CVE-2023-52854: Use After Free

In the Linux kernel, the following vulnerability has been resolved:

padata: Fix refcnt handling in padata_free_shell()

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`.