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

Chen Ridong
2024-12-23T09:00:23Z

On 2024/12/11 3:12, Daniel Jordan wrote:
> Hi Ridong,
>
> On Fri, Dec 06, 2024 at 11:48:36AM +0800, chenridong wrote:
>> On 2024/12/6 7:01, Daniel Jordan wrote:
>>> On Sat, Nov 23, 2024 at 08:05:09AM +0000, Chen Ridong wrote:
>>>> 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?
>
> I think it's possible for reorder_work to be queued even though all
> serial works have finished:
>
> - padata_reorder finds the reorder list nonempty and sees an object from
> padata_find_next, then gets preempted
> - the serial work finishes in another context
> - back in padata_reorder, reorder_work is queued
>
> Not sure this race could actually happen in practice though.
>
> But, I also think reorder_work can be queued when there's an unfinished
> serial work, as you say, but with UAF still happening:
>
> padata_do_serial
> ...
> padata_reorder
> // processes all remaining
> // requests then breaks
> while (1) {
> if (!padata)
> break;
> ...
> }
>
> padata_do_serial
> // new request added
> list_add
> // sees the new request
> queue_work(reorder_work)
> padata_reorder
> queue_work_on(squeue->work)
>
>
>
> <kworker context>
> padata_serial_worker
> // completes new request,
> // no more outstanding
> // requests
> crypto_del_alg
> // free pd
> <kworker context>
> invoke_padata_reorder
> // UAF of pd
>

Sorry for being busy with other work for a while.
Thank you for your patience.
In theory, it does exist. Although I was unable reproduce it(I added
delay helper as below), I noticed that Herbert has reported a UAF issue
occurred in the padata_parallel_worker function. Therefore, it would be
better to fix it in Nicolai's approach.

static void padata_parallel_worker(struct work_struct *parallel_work)
{
+ mdelay(10);
+

Hi, Nicolai, would you resend the patch 3 to fix this issue?
I noticed you sent the patch 2 years ago, but this series has not been
merged.

Or may I send a patch that aligns with your approach to resolve it?
Looking forward your feedback.

>> 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
>
> In other words, in case2 above, reorder_work could be queued, another
> context could complete the request in padata_serial_worker, and then
> invoke_padata_reorder could run and UAF when there aren't any remaining
> serial works.
>
>> I think the patch 3(from Nicolai Stange) can also avoid UAF for pd, but
>> it's complicated.
>
> For fixing the issue you describe, without regard for the reorder work,
> I think the synchronize_rcu from near the end of the patch 3 thread is
> enough. A synchronize_rcu in the slow path seems better than two
> atomics in the fast path.

Thank you. I tested with 'synchronize_rcu', and it can fix the issue I
encountered. As I mentioned, Herbert has provided another stack, which
indicates that case 2 exists. I think it would be better to fix it as
patch 3 did.

Thanks,
Ridong