[PATCH 5.4 141/309] crypto: pcrypt - Avoid deadlock by using per-instance padata queues

[Greg Kroah-Hartman]

From: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

commit bbefa1dd6a6d53537c11624752219e39959d04fb upstream.

If the pcrypt template is used multiple times in an algorithm, then a
deadlock occurs because all pcrypt instances share the same
padata_instance, which completes requests in the order submitted.  That
is, the inner pcrypt request waits for the outer pcrypt request while
the outer request is already waiting for the inner.

This patch fixes this by allocating a set of queues for each pcrypt
instance instead of using two global queues.  In order to maintain
the existing user-space interface, the pinst structure remains global
so any sysfs modifications will apply to every pcrypt instance.

Note that when an update occurs we have to allocate memory for
every pcrypt instance.  Should one of the allocations fail we
will abort the update without rolling back changes already made.

The new per-instance data structure is called padata_shell and is
essentially a wrapper around parallel_data.

Reproducer:

	#include <linux/if_alg.h>
	#include <sys/socket.h>
	#include <unistd.h>

	int main()
	{
		struct sockaddr_alg addr = {
			.salg_type = "aead",
			.salg_name = "pcrypt(pcrypt(rfc4106-gcm-aesni))"
		};
		int algfd, reqfd;
		char buf[32] = { 0 };

		algfd = socket(AF_ALG, SOCK_SEQPACKET, 0);
		bind(algfd, (void *)&addr, sizeof(addr));
		setsockopt(algfd, SOL_ALG, ALG_SET_KEY, buf, 20);
		reqfd = accept(algfd, 0, 0);
		write(reqfd, buf, 32);
		read(reqfd, buf, 16);
	}

Reported-by: syzbot+56c7151cad94eec37c521f0e47d2eee53f9361c4@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 5068c7a883d1 ("crypto: pcrypt - Add pcrypt crypto parallelization wrapper")
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Tested-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 crypto/pcrypt.c        |   36 ++++++-
 include/linux/padata.h |   34 ++++++-
 kernel/padata.c        |  236 ++++++++++++++++++++++++++++++++++---------------
 3 files changed, 227 insertions(+), 79 deletions(-)

--- a/crypto/pcrypt.c
+++ b/crypto/pcrypt.c
@@ -24,6 +24,8 @@ static struct kset           *pcrypt_kse
 
 struct pcrypt_instance_ctx {
 	struct crypto_aead_spawn spawn;
+	struct padata_shell *psenc;
+	struct padata_shell *psdec;
 	atomic_t tfm_count;
 };
 
@@ -32,6 +34,12 @@ struct pcrypt_aead_ctx {
 	unsigned int cb_cpu;
 };
 
+static inline struct pcrypt_instance_ctx *pcrypt_tfm_ictx(
+	struct crypto_aead *tfm)
+{
+	return aead_instance_ctx(aead_alg_instance(tfm));
+}
+
 static int pcrypt_aead_setkey(struct crypto_aead *parent,
 			      const u8 *key, unsigned int keylen)
 {
@@ -90,6 +98,9 @@ static int pcrypt_aead_encrypt(struct ae
 	struct crypto_aead *aead = crypto_aead_reqtfm(req);
 	struct pcrypt_aead_ctx *ctx = crypto_aead_ctx(aead);
 	u32 flags = aead_request_flags(req);
+	struct pcrypt_instance_ctx *ictx;
+
+	ictx = pcrypt_tfm_ictx(aead);
 
 	memset(padata, 0, sizeof(struct padata_priv));
 
@@ -103,7 +114,7 @@ static int pcrypt_aead_encrypt(struct ae
 			       req->cryptlen, req->iv);
 	aead_request_set_ad(creq, req->assoclen);
 
-	err = padata_do_parallel(pencrypt, padata, &ctx->cb_cpu);
+	err = padata_do_parallel(ictx->psenc, padata, &ctx->cb_cpu);
 	if (!err)
 		return -EINPROGRESS;
 
@@ -132,6 +143,9 @@ static int pcrypt_aead_decrypt(struct ae
 	struct crypto_aead *aead = crypto_aead_reqtfm(req);
 	struct pcrypt_aead_ctx *ctx = crypto_aead_ctx(aead);
 	u32 flags = aead_request_flags(req);
+	struct pcrypt_instance_ctx *ictx;
+
+	ictx = pcrypt_tfm_ictx(aead);
 
 	memset(padata, 0, sizeof(struct padata_priv));
 
@@ -145,7 +159,7 @@ static int pcrypt_aead_decrypt(struct ae
 			       req->cryptlen, req->iv);
 	aead_request_set_ad(creq, req->assoclen);
 
-	err = padata_do_parallel(pdecrypt, padata, &ctx->cb_cpu);
+	err = padata_do_parallel(ictx->psdec, padata, &ctx->cb_cpu);
 	if (!err)
 		return -EINPROGRESS;
 
@@ -192,6 +206,8 @@ static void pcrypt_free(struct aead_inst
 	struct pcrypt_instance_ctx *ctx = aead_instance_ctx(inst);
 
 	crypto_drop_aead(&ctx->spawn);
+	padata_free_shell(ctx->psdec);
+	padata_free_shell(ctx->psenc);
 	kfree(inst);
 }
 
