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

Chen Ridong
2024-12-10T09:39:03Z

On 2024/12/6 11:48, chenridong wrote:
>
>
> 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

Hi, Daniel, I am trying to produce the issue 2. However，I failed.
I added 'mdelay' as helper.

static void padata_reorder(struct parallel_data *pd)
{
+ mdelay(10);
struct padata_instance *pinst = pd->ps->pinst;
int cb_cpu;
struct padata_priv *padata;

I believe this can increase the probability of issue 2. But after
testing with pcrypt_aead01, issue 2 cannot be reproduced.
And I don't know whether it exists now.

Looking forward your reply.

Best regards,
Ridong