[PATCH] userns: improve uid/gid map collision detection

[Aristeu Rozanski <aris@redhat.com>]

Initial implementation of the uid/gid maps will enforce that the
maps should be in order and would prevent a use case like this from
being used:
	0	1000	1
	48	500	1

since the second entry both values should be bigger than the previous.
This patch implements a more elaborate collision detection allowing any
order to be used.

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Signed-off-by: Aristeu Rozanski <aris@redhat.com>

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 2b042c4..fb0e492 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -521,6 +521,28 @@ struct seq_operations proc_projid_seq_operations = {
 
 static DEFINE_MUTEX(id_map_mutex);
 
+#define in_range(b,first,len) ((b)>=(first)&&(b)<(first)+(len))
+static inline int extent_collision(struct uid_gid_map *new_map,
+				   struct uid_gid_extent *extent)
+{
+	int i;
+	struct uid_gid_extent *cur;
+
+	for (i = 0; i < new_map->nr_extents; i++) {
+		cur = &new_map->extent[i];
+		if (in_range(extent->first, cur->first, cur->count) ||
+		    in_range(extent->first + extent->count, cur->first,
+			     cur->count))
+			return 1;
+		if (in_range(extent->lower_first, cur->lower_first,
+			     cur->count) ||
+		    in_range(extent->lower_first + extent->count,
+			     cur->lower_first, cur->count))
+			return 1;
+	}
+	return 0;
+}
+
 static ssize_t map_write(struct file *file, const char __user *buf,
 			 size_t count, loff_t *ppos,
 			 int cap_setid,
@@ -634,10 +656,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 		if ((extent->lower_first + extent->count) <= extent->lower_first)
 			goto out;
 
-		/* For now only accept extents that are strictly in order */
-		if (last &&
-		    (((last->first + last->count) > extent->first) ||
-		     ((last->lower_first + last->count) > extent->lower_first)))
+		if (extent_collision(&new_map, extent))
 			goto out;
 
 		new_map.nr_extents++;