fanotify: limit the number of marks in a single fanotify group

There is currently no limit on the number of marks a given fanotify group
can have.  Since fanotify is gated on CAP_SYS_ADMIN this was not seen as
a serious DoS threat.  This patch implements a default of 8192, the same as
inotify to work towards removing the CAP_SYS_ADMIN gating and eliminating
the default DoS'able status.
Signed-off-by: Eric Paris <eparis@redhat.com>

fs/notify/fanotify/fanotify_user.c

@@ -17,6 +17,7 @@
 #include <asm/ioctls.h>
 
 #define FANOTIFY_DEFAULT_MAX_EVENTS	16384
+#define FANOTIFY_DEFAULT_MAX_MARKS	8192
 
 extern const struct fsnotify_ops fanotify_fsnotify_ops;
 
@@ -584,6 +585,9 @@ static int fanotify_add_vfsmount_mark(struct fsnotify_group *group,
 	if (!fsn_mark) {
 		int ret;
 
+		if (atomic_read(&group->num_marks) > group->fanotify_data.max_marks)
+			return -ENOSPC;
+
 		fsn_mark = kmem_cache_alloc(fanotify_mark_cache, GFP_KERNEL);
 		if (!fsn_mark)
 			return -ENOMEM;
@@ -626,6 +630,9 @@ static int fanotify_add_inode_mark(struct fsnotify_group *group,
 	if (!fsn_mark) {
 		int ret;
 
+		if (atomic_read(&group->num_marks) > group->fanotify_data.max_marks)
+			return -ENOSPC;
+
 		fsn_mark = kmem_cache_alloc(fanotify_mark_cache, GFP_KERNEL);
 		if (!fsn_mark)
 			return -ENOMEM;
@@ -700,6 +707,8 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 		group->max_events = FANOTIFY_DEFAULT_MAX_EVENTS;
 	}
 
+	group->fanotify_data.max_marks = FANOTIFY_DEFAULT_MAX_MARKS;
+
 	fd = anon_inode_getfd("[fanotify]", &fanotify_fops, group, f_flags);
 	if (fd < 0)
 		goto out_put_group;

include/linux/fsnotify_backend.h

@@ -169,6 +169,7 @@ struct fsnotify_group {
 			bool bypass_perm; /* protected by access_mutex */
 #endif /* CONFIG_FANOTIFY_ACCESS_PERMISSIONS */
 			int f_flags;
+			unsigned int max_marks;
 		} fanotify_data;
 #endif /* CONFIG_FANOTIFY */
 	};