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padata: Fix refcnt handling in padata_free_shell()

Luis Claudio R. Goncalves requested to merge lgoncalv/centos-stream-9:9.x__RHEL-17617 into main

JIRA: https://issues.redhat.com/browse/RHEL-17617

commit 7ddc21e317b360c3444de3023bcc83b85fabae2f
Author: WangJinchao wangjinchao@xfusion.com
Date: Mon Oct 16 09:15:21 2023 +0800

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`.  

Fixes: 07928d9bfc81 ("padata: Remove broken queue flushing")  

Signed-off-by: WangJinchao <wangjinchao@xfusion.com>  
Acked-by: Daniel Jordan <daniel.m.jordan@oracle.com>  
Acked-by: Daniel Jordan <daniel.m.jordan@oracle.com>  
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Signed-off-by: Luis Claudio R. Goncalves lgoncalv@redhat.com

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