CVE-2023-52854

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, Ill describe the problem scenario using a simplified model:

Suppose theres 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:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24

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, theres the following code:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15

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.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

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