seccomp: Add find_notification helper

This adds a helper which can iterate through a seccomp_filter to
find a notification matching an ID. It removes several replicated
chunks of code.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Tycho Andersen <tycho@tycho.ws>
Cc: Matt Denton <mpdenton@google.com>
Cc: Kees Cook <keescook@google.com>,
Cc: Jann Horn <jannh@google.com>,
Cc: Robert Sesek <rsesek@google.com>,
Cc: Chris Palmer <palmer@google.com>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Tycho Andersen <tycho@tycho.ws>
Link: https://lore.kernel.org/r/20200601112532.150158-1-sargun@sargun.me
Signed-off-by: Kees Cook <keescook@chromium.org>

kernel/seccomp.c

@@ -41,6 +41,7 @@
 #include <linux/tracehook.h>
 #include <linux/uaccess.h>
 #include <linux/anon_inodes.h>
+#include <linux/lockdep.h>
 
 enum notify_state {
 	SECCOMP_NOTIFY_INIT,
@@ -1024,6 +1025,23 @@ static int seccomp_notify_release(struct inode *inode, struct file *file)
 	return 0;
 }
 
+/* must be called with notif_lock held */
+static inline struct seccomp_knotif *
+find_notification(struct seccomp_filter *filter, u64 id)
+{
+	struct seccomp_knotif *cur;
+
+	lockdep_assert_held(&filter->notify_lock);
+
+	list_for_each_entry(cur, &filter->notif->notifications, list) {
+		if (cur->id == id)
+			return cur;
+	}
+
+	return NULL;
+}
+
+
 static long seccomp_notify_recv(struct seccomp_filter *filter,
 				void __user *buf)
 {
@@ -1081,15 +1099,8 @@ static long seccomp_notify_recv(struct seccomp_filter *filter,
 		 * may have died when we released the lock, so we need to make
 		 * sure it's still around.
 		 */
-		knotif = NULL;
 		mutex_lock(&filter->notify_lock);
-		list_for_each_entry(cur, &filter->notif->notifications, list) {
-			if (cur->id == unotif.id) {
-				knotif = cur;
-				break;
-			}
-		}
-
+		knotif = find_notification(filter, unotif.id);
 		if (knotif) {
 			knotif->state = SECCOMP_NOTIFY_INIT;
 			up(&filter->notif->request);
@@ -1104,7 +1115,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
 				void __user *buf)
 {
 	struct seccomp_notif_resp resp = {};
-	struct seccomp_knotif *knotif = NULL, *cur;
+	struct seccomp_knotif *knotif;
 	long ret;
 
 	if (copy_from_user(&resp, buf, sizeof(resp)))
@@ -1121,13 +1132,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
 	if (ret < 0)
 		return ret;
 
-	list_for_each_entry(cur, &filter->notif->notifications, list) {
-		if (cur->id == resp.id) {
-			knotif = cur;
-			break;
-		}
-	}
-
+	knotif = find_notification(filter, resp.id);
 	if (!knotif) {
 		ret = -ENOENT;
 		goto out;
@@ -1153,7 +1158,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
 static long seccomp_notify_id_valid(struct seccomp_filter *filter,
 				    void __user *buf)
 {
-	struct seccomp_knotif *knotif = NULL;
+	struct seccomp_knotif *knotif;
 	u64 id;
 	long ret;
 
@@ -1164,16 +1169,12 @@ static long seccomp_notify_id_valid(struct seccomp_filter *filter,
 	if (ret < 0)
 		return ret;
 
-	ret = -ENOENT;
-	list_for_each_entry(knotif, &filter->notif->notifications, list) {
-		if (knotif->id == id) {
-			if (knotif->state == SECCOMP_NOTIFY_SENT)
-				ret = 0;
-			goto out;
-		}
-	}
+	knotif = find_notification(filter, id);
+	if (knotif && knotif->state == SECCOMP_NOTIFY_SENT)
+		ret = 0;
+	else
+		ret = -ENOENT;
 
-out:
 	mutex_unlock(&filter->notify_lock);
 	return ret;
 }