diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 3fe714da7f5ab3ff7bc582a98f764791c378610e..d6b74b91096b4db0f1fdca10990f9f21bb42d690 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -11,15 +11,24 @@
 #include <linux/sysctl.h>
 #include <linux/err.h>
 
-#define UID_GID_MAP_MAX_EXTENTS 5
+#define UID_GID_MAP_MAX_BASE_EXTENTS 5
+#define UID_GID_MAP_MAX_EXTENTS 340
 
-struct uid_gid_map {	/* 64 bytes -- 1 cache line */
+struct uid_gid_extent {
+	u32 first;
+	u32 lower_first;
+	u32 count;
+};
+
+struct uid_gid_map { /* 64 bytes -- 1 cache line */
 	u32 nr_extents;
-	struct uid_gid_extent {
-		u32 first;
-		u32 lower_first;
-		u32 count;
-	} extent[UID_GID_MAP_MAX_EXTENTS];
+	union {
+		struct uid_gid_extent extent[UID_GID_MAP_MAX_BASE_EXTENTS];
+		struct {
+			struct uid_gid_extent *forward;
+			struct uid_gid_extent *reverse;
+		};
+	};
 };
 
 #define USERNS_SETGROUPS_ALLOWED 1UL
diff --git a/kernel/user.c b/kernel/user.c
index 00281add65b251d484616264dbba8df8b0ae066d..9a20acce460d53cf23f7057c7dc122b685ae639d 100644
--- a/kernel/user.c
+++ b/kernel/user.c
@@ -26,26 +26,32 @@
 struct user_namespace init_user_ns = {
 	.uid_map = {
 		.nr_extents = 1,
-		.extent[0] = {
-			.first = 0,
-			.lower_first = 0,
-			.count = 4294967295U,
+		{
+			.extent[0] = {
+				.first = 0,
+				.lower_first = 0,
+				.count = 4294967295U,
+			},
 		},
 	},
 	.gid_map = {
 		.nr_extents = 1,
-		.extent[0] = {
-			.first = 0,
-			.lower_first = 0,
-			.count = 4294967295U,
+		{
+			.extent[0] = {
+				.first = 0,
+				.lower_first = 0,
+				.count = 4294967295U,
+			},
 		},
 	},
 	.projid_map = {
 		.nr_extents = 1,
-		.extent[0] = {
-			.first = 0,
-			.lower_first = 0,
-			.count = 4294967295U,
+		{
+			.extent[0] = {
+				.first = 0,
+				.lower_first = 0,
+				.count = 4294967295U,
+			},
 		},
 	},
 	.count = ATOMIC_INIT(3),
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index d32b45662fb66828cc907ea42ea4c9ede8a1ef0c..246d4d4ce5c70cde4b5c9ed7fd33f54c1efe5a3e 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -23,6 +23,8 @@
 #include <linux/ctype.h>
 #include <linux/projid.h>
 #include <linux/fs_struct.h>
+#include <linux/bsearch.h>
+#include <linux/sort.h>
 
 static struct kmem_cache *user_ns_cachep __read_mostly;
 static DEFINE_MUTEX(userns_state_mutex);
@@ -181,6 +183,18 @@ static void free_user_ns(struct work_struct *work)
 	do {
 		struct ucounts *ucounts = ns->ucounts;
 		parent = ns->parent;
+		if (ns->gid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
+			kfree(ns->gid_map.forward);
+			kfree(ns->gid_map.reverse);
+		}
+		if (ns->uid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
+			kfree(ns->uid_map.forward);
+			kfree(ns->uid_map.reverse);
+		}
+		if (ns->projid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
+			kfree(ns->projid_map.forward);
+			kfree(ns->projid_map.reverse);
+		}
 		retire_userns_sysctls(ns);
 #ifdef CONFIG_PERSISTENT_KEYRINGS
 		key_put(ns->persistent_keyring_register);
@@ -198,26 +212,101 @@ void __put_user_ns(struct user_namespace *ns)
 }
 EXPORT_SYMBOL(__put_user_ns);
 
