userns: Add a limit on the number of user namespaces

Export the export the maximum number of user namespaces as
/proc/sys/userns/max_user_namespaces.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 53 insertions, 0 deletions

diff --git a/kernel/ucount.c b/kernel/ucount.c
index cbde1dc87851..6c2205c0befd 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -43,7 +43,18 @@ static struct ctl_table_root set_root = {
         .permissions = set_permissions,
 };
 
+static int zero = 0;
+static int int_max = INT_MAX;
 static struct ctl_table userns_table[] = {
+        {
+                .procname       = "max_user_namespaces",
+                .data           = &init_user_ns.max_user_namespaces,
+                .maxlen         = sizeof(init_user_ns.max_user_namespaces),
+                .mode           = 0644,
+                .proc_handler   = proc_dointvec_minmax,
+                .extra1         = &zero,
+                .extra2         = &int_max,
+        },
         { }
 };
 #endif /* CONFIG_SYSCTL */
@@ -55,6 +66,8 @@ bool setup_userns_sysctls(struct user_namespace *ns)
         setup_sysctl_set(&ns->set, &set_root, set_is_seen);
         tbl = kmemdup(userns_table, sizeof(userns_table), GFP_KERNEL);
         if (tbl) {
+                tbl[0].data = &ns->max_user_namespaces;
+
                 ns->sysctls = __register_sysctl_table(&ns->set, "userns", tbl);
         }
         if (!ns->sysctls) {
@@ -78,6 +91,46 @@ void retire_userns_sysctls(struct user_namespace *ns)
 #endif
 }
 
+static inline bool atomic_inc_below(atomic_t *v, int u)
+{
+        int c, old;
+        c = atomic_read(v);
+        for (;;) {
+                if (unlikely(c >= u))
+                        return false;
+                old = atomic_cmpxchg(v, c, c+1);
+                if (likely(old == c))
+                        return true;
+                c = old;
+        }
+}
+
+bool inc_user_namespaces(struct user_namespace *ns)
+{
+        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))
+                        goto fail;
+        }
+        return true;
+fail:
+        bad = pos;
+        for (pos = ns; pos != bad; pos = pos->parent)
+                atomic_dec(&pos->user_namespaces);
+
+        return false;
+}
+
+void dec_user_namespaces(struct user_namespace *ns)
+{
+        struct user_namespace *pos;
+        for (pos = ns; pos; pos = pos->parent) {
+                int dec = atomic_dec_if_positive(&pos->user_namespaces);
+                WARN_ON_ONCE(dec < 0);
+        }
+}
+
 static __init int user_namespace_sysctl_init(void)
 {
 #ifdef CONFIG_SYSCTL