Re: [PATCH v2] padata: Fix the UAF issue related to parallel_data

[Daniel Jordan]

On Thu, Sep 28, 2023 at 08:38:53AM +0800, Wang Jinchao wrote:
> In a high-load arm64 environment, the pcrypt_aead01 test in LTP can lead to
> system UAF (Use-After-Free) issues. Due to the lengthy analysis of the
> pcrypt_aead01 function call, I'll describe the problem scenario using a
> simplified model:
> 
> Suppose there's a user of padata named `user_function` that adheres to
> the padata requirement of calling `padata_free_shell` after `serial()`
> has been invoked, as demonstrated in the following code:
> 
> ```c
> struct request {
>     struct padata_priv padata;
>     struct completion *done;
> };
> 
> void parallel(struct padata_priv *padata) {
>     do_something();
> }
> 
> void serial(struct padata_priv *padata) {
>     struct request *request = container_of(padata, struct request, padata);
>     complete(request->done);
> }
> 
> void user_function() {
>     DECLARE_COMPLETION(done)
>     padata->parallel = parallel;
>     padata->serial = serial;
>     padata_do_parallel();
>     wait_for_completion(&done);
>     padata_free_shell();
> }
> ```
> 
> In the corresponding padata.c file, there's the following code:
> 
> ```c
> static void padata_serial_worker(struct work_struct *serial_work) {
>     ...
>     cnt = 0;
> 
>     while (!list_empty(&local_list)) {
>         ...
>         padata->serial(padata);
>         cnt++;
>     }
> 
>     local_bh_enable();
> 
>     if (refcount_sub_and_test(cnt, &pd->refcnt))
>         padata_free_pd(pd);
> }
> ```
> 
> Because of the high system load and the accumulation of unexecuted
> softirq at this moment, `local_bh_enable()` in padata takes longer
> to execute than usual. Subsequently, when accessing `pd->refcnt`,
> `pd` has already been released by `padata_free_shell()`, resulting
> in a UAF issue with `pd->refcnt`.
> 
> The fix is straightforward: add `refcount_dec_and_test` before calling
> `padata_free_pd` in `padata_free_shell`.

This could use a Fixes tag.  From Nicolai's patch[0] we agreed on

Fixes: 07928d9bfc81 ("padata: Remove broken queue flushing")

With that,

Acked-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>

Thanks!

[0] https://lore.kernel.org/all/87educb7rm.fsf@xxxxxxx/

> Signed-off-by: Wang Jinchao <wangjinchao@xxxxxxxxxxx>
> ---
> V2:
>     To satisfy Sparse, use rcu_dereference_protected.
>     Reported-by: kernel test robot <lkp@xxxxxxxxx>
>     Closes: https://lore.kernel.org/oe-kbuild-all/202309270829.xHgTOMKw-lkp@xxxxxxxxx/
> 
> V1: https://lore.kernel.org/all/ZRE4XvOOhz4HSOgR@fedora/
> 
>  kernel/padata.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/padata.c b/kernel/padata.c
> index 222d60195de6..acef1e703a8b 100644
> --- a/kernel/padata.c
> +++ b/kernel/padata.c
> @@ -1107,7 +1107,9 @@ void padata_free_shell(struct padata_shell *ps)
>  
>  	mutex_lock(&ps->pinst->lock);
>  	list_del(&ps->list);
> -	padata_free_pd(rcu_dereference_protected(ps->pd, 1));
> +	struct parallel_data *pd = rcu_dereference_protected(ps->pd, 1);
> +	if (refcount_dec_and_test(&pd->refcnt))
> +		padata_free_pd(pd);
>  	mutex_unlock(&ps->pinst->lock);
>  
>  	kfree(ps);
> -- 
> 2.40.0
>