Vulnerability CVE-2023-52854
Published: 2024-05-21

Description:
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`.

 References:
https://git.kernel.org/stable/c/41aad9d6953984d134fc50f631f24ef476875d4d
https://git.kernel.org/stable/c/0dd34a7ad395dbcf6ae60e48e9786050e25b9bc5
https://git.kernel.org/stable/c/c7c26d0ef5d20f00dbb2ae3befcabbe0efa77275
https://git.kernel.org/stable/c/1e901bcb8af19416b65f5063a4af7996e5a51d7f
https://git.kernel.org/stable/c/1734a79e951914f1db2c65e635012a35db1c674b
https://git.kernel.org/stable/c/7ddc21e317b360c3444de3023bcc83b85fabae2f
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