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

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() {
    struct request request;
    DECLARE_COMPLETION(done)
    request->done = done;
    padata = &request.padata;
    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.

---
V3:
Include Daniel's ack
V2: https://lore.kernel.org/all/ZRTLHY5A+VqIKhA2@fedora/
To satisfy Sparse, use rcu_dereference_protected.

V1: https://lore.kernel.org/all/ZRE4XvOOhz4HSOgR@fedora/

 kernel/padata.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

index 222d60195de6..79d04a97ded6 100644
@@ -1102,12 +1102,16 @@ EXPORT_SYMBOL(padata_alloc_shell);
  */
 void padata_free_shell(struct padata_shell *ps)
 {
+	struct parallel_data *pd;
+
 	if (!ps)
 		return;
 
 	mutex_lock(&ps->pinst->lock);
 	list_del(&ps->list);
-	padata_free_pd(rcu_dereference_protected(ps->pd, 1));
+	pd = rcu_dereference_protected(ps->pd, 1);
+	if (refcount_dec_and_test(&pd->refcnt))
+		padata_free_pd(rcu_dereference_protected(ps->pd, 1));
 	mutex_unlock(&ps->pinst->lock);
 
 	kfree(ps);

2.40.0

Hi,

The Fixes: tag should be near your Signed-off-by:, and the subject of the patch should be something descriptive like

padata: Fix refcnt handling in padata_free_shell()

Here's some documentation about this:

https://docs.kernel.org/process/5.Posting.html#patch-formatting-and-changelogs

These two tags can also go near your SoB.

Updated in patch v4.

From intel kernel test robot:
If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <[email protected]>
| Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
So I updated the patch without these tags.

Thanks for your and Herbert's acks, which were included in patch v4.

I know Herbert has acked my patch from your email; however, I have not received Herbert's acknowledgment email, and I do not know why.