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

[Wang Jinchao <wangjinchao@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_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`.

Signed-off-by: Wang Jinchao <wangjinchao@xfusion.com>
---
 kernel/padata.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/padata.c b/kernel/padata.c
index 222d60195de6..15dbcf7ce623 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -1107,7 +1107,8 @@ 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));
+	if (refcount_dec_and_test(&ps->pd->refcnt))
+		padata_free_pd(rcu_dereference_protected(ps->pd, 1));
 	mutex_unlock(&ps->pinst->lock);
 
 	kfree(ps);
-- 
2.40.0