audit_enabled && (current->signal->audit_tty & AUDIT_TTY_ENABLE)) {
 		kuid_t auid;
 		unsigned int sessionid;
 
@@ -177,29 +174,25 @@ void tty_audit_tiocsti(struct tty_struct *tty, char ch)
  */
 int tty_audit_push(void)
 {
-	struct tty_audit_buf *buf = ERR_PTR(-EPERM);
+	struct tty_audit_buf *buf;
 	unsigned long flags;
 
+	if (~current->signal->audit_tty & AUDIT_TTY_ENABLE)
+		return -EPERM;
+
 	spin_lock_irqsave(&current->sighand->siglock, flags);
-	if (current->signal->audit_tty) {
-		buf = current->signal->tty_audit_buf;
-		if (buf)
-			atomic_inc(&buf->count);
-	}
+	buf = current->signal->tty_audit_buf;
+	if (buf)
+		atomic_inc(&buf->count);
 	spin_unlock_irqrestore(&current->sighand->siglock, flags);
 
-	/*
-	 * Return 0 when signal->audit_tty set
-	 * but current->signal->tty_audit_buf == NULL.
-	 */
-	if (!buf || IS_ERR(buf))
-		return PTR_ERR(buf);
-
-	mutex_lock(&buf->mutex);
-	tty_audit_buf_push(buf);
-	mutex_unlock(&buf->mutex);
+	if (buf) {
+		mutex_lock(&buf->mutex);
+		tty_audit_buf_push(buf);
+		mutex_unlock(&buf->mutex);
 
-	tty_audit_buf_put(buf);
+		tty_audit_buf_put(buf);
+	}
 	return 0;
 }
 
@@ -218,8 +211,6 @@ static struct tty_audit_buf *tty_audit_buf_get(void)
 	buf = NULL;
 	buf2 = NULL;
 	spin_lock_irqsave(&current->sighand->siglock, flags);
-	if (likely(!current->signal->audit_tty))
-		goto out;
 	buf = current->signal->tty_audit_buf;
 	if (buf) {
 		atomic_inc(&buf->count);
@@ -233,9 +224,10 @@ static struct tty_audit_buf *tty_audit_buf_get(void)
 		return NULL;
 	}
 
-	spin_lock_irqsave(&current->sighand->siglock, flags);
-	if (!current->signal->audit_tty)
+	if (~current->signal->audit_tty & AUDIT_TTY_ENABLE)
 		goto out;
+
+	spin_lock_irqsave(&current->sighand->siglock, flags);
 	buf = current->signal->tty_audit_buf;
 	if (!buf) {
 		current->signal->tty_audit_buf = buf2;
@@ -259,9 +251,8 @@ static struct tty_audit_buf *tty_audit_buf_get(void)
 void tty_audit_add_data(struct tty_struct *tty, const void *data, size_t size)
 {
 	struct tty_audit_buf *buf;
-	int audit_log_tty_passwd;
-	unsigned long flags;
 	unsigned int icanon = !!L_ICANON(tty);
+	unsigned int audit_tty;
 	dev_t dev;
 
 	if (unlikely(size == 0))
@@ -271,10 +262,10 @@ void tty_audit_add_data(struct tty_struct *tty, const void *data, size_t size)
 	    && tty->driver->subtype == PTY_TYPE_MASTER)
 		return;
 
-	spin_lock_irqsave(&current->sighand->siglock, flags);
-	audit_log_tty_passwd = current->signal->audit_tty_log_passwd;
-	spin_unlock_irqrestore(&current->sighand->siglock, flags);
-	if (!audit_log_tty_passwd && icanon && !L_ECHO(tty))
+	audit_tty = READ_ONCE(current->signal->audit_tty);
+	if (~audit_tty & AUDIT_TTY_ENABLE)
+		return;
+	if ((~audit_tty & AUDIT_TTY_LOG_PASSWD) && icanon && !L_ECHO(tty))
 		return;
 
 	buf = tty_audit_buf_get();
diff --git a/include/linux/audit.h b/include/linux/audit.h
index 20eba1e..9ed7254 100644
--- a/include/linux/audit.h
+++ b/include/linux/audit.h
@@ -109,6 +109,10 @@ extern int audit_classify_compat_syscall(int abi, unsigned syscall);
 /* maximized args number that audit_socketcall can process */
 #define AUDITSC_ARGS		6
 
+/* bit values for ->signal->audit_tty */
+#define AUDIT_TTY_ENABLE	BIT(0)
+#define AUDIT_TTY_LOG_PASSWD	BIT(1)
+
 struct filename;
 
 extern void audit_log_session_info(struct audit_buffer *ab);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index edad7a4..400a738 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -771,7 +771,6 @@ struct signal_struct {
 #endif
 #ifdef CONFIG_AUDIT
 	unsigned audit_tty;
-	unsigned audit_tty_log_passwd;
 	struct tty_audit_buf *tty_audit_buf;
 #endif
 
diff --git a/kernel/audit.c b/kernel/audit.c
index 270dfb9..dfaa8e7 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -1031,20 +1031,19 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 		break;
 	case AUDIT_TTY_GET: {
 		struct audit_tty_status s;
-		struct task_struct *tsk = current;
+		unsigned int t;
 
-		spin_lock(&tsk->sighand->siglock);
-		s.enabled = tsk->signal->audit_tty;
-		s.log_passwd = tsk->signal->audit_tty_log_passwd;
-		spin_unlock(&tsk->sighand->siglock);
+		t = READ_ONCE(current->signal->audit_tty);
+		s.enabled = t & AUDIT_TTY_ENABLE;
+		s.log_passwd = !!(t & AUDIT_TTY_LOG_PASSWD);
 
 		audit_send_reply(skb, seq, AUDIT_TTY_GET, 0, 0, &s, sizeof(s));
 		break;
 	}
 	case AUDIT_TTY_SET: {
 		struct audit_tty_status s, old;
-		struct task_struct *tsk = current;
 		struct audit_buffer	*ab;
+		unsigned int t;
 
 		memset(&s, 0, sizeof(s));
 		/* guard against past and future API changes */
@@ -1054,14 +1053,14 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 		    (s.log_passwd != 0 && s.log_passwd != 1))
 			err = -EINVAL;
 
-		spin_lock(&tsk->sighand->siglock);
-		old.enabled = tsk->signal->audit_tty;
-		old.log_passwd = tsk->signal->audit_tty_log_passwd;
-		if (!err) {
-			tsk->signal->audit_tty = s.enabled;
-			tsk->signal->audit_tty_log_passwd = s.log_passwd;
+		if (err)
+			t = READ_ONCE(current->signal->audit_tty);
+		else {
+			t = s.enabled | (-s.log_passwd & AUDIT_TTY_LOG_PASSWD);
+			t = xchg(&current->signal->audit_tty, t);
 		}
-		spin_unlock(&tsk->sighand->siglock);
+		old.enabled = t & AUDIT_TTY_ENABLE;
+		old.log_passwd = !!(t & AUDIT_TTY_LOG_PASSWD);
 
 		audit_log_common_recv_msg(&ab, AUDIT_CONFIG_CHANGE);
 		audit_log_format(ab, " op=tty_set old-enabled=%d new-enabled=%d"
-- 
2.7.0