Quoting Eric W. Biederman (ebied...@xmission.com):
> Export the export the maximum number of user namespaces as
^ note if you resend, duplicate "export the"
> /proc/sys/userns/max_user_namespaces.
>
> Signed-off-by: "Eric W. Biederman" <ebied...@xmission.com>
Acked-by: Serge Hallyn <se...@hallyn.com>
> ---
> include/linux/user_namespace.h | 2 ++
> kernel/fork.c | 2 ++
> kernel/user_namespace.c | 69
> +++++++++++++++++++++++++++++++++++++-----
> 3 files changed, 65 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index 7d59af1f08f1..ba6a995178f9 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -43,6 +43,8 @@ struct user_namespace {
> struct ctl_table_set set;
> struct ctl_table_header *sysctls;
> #endif
> + int max_user_namespaces;
> + atomic_t user_namespaces;
> };
>
> extern struct user_namespace init_user_ns;
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 5c2c355aa97f..95d5498c463f 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -323,6 +323,8 @@ void __init fork_init(void)
> init_task.signal->rlim[RLIMIT_NPROC].rlim_max = max_threads/2;
> init_task.signal->rlim[RLIMIT_SIGPENDING] =
> init_task.signal->rlim[RLIMIT_NPROC];
> +
> + init_user_ns.max_user_namespaces = max_threads;
> }
>
> int __weak arch_dup_task_struct(struct task_struct *dst,
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 10afbb55dfc2..0061550e3282 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -29,6 +29,7 @@ static DEFINE_MUTEX(userns_state_mutex);
> static bool new_idmap_permitted(const struct file *file,
> struct user_namespace *ns, int cap_setid,
> struct uid_gid_map *map);
> +#define COUNT_MAX (INT_MAX - 1)
>
> #ifdef CONFIG_SYSCTL
> static struct ctl_table_set *
> @@ -63,7 +64,18 @@ static struct ctl_table_root set_root = {
> .permissions = set_permissions,
> };
>
> +static int zero = 0;
> +static int count_max = COUNT_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 = &count_max,
> + },
> { }
> };
> #endif /* CONFIG_SYSCTL */
> @@ -75,6 +87,8 @@ static 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) {
> @@ -113,6 +127,34 @@ static void set_cred_user_ns(struct cred *cred, struct
> user_namespace *user_ns)
> cred->user_ns = user_ns;
> }
>
> +static 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);
> + int sum = atomic_inc_return(&pos->user_namespaces);
> + if (sum > max)
> + goto fail;
> + }
> + return true;
> +fail:
> + bad = pos;
> + atomic_dec(&pos->user_namespaces);
> + for (pos = ns; pos != bad; pos = pos->parent)
> + atomic_dec(&pos->user_namespaces);
> +
> + return false;
> +}
> +
> +static 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);
> + }
> +}
> +
> /*
> * Create a new user namespace, deriving the creator from the user in the
> * passed credentials, and replacing that user with the new root user for the
> @@ -128,8 +170,12 @@ int create_user_ns(struct cred *new)
> kgid_t group = new->egid;
> int ret;
>
> + ret = -EUSERS;
> if (parent_ns->level > 32)
> - return -EUSERS;
> + goto fail;
> +
> + if (!inc_user_namespaces(parent_ns))
> + goto fail;
>
> /*
> * Verify that we can not violate the policy of which files
> @@ -137,26 +183,27 @@ int create_user_ns(struct cred *new)
> * by verifing that the root directory is at the root of the
> * mount namespace which allows all files to be accessed.
> */
> + ret = -EPERM;
> if (current_chrooted())
> - return -EPERM;
> + goto fail_dec;
>
> /* The creator needs a mapping in the parent user namespace
> * or else we won't be able to reasonably tell userspace who
> * created a user_namespace.
> */
> + ret = -EPERM;
> if (!kuid_has_mapping(parent_ns, owner) ||
> !kgid_has_mapping(parent_ns, group))
> - return -EPERM;
> + goto fail_dec;
>
> + ret = -ENOMEM;
> ns = kmem_cache_zalloc(user_ns_cachep, GFP_KERNEL);
> if (!ns)
> - return -ENOMEM;
> + goto fail_dec;
>
> ret = ns_alloc_inum(&ns->ns);
> - if (ret) {
> - kmem_cache_free(user_ns_cachep, ns);
> - return ret;
> - }
> + if (ret)
> + goto fail_free;
> ns->ns.ops = &userns_operations;
>
> atomic_set(&ns->count, 1);
> @@ -165,6 +212,7 @@ int create_user_ns(struct cred *new)
> ns->level = parent_ns->level + 1;
> ns->owner = owner;
> ns->group = group;
> + ns->max_user_namespaces = COUNT_MAX;
>
> /* Inherit USERNS_SETGROUPS_ALLOWED from our parent */
> mutex_lock(&userns_state_mutex);
> @@ -185,7 +233,11 @@ fail_keyring:
> key_put(ns->persistent_keyring_register);
> #endif
> ns_free_inum(&ns->ns);
> +fail_free:
> kmem_cache_free(user_ns_cachep, ns);
> +fail_dec:
> + dec_user_namespaces(parent_ns);
> +fail:
> return ret;
> }
>
> @@ -221,6 +273,7 @@ void free_user_ns(struct user_namespace *ns)
> #endif
> ns_free_inum(&ns->ns);
> kmem_cache_free(user_ns_cachep, ns);
> + dec_user_namespaces(parent);
> ns = parent;
> } while (atomic_dec_and_test(&parent->count));
> }
> --
> 2.8.3
