Re: [PATCH 2/2] padata: fix UAF in padata_reorder

[chenridong <chenridong@huawei.com>]

On 2024/12/6 7:01, Daniel Jordan wrote:
> Hello Ridong,
> 
> On Sat, Nov 23, 2024 at 08:05:09AM +0000, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@huawei.com>
>>
>> A bug was found when run ltp test:
> ...snip...
>> This can be explained as bellow:
>>
>> pcrypt_aead_encrypt
>> ...
>> padata_do_parallel
>> refcount_inc(&pd->refcnt); // add refcnt
>> ...
>> padata_do_serial
>> padata_reorder // pd
>> while (1) {
>> padata_find_next(pd, true); // using pd
>> queue_work_on
>> ...
>> padata_serial_worker				crypto_del_alg
>> padata_put_pd_cnt // sub refcnt
>> 						padata_free_shell
>> 						padata_put_pd(ps->pd);
>> 						// pd is freed
>> // loop again, but pd is freed
>> // call padata_find_next, UAF
>> }
> 
> Thanks for the fix and clear explanation.
> 
>> diff --git a/kernel/padata.c b/kernel/padata.c
>> index 5d8e18cdcb25..627014825266 100644
>> --- a/kernel/padata.c
>> +++ b/kernel/padata.c
>> @@ -319,6 +319,7 @@ static void padata_reorder(struct parallel_data *pd)
>>  	if (!spin_trylock_bh(&pd->lock))
>>  		return;
>>  
>> +	padata_get_pd(pd);
>>  	while (1) {
>>  		padata = padata_find_next(pd, true);
>>  
>> @@ -355,6 +356,7 @@ 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);
>> +	padata_put_pd(pd);
> 
> Putting the ref unconditionally here doesn't cover the case where reorder_work
> is queued and accesses the freed pd.
> 
> The review of patches 3-5 from this series has a potential solution for
> this that also keeps some of these refcount operations out of the fast
> path:
> 
>     https://lore.kernel.org/all/20221019083708.27138-1-nstange@suse.de/
> 

Thank you for your review.

IIUC, patches 3-5 from this series aim to fix two issue.
1. Avoid UAF for pd(the patch 3).
2. Avoid UAF for ps(the patch 4-5).
What my patch 2 intends to fix is the issue 1.

Let's focus on issue 1.
As shown bellow, if reorder_work is queued, the refcnt must greater than
0, since its serial work have not be finished yet. Do your agree with that?

pcrypt_aead_encrypt/pcrypt_aead_decrypt
padata_do_parallel 			// refcount_inc(&pd->refcnt);
padata_parallel_worker	
padata->parallel(padata);
padata_do_serial(padata);		
// pd->reorder_list 			// enque reorder_list
padata_reorder
 - case1:squeue->work
	padata_serial_worker		// sub refcnt cnt
 - case2:pd->reorder_work		// reorder->list is not empty
	invoke_padata_reorder 		// this means refcnt > 0
	...
	padata_serial_worker

I think the patch 3(from Nicolai Stange) can also avoid UAF for pd, but
it's complicated. IIUC, the issue 1 can only occur in the scenario what
I mentioned is my commit message. How I fix issue 1 is by adding and
putting the refcnt in the padata_reorder function, which is simple and
clear.

If understand something uncorrectly, please let me know.

As the issue 2, I have not encountered it, but it exists in theory.

Thanks,
Ridong