Re: [PATCH 3/5] padata: grab parallel_data refcnt for reorder

[Nicolai Stange]

Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> writes:

> On Wed, Oct 19, 2022 at 10:37:06AM +0200, Nicolai Stange wrote:
>> On entry of padata_do_serial(), the in-flight padata_priv owns a reference
>> to the associated parallel_data instance.
>> 
>> However, as soon as the padata_priv got enqueued on the reorder list, it
>> can be completed from a different context, causing the reference to get
>> released in the course.
>> 
>> This would potentially cause UAFs from the subsequent padata_reorder()
>> operations invoked from the enqueueing padata_do_serial() or from the
>> reorder work.
>> 

<snip>

>> ---
>>  kernel/padata.c | 26 +++++++++++++++++++++-----
>>  1 file changed, 21 insertions(+), 5 deletions(-)
>> 
>> diff --git a/kernel/padata.c b/kernel/padata.c
>> index 0bf8c80dad5a..b79226727ef7 100644
>> --- a/kernel/padata.c
>> +++ b/kernel/padata.c
>> @@ -275,7 +275,7 @@ static struct padata_priv *padata_find_next(struct parallel_data *pd,
>>  	return padata;
>>  }
>>  
>> -static void padata_reorder(struct parallel_data *pd)
>> +static bool padata_reorder(struct parallel_data *pd)
>>  {
>>  	struct padata_instance *pinst = pd->ps->pinst;
>>  	int cb_cpu;
>> @@ -294,7 +294,7 @@ static void padata_reorder(struct parallel_data *pd)
>>  	 * care for all the objects enqueued during the holdtime of the lock.
>>  	 */
>>  	if (!spin_trylock_bh(&pd->lock))
>> -		return;
>> +		return false;
>>  
>>  	while (1) {
>>  		padata = padata_find_next(pd, true);
>> @@ -331,17 +331,23 @@ static void padata_reorder(struct parallel_data *pd)
>>  
>>  	reorder = per_cpu_ptr(pd->reorder_list, pd->cpu);
>>  	if (!list_empty(&reorder->list) && padata_find_next(pd, false))
>> -		queue_work(pinst->serial_wq, &pd->reorder_work);
>> +		return queue_work(pinst->serial_wq, &pd->reorder_work);
>> +
>> +	return false;
>>  }
>>  
>>  static void invoke_padata_reorder(struct work_struct *work)
>>  {
>>  	struct parallel_data *pd;
>> +	bool keep_refcnt;
>>  
>>  	local_bh_disable();
>>  	pd = container_of(work, struct parallel_data, reorder_work);
>> -	padata_reorder(pd);
>> +	keep_refcnt = padata_reorder(pd);
>>  	local_bh_enable();
>> +
>> +	if (!keep_refcnt)
>> +		padata_put_pd(pd);
>>  }
>>  
>>  static void padata_serial_worker(struct work_struct *serial_work)
>> @@ -392,6 +398,15 @@ void padata_do_serial(struct padata_priv *padata)
>>  	struct padata_list *reorder = per_cpu_ptr(pd->reorder_list, hashed_cpu);
>>  	struct padata_priv *cur;
>>  
>> +	/*
>> +	 * The in-flight padata owns a reference on pd. However, as
>> +	 * soon as it's been enqueued on the reorder list, another
>> +	 * task can dequeue and complete it, thereby dropping the
>> +	 * reference. Grab another reference here, it will eventually
>> +	 * be released from a reorder work, if any, or below.
>> +	 */
>> +	padata_get_pd(pd);
>> +
>>  	spin_lock(&reorder->lock);
>>  	/* Sort in ascending order of sequence number. */
>>  	list_for_each_entry_reverse(cur, &reorder->list, list)
>> @@ -407,7 +422,8 @@ void padata_do_serial(struct padata_priv *padata)
>>  	 */
>>  	smp_mb();
>>  
>> -	padata_reorder(pd);
>> +	if (!padata_reorder(pd))
>> +		padata_put_pd(pd);
>
> do_serial is supposed to be called with BHs disabled and will be in
> every case after a fix for a separate issue that I plan to send this
> cycle.  Given that it (will soon...) always happen under RCU protection,
> part of this issue could be addressed like this, which puts the expense
> of dealing with this rare problem in the slow path:
>
> diff --git a/kernel/padata.c b/kernel/padata.c
> index 0bf8c80dad5a..cd6740ae6629 100644
> --- a/kernel/padata.c
> +++ b/kernel/padata.c
> @@ -1110,6 +1110,12 @@ void padata_free_shell(struct padata_shell *ps)
>  	if (!ps)
>  		return;
>  
> +	/*
> +	 * Wait for all _do_serial calls to finish to avoid touching freed pd's
> +	 * and ps's.
> +	 */
> +	synchronize_rcu();
> +
>  	mutex_lock(&ps->pinst->lock);
>  	list_del(&ps->list);
>  	padata_put_pd(rcu_dereference_protected(ps->pd, 1));
>
> pcrypt calls padata_free_shell() when all outstanding transforms (and
> thus requests, I think) have been freed/completed, so no new task can
> come into padata_reorder.  synchronize_rcu() then flushes out any
> remaining _reorder calls.
>
> This doesn't deal with pending reorder_work items, but we can fix it
> later in the series.
>
> What do you think?

Yes, I think that would work. Will you handle it alongside that fix for
a separate issue you mentioned above? Or shall I once this fix has
landed?

Thanks!

Nicolai

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