On Thu, Jul 30, 2015 at 06:15:23AM +0000, Nicholas A. Bellinger wrote:
> From: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
>
> This patch addresses a v4.2-rc1 regression where backend driver
> struct module unload immediately after ->free_device() has done
> an internal call_rcu(), results in IRQ rcu_process_callbacks()
> use-after-free paging OOPsen.
>
> It adds a explicit synchronize_rcu() in target_backend_unregister()
> to wait a full RCU grace period before releasing target_backend_ops
> memory, and allowing TBO->module exit to proceed.
Good catch, but...
You need rcu_barrier() rather than synchronize_rcu() in this case.
All that synchronize_rcu() does is wait for pre-existing RCU readers,
when what is needed is to wait for all pre-existing RCU callbacks
to be invoked.
So please replace the two synchronize_rcu() calls with rcu_barrier().
Thanx, Paul
> Also, go ahead and do the same for target_unregister_template()
> to ensure se_deve_entry->rcu_head -> kfree_rcu() grace period has
> passed, before allowing target_core_fabric_ops->owner module exit
> to proceed.
>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Hannes Reinecke <hare@xxxxxxx>
> Cc: Sagi Grimberg <sagig@xxxxxxxxxxxx>
> Signed-off-by: Nicholas Bellinger <nab@xxxxxxxxxxxxxxx>
> ---
> drivers/target/target_core_configfs.c | 10 +++++++++-
> drivers/target/target_core_hba.c | 10 +++++++++-
> 2 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/target/target_core_configfs.c b/drivers/target/target_core_configfs.c
> index c2e9fea..b4c3ae0 100644
> --- a/drivers/target/target_core_configfs.c
> +++ b/drivers/target/target_core_configfs.c
> @@ -457,8 +457,16 @@ void target_unregister_template(const struct target_core_fabric_ops *fo)
> if (!strcmp(t->tf_ops->name, fo->name)) {
> BUG_ON(atomic_read(&t->tf_access_cnt));
> list_del(&t->tf_list);
> + mutex_unlock(&g_tf_lock);
> + /*
> + * Allow any outstanding fabric se_deve_entry->rcu_head
> + * grace periods to expire post kfree_rcu(), before allowing
> + * fabric driver unload of target_core_fabric_ops->module
> + * to proceed.
> + */
> + synchronize_rcu();
> kfree(t);
> - break;
> + return;
> }
> }
> mutex_unlock(&g_tf_lock);
> diff --git a/drivers/target/target_core_hba.c b/drivers/target/target_core_hba.c
> index 62ea4e8..0fb830b 100644
> --- a/drivers/target/target_core_hba.c
> +++ b/drivers/target/target_core_hba.c
> @@ -84,8 +84,16 @@ void target_backend_unregister(const struct target_backend_ops *ops)
> list_for_each_entry(tb, &backend_list, list) {
> if (tb->ops == ops) {
> list_del(&tb->list);
> + mutex_unlock(&backend_mutex);
> + /*
> + * Allow any outstanding backend driver ->rcu_head grace
> + * period to expire post ->free_device() -> call_rcu(),
> + * before allowing backend driver module unload of
> + * target_backend_ops->owner to proceed.
> + */
> + synchronize_rcu();
> kfree(tb);
> - break;
> + return;
> }
> }
> mutex_unlock(&backend_mutex);
> --
> 1.9.1
>
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