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

[Daniel Jordan]

Hello Ridong,

On Sat, Nov 23, 2024 at 08:05:09AM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@xxxxxxxxxx>
> 
> 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@xxxxxxx/