-static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
+/**
+ * idmap_key struct holds the information necessary to find an idmapping in a
+ * sorted idmap array. It is passed to cmp_map_id() as first argument.
+ */
+struct idmap_key {
+	bool map_up; /* true  -> id from kid; false -> kid from id */
+	u32 id; /* id to find */
+	u32 count; /* == 0 unless used with map_id_range_down() */
+};
+
+/**
+ * cmp_map_id - Function to be passed to bsearch() to find the requested
+ * idmapping. Expects struct idmap_key to be passed via @k.
+ */
+static int cmp_map_id(const void *k, const void *e)
 {
-	unsigned idx, extents;
+	u32 first, last, id2;
+	const struct idmap_key *key = k;
+	const struct uid_gid_extent *el = e;
+
+	id2 = key->id + key->count - 1;
+
+	/* handle map_id_{down,up}() */
+	if (key->map_up)
+		first = el->lower_first;
+	else
+		first = el->first;
+
+	last = first + el->count - 1;
+
+	if (key->id >= first && key->id <= last &&
+	    (id2 >= first && id2 <= last))
+		return 0;
+
+	if (key->id < first || id2 < first)
+		return -1;
+
+	return 1;
+}
+
+/**
+ * map_id_range_down_max - Find idmap via binary search in ordered idmap array.
+ * Can only be called if number of mappings exceeds UID_GID_MAP_MAX_BASE_EXTENTS.
+ */
+static struct uid_gid_extent *
+map_id_range_down_max(unsigned extents, struct uid_gid_map *map, u32 id, u32 count)
+{
+	struct idmap_key key;
+
+	key.map_up = false;
+	key.count = count;
+	key.id = id;
+
+	return bsearch(&key, map->forward, extents,
+		       sizeof(struct uid_gid_extent), cmp_map_id);
+}
+
+/**
+ * map_id_range_down_base - Find idmap via binary search in static extent array.
+ * Can only be called if number of mappings is equal or less than
+ * UID_GID_MAP_MAX_BASE_EXTENTS.
+ */
+static struct uid_gid_extent *
+map_id_range_down_base(unsigned extents, struct uid_gid_map *map, u32 id, u32 count)
+{
+	unsigned idx;
 	u32 first, last, id2;
 
 	id2 = id + count - 1;
 
 	/* Find the matching extent */
-	extents = map->nr_extents;
-	smp_rmb();
 	for (idx = 0; idx < extents; idx++) {
 		first = map->extent[idx].first;
 		last = first + map->extent[idx].count - 1;
 		if (id >= first && id <= last &&
 		    (id2 >= first && id2 <= last))
-			break;
+			return &map->extent[idx];
 	}
+	return NULL;
+}
+
+static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
+{
+	struct uid_gid_extent *extent;
+	unsigned extents = map->nr_extents;
+	smp_rmb();
+
+	if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+		extent = map_id_range_down_base(extents, map, id, count);
+	else
+		extent = map_id_range_down_max(extents, map, id, count);
+
 	/* Map the id or note failure */
-	if (idx < extents)
-		id = (id - first) + map->extent[idx].lower_first;
+	if (extent)
+		id = (id - extent->first) + extent->lower_first;
 	else
 		id = (u32) -1;
 
@@ -226,44 +315,61 @@ static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
 
 static u32 map_id_down(struct uid_gid_map *map, u32 id)
 {
-	unsigned idx, extents;
+	return map_id_range_down(map, id, 1);
+}
+
+/**
+ * map_id_up_base - Find idmap via binary search in static extent array.
+ * Can only be called if number of mappings is equal or less than
+ * UID_GID_MAP_MAX_BASE_EXTENTS.
+ */
+static struct uid_gid_extent *
+map_id_up_base(unsigned extents, struct uid_gid_map *map, u32 id)
+{
+	unsigned idx;
 	u32 first, last;
 
