Re: [PATCH RFC v4 1/3] page_pool: fix timing for checking and disabling napi_local

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/12/3 10:49, Jakub Kicinski wrote:
> On Wed, 20 Nov 2024 18:34:53 +0800 Yunsheng Lin wrote:
>> page_pool page may be freed from skb_defer_free_flush() in
>> softirq context without binding to any specific napi, it
>> may cause use-after-free problem due to the below time window,
>> as below, CPU1 may still access napi->list_owner after CPU0
>> free the napi memory:
>>
>>             CPU 0                           CPU1
>>       page_pool_destroy()          skb_defer_free_flush()
>>              .                               .
>>              .                napi = READ_ONCE(pool->p.napi);
>>              .                               .
>> page_pool_disable_direct_recycling()         .
>>    driver free napi memory                   .
>>              .                               .
>>              .       napi && READ_ONCE(napi->list_owner) == cpuid
>>              .                               .
>>
>> Use rcu mechanism to avoid the above problem.
>>
>> Note, the above was found during code reviewing on how to fix
>> the problem in [1].
>>
>> 1. https://lore.kernel.org/lkml/8067f204-1380-4d37-8ffd-007fc6f26738@kernel.org/T/
> 
> Please split it from the rest of the series, it's related but not
> really logically connected (dma mappings and NAPI recycling are 
> different features of page pool).
> 
>> @@ -1126,6 +1133,12 @@ void page_pool_destroy(struct page_pool *pool)
>>  	if (!page_pool_release(pool))
>>  		return;
>>  
>> +	/* Paired with rcu lock in page_pool_napi_local() to enable clearing
>> +	 * of pool->p.napi in page_pool_disable_direct_recycling() is seen
>> +	 * before returning to driver to free the napi instance.
>> +	 */
>> +	synchronize_rcu();
> 
> I don't think this is in the right place.
> Why not inside page_pool_disable_direct_recycling() ?

It is in page_pool_destroy() mostly because:
1. Only call synchronize_rcu() when there is inflight pages, which should
   be an unlikely case, and  synchronize_rcu() might need to be called at
   least for the case of pool->p.napi not being NULL if it is called inside
   page_pool_disable_direct_recycling().
2. the above synchronize_rcu() in page_pool_destroy() is also reused
   for the fixing of dma unmappings.

> 
> Hopefully this doesn't cause long stalls during reconfig, normally
> NAPIs and page pools are freed together, and NAPI removal implies
> synchronize_rcu(). That's why it's not really a problem for majority
> of drivers. You should perhaps make a note of this in the commit
> message.
>