Use After Free Affecting kernel-rt-debug-kvm package, versions *

Note: Versions mentioned in the description apply only to the upstream kernel-rt-debug-kvm package and not the kernel-rt-debug-kvm package as distributed by RHEL. See How to fix? for RHEL:8 relevant fixed versions and status.

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:

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:

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.

• https://access.redhat.com/security/cve/CVE-2023-52854
• 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