 	/* Find the matching extent */
-	extents = map->nr_extents;
-	smp_rmb();
 	for (idx = 0; idx < extents; idx++) {
-		first = map->extent[idx].first;
+		first = map->extent[idx].lower_first;
 		last = first + map->extent[idx].count - 1;
 		if (id >= first && id <= last)
-			break;
+			return &map->extent[idx];
 	}
-	/* Map the id or note failure */
-	if (idx < extents)
-		id = (id - first) + map->extent[idx].lower_first;
-	else
-		id = (u32) -1;
+	return NULL;
+}
 
-	return id;
+/**
+ * map_id_up_max - Find idmap via binary search in ordered idmap array.
+ * Can only be called if number of mappings exceeds UID_GID_MAP_MAX_BASE_EXTENTS.
+ */
+static struct uid_gid_extent *
+map_id_up_max(unsigned extents, struct uid_gid_map *map, u32 id)
+{
+	struct idmap_key key;
+
+	key.map_up = true;
+	key.count = 1;
+	key.id = id;
+
+	return bsearch(&key, map->reverse, extents,
+		       sizeof(struct uid_gid_extent), cmp_map_id);
 }
 
 static u32 map_id_up(struct uid_gid_map *map, u32 id)
 {
-	unsigned idx, extents;
-	u32 first, last;
-
-	/* Find the matching extent */
-	extents = map->nr_extents;
+	struct uid_gid_extent *extent;
+	unsigned extents = map->nr_extents;
 	smp_rmb();
-	for (idx = 0; idx < extents; idx++) {
-		first = map->extent[idx].lower_first;
-		last = first + map->extent[idx].count - 1;
-		if (id >= first && id <= last)
-			break;
-	}
+
+	if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+		extent = map_id_up_base(extents, map, id);
+	else
+		extent = map_id_up_max(extents, map, id);
+
 	/* Map the id or note failure */
-	if (idx < extents)
-		id = (id - first) + map->extent[idx].first;
+	if (extent)
+		id = (id - extent->lower_first) + extent->first;
 	else
 		id = (u32) -1;
 
@@ -540,13 +646,17 @@ static int projid_m_show(struct seq_file *seq, void *v)
 static void *m_start(struct seq_file *seq, loff_t *ppos,
 		     struct uid_gid_map *map)
 {
-	struct uid_gid_extent *extent = NULL;
 	loff_t pos = *ppos;
+	unsigned extents = map->nr_extents;
+	smp_rmb();
 
-	if (pos < map->nr_extents)
-		extent = &map->extent[pos];
+	if (pos >= extents)
+		return NULL;
 
-	return extent;
+	if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+		return &map->extent[pos];
+
+	return &map->forward[pos];
 }
 
 static void *uid_m_start(struct seq_file *seq, loff_t *ppos)
@@ -618,7 +728,10 @@ static bool mappings_overlap(struct uid_gid_map *new_map,
 		u32 prev_upper_last, prev_lower_last;
 		struct uid_gid_extent *prev;
 
-		prev = &new_map->extent[idx];
+		if (new_map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+			prev = &new_map->extent[idx];
+		else
+			prev = &new_map->forward[idx];
 
 		prev_upper_first = prev->first;
 		prev_lower_first = prev->lower_first;
@@ -638,6 +751,101 @@ static bool mappings_overlap(struct uid_gid_map *new_map,
 	return false;
 }
 
