NACK: [SRU][F][PATCH 1/1] padata: Fix refcnt handling in padata_free_shell()

[Alice Munduruca]

Focal is now ESM. Directing to other mailing list.

On Mon, 11 Aug 2025 at 14:39, <alice.munduruca at canonical.com> wrote:
>
> From: WangJinchao <wangjinchao at xfusion.com>
>
> 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`.
>
> Fixes: 07928d9bfc81 ("padata: Remove broken queue flushing")
>
> Signed-off-by: WangJinchao <wangjinchao at xfusion.com>
> Acked-by: Daniel Jordan <daniel.m.jordan at oracle.com>
> Acked-by: Daniel Jordan <daniel.m.jordan at oracle.com>
> Signed-off-by: Herbert Xu <herbert at gondor.apana.org.au>
> (backported from commit 7ddc21e317b360c3444de3023bcc83b85fabae2f)
> [cremfuelled: Removed the local variable `pinst` as it only referred to a readily-available attribute anyways and changed refcount function into an atomic function given context]
> CVE-2023-52854
> Signed-off-by: Alice C. Munduruca <alice.munduruca at canonical.com>
> ---
>  kernel/padata.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/padata.c b/kernel/padata.c
> index 47aebcda65d5..f1eb0d4666e9 100644
> --- a/kernel/padata.c
> +++ b/kernel/padata.c
> @@ -1119,12 +1119,17 @@ EXPORT_SYMBOL(padata_alloc_shell);
>   */
>  void padata_free_shell(struct padata_shell *ps)
>  {
> -       struct padata_instance *pinst = ps->pinst;
> +       struct parallel_data *pd;
>
> -       mutex_lock(&pinst->lock);
> +       if (!ps)
> +               return;
> +
> +       mutex_lock(&ps->pinst->lock);
>         list_del(&ps->list);
> -       padata_free_pd(rcu_dereference_protected(ps->pd, 1));
> -       mutex_unlock(&pinst->lock);
> +       pd = rcu_dereference_protected(ps->pd, 1);
> +       if (atomic_dec_and_test(&pd->refcnt))
> +               padata_free_pd(pd);
> +       mutex_unlock(&ps->pinst->lock);
>
>         kfree(ps);
>  }
> --
> 2.48.1
>