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SubjectRe: [PATCH] cfq-iosched: fix ioc_data leak
On Fri, Apr 04 2008, Fabio Checconi wrote:
> Hi,
> stress testing module loading and unloading on a derived scheduler, I
> had crashes on cfq caused by what seemed to be an error in the caching of
> cic lookup results in the ioc_data field of io contexts.
>
> As what's happening is a little bit involved (at least for me), I've put
> together some of the collected oopses with and without debug patches, the
> patches themselves, a script used to reproduce the problem and the .config
> used on kvm/qemu here, hoping that they can explain the problem better than
> words:
>
> http://feanor.sssup.it/~fabio/linux/cfq-ioc-data/
>
> The patch below should fix the problem.
>
>
> Subject: cfq-iosched: do not leak ioc_data across iosched switches
>
> When switching scheduler from cfq, cfq_exit_queue() does not clear
> ioc->ioc_data, leaving a dangling pointer that can deceive the following
> lookups when the iosched is switched back to cfq. The pattern that can
> trigger that is the following:
>
> - elevator switch from cfq to something else;
> - module unloading, with elv_unregister() that calls cfq_free_io_context()
> on ioc freeing the cic (via the .trim op);
> - module gets reloaded and the elevator switches back to cfq;
> - reallocation of a cic at the same address as before (with a valid key).
>
> To fix it just assign NULL to ioc_data in __cfq_exit_single_io_context(),
> that is called from the regular exit path and from the elevator switching
> code. The only path that frees a cic and is not covered is the error handling
> one, but cic's freed in this way are never cached in ioc_data.
>
> Signed-off-by: Fabio Checconi <fabio@gandalf.sssup.it>
> ---
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 0f962ec..67cd023 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1207,6 +1207,8 @@ static void cfq_exit_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq)
> static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
> struct cfq_io_context *cic)
> {
> + struct io_context *ioc = cic->ioc;
> +
> list_del_init(&cic->queue_list);
>
> /*
> @@ -1216,6 +1218,9 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
> cic->dead_key = (unsigned long) cic->key;
> cic->key = NULL;
>
> + if (ioc->ioc_data == cic)
> + rcu_assign_pointer(ioc->ioc_data, NULL);
> +
> if (cic->cfqq[ASYNC]) {
> cfq_exit_cfqq(cfqd, cic->cfqq[ASYNC]);
> cic->cfqq[ASYNC] = NULL;
> @@ -1248,8 +1253,7 @@ static void cfq_exit_single_io_context(struct io_context *ioc,
> */
> static void cfq_exit_io_context(struct io_context *ioc)
> {
> - rcu_assign_pointer(ioc->ioc_data, NULL);
> call_for_each_cic(ioc, cfq_exit_single_io_context);
> }
>
> static struct cfq_io_context *
> @@ -1480,8 +1485,7 @@ cfq_drop_dead_cic(struct cfq_data *cfqd, struct io_context *ioc,
>
> spin_lock_irqsave(&ioc->lock, flags);
>
> - if (ioc->ioc_data == cic)
> - rcu_assign_pointer(ioc->ioc_data, NULL);
> + BUG_ON(ioc->ioc_data == cic);
>
> radix_tree_delete(&ioc->radix_root, (unsigned long) cfqd);
> hlist_del_rcu(&cic->cic_list);

Your analysis and fix looks correct, thanks a lot!

--
Jens Axboe



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