Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
>
> When freeing a deeply nested user namespace free_user_ns calls
> put_user_ns on it's parent which may in turn call free_user_ns again.
> When -fno-optimize-sibling-calls is passed to gcc one stack frame per
> user namespace is left on the stack, potentially overflowing the
> kernel stack. CONFIG_FRAME_POINTER forces -fno-optimize-sibling-calls
> so we can't count on gcc to optimize this code.
>
> Remove struct kref and use a plain atomic_t. Making the code more
> flexible and easier to comprehend. Make the loop in free_user_ns
> explict to guarantee that the stack does not overflow with
> CONFIG_FRAME_POINTER enabled.
>
> I have tested this fix with a simple program that uses unshare to
> create a deeply nested user namespace structure and then calls exit.
> With 1000 nesteuser namespaces before this change running my test
> program causes the kernel to die a horrible death. With 10,000,000
> nested user namespaces after this change my test program runs to
> completion and causes no harm.
>
> Pointed-out-by: Vasily Kulikov <segoon@xxxxxxxxxxxx>
Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
> include/linux/user_namespace.h | 10 +++++-----
> kernel/user.c | 4 +---
> kernel/user_namespace.c | 17 +++++++++--------
> 3 files changed, 15 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index b9bd2e6..4ce0093 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -21,7 +21,7 @@ struct user_namespace {
> struct uid_gid_map uid_map;
> struct uid_gid_map gid_map;
> struct uid_gid_map projid_map;
> - struct kref kref;
> + atomic_t count;
> struct user_namespace *parent;
> kuid_t owner;
> kgid_t group;
> @@ -35,18 +35,18 @@ extern struct user_namespace init_user_ns;
> static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
> {
> if (ns)
> - kref_get(&ns->kref);
> + atomic_inc(&ns->count);
> return ns;
> }
>
> extern int create_user_ns(struct cred *new);
> extern int unshare_userns(unsigned long unshare_flags, struct cred **new_cred);
> -extern void free_user_ns(struct kref *kref);
> +extern void free_user_ns(struct user_namespace *ns);
>
> static inline void put_user_ns(struct user_namespace *ns)
> {
> - if (ns)
> - kref_put(&ns->kref, free_user_ns);
> + if (ns && atomic_dec_and_test(&ns->count))
> + free_user_ns(ns);
> }
>
> struct seq_operations;
> diff --git a/kernel/user.c b/kernel/user.c
> index 33acb5e..57ebfd4 100644
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -47,9 +47,7 @@ struct user_namespace init_user_ns = {
> .count = 4294967295U,
> },
> },
> - .kref = {
> - .refcount = ATOMIC_INIT(3),
> - },
> + .count = ATOMIC_INIT(3),
> .owner = GLOBAL_ROOT_UID,
> .group = GLOBAL_ROOT_GID,
> .proc_inum = PROC_USER_INIT_INO,
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 2b042c4..24f8ec3 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -78,7 +78,7 @@ int create_user_ns(struct cred *new)
> return ret;
> }
>
> - kref_init(&ns->kref);
> + atomic_set(&ns->count, 1);
> /* Leave the new->user_ns reference with the new user namespace. */
> ns->parent = parent_ns;
> ns->owner = owner;
> @@ -104,15 +104,16 @@ int unshare_userns(unsigned long unshare_flags, struct cred **new_cred)
> return create_user_ns(cred);
> }
>
> -void free_user_ns(struct kref *kref)
> +void free_user_ns(struct user_namespace *ns)
> {
> - struct user_namespace *parent, *ns =
> - container_of(kref, struct user_namespace, kref);
> + struct user_namespace *parent;
>
> - parent = ns->parent;
> - proc_free_inum(ns->proc_inum);
> - kmem_cache_free(user_ns_cachep, ns);
> - put_user_ns(parent);
> + do {
> + parent = ns->parent;
> + proc_free_inum(ns->proc_inum);
> + kmem_cache_free(user_ns_cachep, ns);
> + ns = parent;
> + } while (atomic_dec_and_test(&parent->count));
> }
> EXPORT_SYMBOL(free_user_ns);
>
> --
> 1.7.5.4
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