userns: Add a limit on the number of user namespaces

[cascardo/linux.git] / kernel / ucount.c

diff --git a/kernel/ucount.c b/kernel/ucount.c
index cbde1dc..6c2205c 100644 (file)
--- 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