UBUNTU-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, 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.