userns: Make the count of user namespaces per user

Add a structure that is per user and per user ns and use it to hold
the count of user namespaces.  This makes prevents one user from
creating denying service to another user by creating the maximum
number of user namespaces.

Rename the sysctl export of the maximum count from
/proc/sys/userns/max_user_namespaces to /proc/sys/user/max_user_namespaces
to reflect that the count is now per user.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 95 insertions, 21 deletions

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 6c2205c..33c4187 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -8,8 +8,20 @@
 #include <linux/stat.h>
 #include <linux/sysctl.h>
 #include <linux/slab.h>
+#include <linux/hash.h>
 #include <linux/user_namespace.h>
 
+#define UCOUNTS_HASHTABLE_BITS 10
+static struct hlist_head ucounts_hashtable[(1 << UCOUNTS_HASHTABLE_BITS)];
+static DEFINE_SPINLOCK(ucounts_lock);
+
+#define ucounts_hashfn(ns, uid)						\
+	hash_long((unsigned long)__kuid_val(uid) + (unsigned long)(ns), \
+		  UCOUNTS_HASHTABLE_BITS)
+#define ucounts_hashentry(ns, uid)	\
+	(ucounts_hashtable + ucounts_hashfn(ns, uid))
+
+
 #ifdef CONFIG_SYSCTL
 static struct ctl_table_set *
 set_lookup(struct ctl_table_root *root)
@@ -45,7 +57,7 @@ static struct ctl_table_root set_root = {
 
 static int zero = 0;
 static int int_max = INT_MAX;
-static struct ctl_table userns_table[] = {
+static struct ctl_table user_table[] = {
 	{
 		.procname	= "max_user_namespaces",
 		.data		= &init_user_ns.max_user_namespaces,
@@ -64,11 +76,11 @@ bool setup_userns_sysctls(struct user_namespace *ns)
 #ifdef CONFIG_SYSCTL
 	struct ctl_table *tbl;
 	setup_sysctl_set(&ns->set, &set_root, set_is_seen);
-	tbl = kmemdup(userns_table, sizeof(userns_table), GFP_KERNEL);
+	tbl = kmemdup(user_table, sizeof(user_table), GFP_KERNEL);
 	if (tbl) {
 		tbl[0].data = &ns->max_user_namespaces;
 
-		ns->sysctls = __register_sysctl_table(&ns->set, "userns", tbl);
+		ns->sysctls = __register_sysctl_table(&ns->set, "user", tbl);
 	}
 	if (!ns->sysctls) {
 		kfree(tbl);
@@ -91,6 +103,61 @@ void retire_userns_sysctls(struct user_namespace *ns)
 #endif
 }
 
+static struct ucounts *find_ucounts(struct user_namespace *ns, kuid_t uid, struct hlist_head *hashent)
+{
+	struct ucounts *ucounts;
+
+	hlist_for_each_entry(ucounts, hashent, node) {
+		if (uid_eq(ucounts->uid, uid) && (ucounts->ns == ns))
+			return ucounts;
+	}
+	return NULL;
+}
+
+static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
+{
+	struct hlist_head *hashent = ucounts_hashentry(ns, uid);
+	struct ucounts *ucounts, *new;
+
+	spin_lock(&ucounts_lock);
+	ucounts = find_ucounts(ns, uid, hashent);
+	if (!ucounts) {
+		spin_unlock(&ucounts_lock);
+
+		new = kzalloc(sizeof(*new), GFP_KERNEL);
+		if (!new)
+			return NULL;
+
+		new->ns = ns;
+		new->uid = uid;
+		atomic_set(&new->count, 0);
+
+		spin_lock(&ucounts_lock);
+		ucounts = find_ucounts(ns, uid, hashent);
+		if (ucounts) {
+			kfree(new);
+		} else {
+			hlist_add_head(&new->node, hashent);
+			ucounts = new;
+		}
+	}
+	if (!atomic_add_unless(&ucounts->count, 1, INT_MAX))
+		ucounts = NULL;
+	spin_unlock(&ucounts_lock);
+	return ucounts;
+}
+
+static void put_ucounts(struct ucounts *ucounts)
+{
+	if (atomic_dec_and_test(&ucounts->count)) {
+		spin_lock(&ucounts_lock);
+		hlist_del_init(&ucounts->node);
+		spin_unlock(&ucounts_lock);
+
+		kfree(ucounts);
+	}
+}
+
 static inline bool atomic_inc_below(atomic_t *v, int u)
 {
 	int c, old;
@@ -105,44 +172,51 @@ static inline bool atomic_inc_below(atomic_t *v, int u)
 	}
 }
 
-bool inc_user_namespaces(struct user_namespace *ns)
+struct ucounts *inc_user_namespaces(struct user_namespace *ns, kuid_t uid)
 {
-	struct user_namespace *pos, *bad;
-	for (pos = ns; pos; pos = pos->parent) {
-		int max = READ_ONCE(pos->max_user_namespaces);
-		if (!atomic_inc_below(&pos->user_namespaces, max))
+	struct ucounts *ucounts, *iter, *bad;
+	struct user_namespace *tns;
+	ucounts = get_ucounts(ns, uid);
+	for (iter = ucounts; iter; iter = tns->ucounts) {
+		int max;
+		tns = iter->ns;
+		max = READ_ONCE(tns->max_user_namespaces);
+		if (!atomic_inc_below(&iter->user_namespaces, max))
 			goto fail;
 	}
-	return true;
+	return ucounts;
 fail:
-	bad = pos;
-	for (pos = ns; pos != bad; pos = pos->parent)
-		atomic_dec(&pos->user_namespaces);
+	bad = iter;
+	for (iter = ucounts; iter != bad; iter = iter->ns->ucounts)
+		atomic_dec(&iter->user_namespaces);
 
-	return false;
+	put_ucounts(ucounts);
+	return NULL;
 }
 
-void dec_user_namespaces(struct user_namespace *ns)
+void dec_user_namespaces(struct ucounts *ucounts)
 {
-	struct user_namespace *pos;
-	for (pos = ns; pos; pos = pos->parent) {
-		int dec = atomic_dec_if_positive(&pos->user_namespaces);
+	struct ucounts *iter;
+	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
+		int dec = atomic_dec_if_positive(&iter->user_namespaces);
 		WARN_ON_ONCE(dec < 0);
 	}
+	put_ucounts(ucounts);
 }
 
+
 static __init int user_namespace_sysctl_init(void)
 {
 #ifdef CONFIG_SYSCTL
-	static struct ctl_table_header *userns_header;
+	static struct ctl_table_header *user_header;
 	static struct ctl_table empty[1];
 	/*
-	 * It is necessary to register the userns directory in the
+	 * It is necessary to register the user directory in the
 	 * default set so that registrations in the child sets work
 	 * properly.
 	 */
-	userns_header = register_sysctl("userns", empty);
-	BUG_ON(!userns_header);
+	user_header = register_sysctl("user", empty);
+	BUG_ON(!user_header);
 	BUG_ON(!setup_userns_sysctls(&init_user_ns));
 #endif
 	return 0;