Re: [PATCH] Fix recursive lock in free_uid()/free_user_ns()

From: Serge E. Hallyn
Date: Mon Feb 23 2009 - 15:34:26 EST


Quoting David Howells (dhowells@xxxxxxxxxx):
> free_uid() and free_user_ns() are corecursive when CONFIG_USER_SCHED=n, but
> free_user_ns() is called from free_uid() by way of uid_hash_remove(), which
> requires uidhash_lock to be held. free_user_ns() then calls free_uid() to
> complete the destruction.
>
> Fix this by deferring the destruction of the user_namespace.

Crap. I think when I did this I looked at put_user_ns() I though it
was calling free_user_ns() through an rcu callback (mis-read kref_put).

> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>

Acked-by: Serge Hallyn <serue@xxxxxxxxxx>

Thanks, David and Alexey.

-serge

> ---
>
> include/linux/user_namespace.h | 1 +
> kernel/user_namespace.c | 21 +++++++++++++++++----
> 2 files changed, 18 insertions(+), 4 deletions(-)
>
>
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index 315bcd3..cc4f453 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -13,6 +13,7 @@ struct user_namespace {
> struct kref kref;
> struct hlist_head uidhash_table[UIDHASH_SZ];
> struct user_struct *creator;
> + struct work_struct destroyer;
> };
>
> extern struct user_namespace init_user_ns;
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 7908431..076c7c8 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -60,12 +60,25 @@ int create_user_ns(struct cred *new)
> return 0;
> }
>
> -void free_user_ns(struct kref *kref)
> +/*
> + * Deferred destructor for a user namespace. This is required because
> + * free_user_ns() may be called with uidhash_lock held, but we need to call
> + * back to free_uid() which will want to take the lock again.
> + */
> +static void free_user_ns_work(struct work_struct *work)
> {
> - struct user_namespace *ns;
> -
> - ns = container_of(kref, struct user_namespace, kref);
> + struct user_namespace *ns =
> + container_of(work, struct user_namespace, destroyer);
> free_uid(ns->creator);
> kfree(ns);
> }
> +
> +void free_user_ns(struct kref *kref)
> +{
> + struct user_namespace *ns =
> + container_of(kref, struct user_namespace, kref);
> +
> + INIT_WORK(&ns->destroyer, free_user_ns_work);
> + schedule_work(&ns->destroyer);
> +}
> EXPORT_SYMBOL(free_user_ns);
--
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