Subject: CVE-2023-52854: padata: Fix refcnt handling in padata_free_shell()

From: Greg Kroah-Hartman <[email-masked]>

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`.

The Linux kernel CVE team has assigned CVE-2023-52854 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.6 with commit 07928d9bfc81 and fixed in 5.10.201 with commit 41aad9d69539
	Issue introduced in 5.6 with commit 07928d9bfc81 and fixed in 5.15.139 with commit 0dd34a7ad395
	Issue introduced in 5.6 with commit 07928d9bfc81 and fixed in 6.1.63 with commit c7c26d0ef5d2
	Issue introduced in 5.6 with commit 07928d9bfc81 and fixed in 6.5.12 with commit 1e901bcb8af1
	Issue introduced in 5.6 with commit 07928d9bfc81 and fixed in 6.6.2 with commit 1734a79e9519
	Issue introduced in 5.6 with commit 07928d9bfc81 and fixed in 6.7 with commit 7ddc21e317b3
	Issue introduced in 3.16.84 with commit 13721e447acc
	Issue introduced in 4.4.215 with commit 7a2ccb65f901
	Issue introduced in 4.9.215 with commit 928cf3d733c4
	Issue introduced in 4.14.172 with commit c9da8ee14917
	Issue introduced in 4.19.103 with commit dc34710a7aba
	Issue introduced in 5.4.19 with commit 5fefc9b3e358
	Issue introduced in 5.5.3 with commit 26daf8e6515c

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2023-52854
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	kernel/padata.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	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