Merge: CVE-2025-21726: padata: avoid UAF for reorder_work

MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/6499

JIRA: https://issues.redhat.com/browse/RHEL-81522
CVE: CVE-2025-21726
MR: https://gitlab.com/redhat/centos-stream/src/kernel/centos-stream-9/-/merge_requests/6499

This MR backports the 3-patch series that includes the fix to CVE-2025-21726
as well as two other minor cleanup and fix patches.

Signed-off-by: Waiman Long <longman@redhat.com>

Approved-by: Herton R. Krzesinski <herton@redhat.com>
Approved-by: Rafael Aquini <raquini@redhat.com>
Approved-by: CKI KWF Bot <cki-ci-bot+kwf-gitlab-com@redhat.com>

Merged-by: Augusto Caringi <acaringi@redhat.com>

Author: caringi

kernel/padata.c

@@ -47,6 +47,22 @@ struct padata_mt_job_state {
 static void padata_free_pd(struct parallel_data *pd);
 static void __init padata_mt_helper(struct work_struct *work);
 
+static inline void padata_get_pd(struct parallel_data *pd)
+{
+	refcount_inc(&pd->refcnt);
+}
+
+static inline void padata_put_pd_cnt(struct parallel_data *pd, int cnt)
+{
+	if (refcount_sub_and_test(cnt, &pd->refcnt))
+		padata_free_pd(pd);
+}
+
+static inline void padata_put_pd(struct parallel_data *pd)
+{
+	padata_put_pd_cnt(pd, 1);
+}
+
 static int padata_index_to_cpu(struct parallel_data *pd, int cpu_index)
 {
 	int cpu, target_cpu;
@@ -206,7 +222,7 @@ int padata_do_parallel(struct padata_shell *ps,
 	if ((pinst->flags & PADATA_RESET))
 		goto out;
 
-	refcount_inc(&pd->refcnt);
+	padata_get_pd(pd);
 	padata->pd = pd;
 	padata->cb_cpu = *cb_cpu;
 
@@ -336,8 +352,14 @@ static void padata_reorder(struct parallel_data *pd)
 	smp_mb();
 
 	reorder = per_cpu_ptr(pd->reorder_list, pd->cpu);
-	if (!list_empty(&reorder->list) && padata_find_next(pd, false))
+	if (!list_empty(&reorder->list) && padata_find_next(pd, false)) {
+		/*
+		 * Other context(eg. the padata_serial_worker) can finish the request.
+		 * To avoid UAF issue, add pd ref here, and put pd ref after reorder_work finish.
+		 */
+		padata_get_pd(pd);
 		queue_work(pinst->serial_wq, &pd->reorder_work);
+	}
 }
 
 static void invoke_padata_reorder(struct work_struct *work)
@@ -348,6 +370,8 @@ static void invoke_padata_reorder(struct work_struct *work)
 	pd = container_of(work, struct parallel_data, reorder_work);
 	padata_reorder(pd);
 	local_bh_enable();
+	/* Pairs with putting the reorder_work in the serial_wq */
+	padata_put_pd(pd);
 }
 
 static void padata_serial_worker(struct work_struct *serial_work)
@@ -380,8 +404,7 @@ static void padata_serial_worker(struct work_struct *serial_work)
 	}
 	local_bh_enable();
 
-	if (refcount_sub_and_test(cnt, &pd->refcnt))
-		padata_free_pd(pd);
+	padata_put_pd_cnt(pd, cnt);
 }
 
 /**
@@ -521,13 +544,6 @@ void __init padata_do_multithreaded(struct padata_mt_job *job)
 	ps.chunk_size = max(ps.chunk_size, 1ul);
 	ps.chunk_size = roundup(ps.chunk_size, job->align);
 
-	/*
-	 * chunk_size can be 0 if the caller sets min_chunk to 0. So force it
-	 * to at least 1 to prevent divide-by-0 panic in padata_mt_helper().`
-	 */
-	if (!ps.chunk_size)
-		ps.chunk_size = 1U;
-
 	list_for_each_entry(pw, &works, pw_list)
 		if (job->numa_aware) {
 			int old_node = atomic_read(&last_used_nid);
@@ -688,8 +704,7 @@ static int padata_replace(struct padata_instance *pinst)
 	synchronize_rcu();
 
 	list_for_each_entry_continue_reverse(ps, &pinst->pslist, list)
-		if (refcount_dec_and_test(&ps->opd->refcnt))
-			padata_free_pd(ps->opd);
+		padata_put_pd(ps->opd);
 
 	pinst->flags &= ~PADATA_RESET;
 
@@ -977,7 +992,7 @@ static ssize_t padata_sysfs_store(struct kobject *kobj, struct attribute *attr,
 
 	pinst = kobj2pinst(kobj);
 	pentry = attr2pentry(attr);
-	if (pentry->show)
+	if (pentry->store)
 		ret = pentry->store(pinst, attr, buf, count);
 
 	return ret;
@@ -1128,11 +1143,16 @@ void padata_free_shell(struct padata_shell *ps)
 	if (!ps)
 		return;
 
+	/*
+	 * Wait for all _do_serial calls to finish to avoid touching
+	 * freed pd's and ps's.
+	 */
+	synchronize_rcu();
+
 	mutex_lock(&ps->pinst->lock);
 	list_del(&ps->list);
 	pd = rcu_dereference_protected(ps->pd, 1);
-	if (refcount_dec_and_test(&pd->refcnt))
-		padata_free_pd(pd);
+	padata_put_pd(pd);
 	mutex_unlock(&ps->pinst->lock);
 
 	kfree(ps);