+/**
+ * insert_extent - Safely insert a new idmap extent into struct uid_gid_map.
+ * Takes care to allocate a 4K block of memory if the number of mappings exceeds
+ * UID_GID_MAP_MAX_BASE_EXTENTS.
+ */
+static int insert_extent(struct uid_gid_map *map, struct uid_gid_extent *extent)
+{
+	struct uid_gid_extent *dest;
+
+	if (map->nr_extents == UID_GID_MAP_MAX_BASE_EXTENTS) {
+		struct uid_gid_extent *forward;
+
+		/* Allocate memory for 340 mappings. */
+		forward = kmalloc(sizeof(struct uid_gid_extent) *
+				 UID_GID_MAP_MAX_EXTENTS, GFP_KERNEL);
+		if (!forward)
+			return -ENOMEM;
+
+		/* Copy over memory. Only set up memory for the forward pointer.
+		 * Defer the memory setup for the reverse pointer.
+		 */
+		memcpy(forward, map->extent,
+		       map->nr_extents * sizeof(map->extent[0]));
+
+		map->forward = forward;
+		map->reverse = NULL;
+	}
+
+	if (map->nr_extents < UID_GID_MAP_MAX_BASE_EXTENTS)
+		dest = &map->extent[map->nr_extents];
+	else
+		dest = &map->forward[map->nr_extents];
+
+	*dest = *extent;
+	map->nr_extents++;
+	return 0;
+}
+
+/* cmp function to sort() forward mappings */
+static int cmp_extents_forward(const void *a, const void *b)
+{
+	const struct uid_gid_extent *e1 = a;
+	const struct uid_gid_extent *e2 = b;
+
+	if (e1->first < e2->first)
+		return -1;
+
+	if (e1->first > e2->first)
+		return 1;
+
+	return 0;
+}
+
+/* cmp function to sort() reverse mappings */
+static int cmp_extents_reverse(const void *a, const void *b)
+{
+	const struct uid_gid_extent *e1 = a;
+	const struct uid_gid_extent *e2 = b;
+
+	if (e1->lower_first < e2->lower_first)
+		return -1;
+
+	if (e1->lower_first > e2->lower_first)
+		return 1;
+
+	return 0;
+}
+
+/**
+ * sort_idmaps - Sorts an array of idmap entries.
+ * Can only be called if number of mappings exceeds UID_GID_MAP_MAX_BASE_EXTENTS.
+ */
+static int sort_idmaps(struct uid_gid_map *map)
+{
+	if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+		return 0;
+
+	/* Sort forward array. */
+	sort(map->forward, map->nr_extents, sizeof(struct uid_gid_extent),
+	     cmp_extents_forward, NULL);
+
+	/* Only copy the memory from forward we actually need. */
+	map->reverse = kmemdup(map->forward,
+			       map->nr_extents * sizeof(struct uid_gid_extent),
+			       GFP_KERNEL);
+	if (!map->reverse)
+		return -ENOMEM;
+
+	/* Sort reverse array. */
+	sort(map->reverse, map->nr_extents, sizeof(struct uid_gid_extent),
+	     cmp_extents_reverse, NULL);
+
+	return 0;
+}
+
 static ssize_t map_write(struct file *file, const char __user *buf,
 			 size_t count, loff_t *ppos,
 			 int cap_setid,
@@ -648,7 +856,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 	struct user_namespace *ns = seq->private;
 	struct uid_gid_map new_map;
 	unsigned idx;
-	struct uid_gid_extent *extent = NULL;
+	struct uid_gid_extent extent;
 	char *kbuf = NULL, *pos, *next_line;
 	ssize_t ret = -EINVAL;
 
@@ -673,6 +881,8 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 	 */
 	mutex_lock(&userns_state_mutex);
 
+	memset(&new_map, 0, sizeof(struct uid_gid_map));
+
 	ret = -EPERM;
 	/* Only allow one successful write to the map */
 	if (map->nr_extents != 0)
@@ -700,9 +910,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 	/* Parse the user data */
 	ret = -EINVAL;
 	pos = kbuf;
-	new_map.nr_extents = 0;
 	for (; pos; pos = next_line) {
-		extent = &new_map.extent[new_map.nr_extents];
 
 		/* Find the end of line and ensure I don't look past it */
 		next_line = strchr(pos, '\n');
@@ -714,17 +922,17 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 		}
 
 		pos = skip_spaces(pos);
-		extent->first = simple_strtoul(pos, &pos, 10);
+		extent.first = simple_strtoul(pos, &pos, 10);
 		if (!isspace(*pos))
 			goto out;
 
 		pos = skip_spaces(pos);
-		extent->lower_first = simple_strtoul(pos, &pos, 10);
+		extent.lower_first = simple_strtoul(pos, &pos, 10);
 		if (!isspace(*pos))
 			goto out;
 
 		pos = skip_spaces(pos);
-		extent->count = simple_strtoul(pos, &pos, 10);
+		extent.count = simple_strtoul(pos, &pos, 10);
 		if (*pos && !isspace(*pos))
 			goto out;
 
@@ -734,29 +942,31 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 			goto out;
 
 		/* Verify we have been given valid starting values */
-		if ((extent->first == (u32) -1) ||
-		    (extent->lower_first == (u32) -1))
+		if ((extent.first == (u32) -1) ||
+		    (extent.lower_first == (u32) -1))
 			goto out;
 
 		/* Verify count is not zero and does not cause the
 		 * extent to wrap
 		 */
-		if ((extent->first + extent->count) <= extent->first)
+		if ((extent.first + extent.count) <= extent.first)
 			goto out;
-		if ((extent->lower_first + extent->count) <=
-		     extent->lower_first)
+		if ((extent.lower_first + extent.count) <=
+		     extent.lower_first)
 			goto out;
 
 		/* Do the ranges in extent overlap any previous extents? */
-		if (mappings_overlap(&new_map, extent))
+		if (mappings_overlap(&new_map, &extent))
 			goto out;
 
-		new_map.nr_extents++;
-
-		/* Fail if the file contains too many extents */
-		if ((new_map.nr_extents == UID_GID_MAP_MAX_EXTENTS) &&
+		if ((new_map.nr_extents + 1) == UID_GID_MAP_MAX_EXTENTS &&
 		    (next_line != NULL))
 			goto out;
+
+		ret = insert_extent(&new_map, &extent);
+		if (ret < 0)
+			goto out;
+		ret = -EINVAL;
 	}
 	/* Be very certaint the new map actually exists */
 	if (new_map.nr_extents == 0)
@@ -767,16 +977,26 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 	if (!new_idmap_permitted(file, ns, cap_setid, &new_map))
 		goto out;
 
+	ret = sort_idmaps(&new_map);
+	if (ret < 0)
+		goto out;
+
+	ret = -EPERM;
 	/* Map the lower ids from the parent user namespace to the
 	 * kernel global id space.
 	 */
 	for (idx = 0; idx < new_map.nr_extents; idx++) {
+		struct uid_gid_extent *e;
 		u32 lower_first;
-		extent = &new_map.extent[idx];
+
+		if (new_map.nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+			e = &new_map.extent[idx];
+		else
+			e = &new_map.forward[idx];
 
 		lower_first = map_id_range_down(parent_map,
-						extent->lower_first,
-						extent->count);
+						e->lower_first,
+						e->count);
 
 		/* Fail if we can not map the specified extent to
 		 * the kernel global id space.
@@ -784,18 +1004,31 @@ static ssize_t map_write(struct file *file, const char __user *buf,
 		if (lower_first == (u32) -1)
 			goto out;
 
-		extent->lower_first = lower_first;
+		e->lower_first = lower_first;
 	}
 
 	/* Install the map */
-	memcpy(map->extent, new_map.extent,
-		new_map.nr_extents*sizeof(new_map.extent[0]));
+	if (new_map.nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) {
+		memcpy(map->extent, new_map.extent,
+		       new_map.nr_extents * sizeof(new_map.extent[0]));
+	} else {
+		map->forward = new_map.forward;
+		map->reverse = new_map.reverse;
+	}
 	smp_wmb();
 	map->nr_extents = new_map.nr_extents;
 
 	*ppos = count;
 	ret = count;
 out:
+	if (ret < 0 && new_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
+		kfree(new_map.forward);
+		kfree(new_map.reverse);
+		map->forward = NULL;
+		map->reverse = NULL;
+		map->nr_extents = 0;
+	}
+
 	mutex_unlock(&userns_state_mutex);
 	kfree(kbuf);
 	return ret;