@@ -233,12 +249,22 @@ static int pcrypt_create_aead(struct cry
 	if (!inst)
 		return -ENOMEM;
 
+	err = -ENOMEM;
+
 	ctx = aead_instance_ctx(inst);
+	ctx->psenc = padata_alloc_shell(pencrypt);
+	if (!ctx->psenc)
+		goto out_free_inst;
+
+	ctx->psdec = padata_alloc_shell(pdecrypt);
+	if (!ctx->psdec)
+		goto out_free_psenc;
+
 	crypto_set_aead_spawn(&ctx->spawn, aead_crypto_instance(inst));
 
 	err = crypto_grab_aead(&ctx->spawn, name, 0, 0);
 	if (err)
-		goto out_free_inst;
+		goto out_free_psdec;
 
 	alg = crypto_spawn_aead_alg(&ctx->spawn);
 	err = pcrypt_init_instance(aead_crypto_instance(inst), &alg->base);
@@ -271,6 +297,10 @@ out:
 
 out_drop_aead:
 	crypto_drop_aead(&ctx->spawn);
+out_free_psdec:
+	padata_free_shell(ctx->psdec);
+out_free_psenc:
+	padata_free_shell(ctx->psenc);
 out_free_inst:
 	kfree(inst);
 	goto out;
--- a/include/linux/padata.h
+++ b/include/linux/padata.h
@@ -9,6 +9,7 @@
 #ifndef PADATA_H
 #define PADATA_H
 
+#include <linux/compiler_types.h>
 #include <linux/workqueue.h>
 #include <linux/spinlock.h>
 #include <linux/list.h>
@@ -98,7 +99,7 @@ struct padata_cpumask {
  * struct parallel_data - Internal control structure, covers everything
  * that depends on the cpumask in use.
  *
- * @pinst: padata instance.
+ * @sh: padata_shell object.
  * @pqueue: percpu padata queues used for parallelization.
  * @squeue: percpu padata queues used for serialuzation.
  * @reorder_objects: Number of objects waiting in the reorder queues.
@@ -111,7 +112,7 @@ struct padata_cpumask {
  * @lock: Reorder lock.
  */
 struct parallel_data {
-	struct padata_instance		*pinst;
+	struct padata_shell		*ps;
 	struct padata_parallel_queue	__percpu *pqueue;
 	struct padata_serial_queue	__percpu *squeue;
 	atomic_t			reorder_objects;
@@ -125,13 +126,32 @@ struct parallel_data {
 };
 
 /**
+ * struct padata_shell - Wrapper around struct parallel_data, its
+ * purpose is to allow the underlying control structure to be replaced
+ * on the fly using RCU.
+ *
+ * @pinst: padat instance.
+ * @pd: Actual parallel_data structure which may be substituted on the fly.
+ * @opd: Pointer to old pd to be freed by padata_replace.
+ * @list: List entry in padata_instance list.
+ */
+struct padata_shell {
+	struct padata_instance		*pinst;
+	struct parallel_data __rcu	*pd;
+	struct parallel_data		*opd;
+	struct list_head		list;
+};
+
+/**
  * struct padata_instance - The overall control structure.
  *
  * @cpu_notifier: cpu hotplug notifier.
  * @parallel_wq: The workqueue used for parallel work.
  * @serial_wq: The workqueue used for serial work.
- * @pd: The internal control structure.
+ * @pslist: List of padata_shell objects attached to this instance.
  * @cpumask: User supplied cpumasks for parallel and serial works.
+ * @rcpumask: Actual cpumasks based on user cpumask and cpu_online_mask.
+ * @omask: Temporary storage used to compute the notification mask.
  * @cpumask_change_notifier: Notifiers chain for user-defined notify
  *            callbacks that will be called when either @pcpu or @cbcpu
  *            or both cpumasks change.
@@ -143,8 +163,10 @@ struct padata_instance {
 	struct hlist_node		 node;
 	struct workqueue_struct		*parallel_wq;
 	struct workqueue_struct		*serial_wq;
-	struct parallel_data		*pd;
+	struct list_head		pslist;
 	struct padata_cpumask		cpumask;
+	struct padata_cpumask		rcpumask;
+	cpumask_var_t			omask;
 	struct blocking_notifier_head	 cpumask_change_notifier;
 	struct kobject                   kobj;
 	struct mutex			 lock;
@@ -156,7 +178,9 @@ struct padata_instance {
 
 extern struct padata_instance *padata_alloc_possible(const char *name);
 extern void padata_free(struct padata_instance *pinst);
-extern int padata_do_parallel(struct padata_instance *pinst,
+extern struct padata_shell *padata_alloc_shell(struct padata_instance *pinst);
+extern void padata_free_shell(struct padata_shell *ps);
+extern int padata_do_parallel(struct padata_shell *ps,
 			      struct padata_priv *padata, int *cb_cpu);
 extern void padata_do_serial(struct padata_priv *padata);
 extern int padata_set_cpumask(struct padata_instance *pinst, int cpumask_type,
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -89,7 +89,7 @@ static void padata_parallel_worker(struc
 /**
  * padata_do_parallel - padata parallelization function
  *
- * @pinst: padata instance
+ * @ps: padatashell
  * @padata: object to be parallelized
  * @cb_cpu: pointer to the CPU that the serialization callback function should
  *          run on.  If it's not in the serial cpumask of @pinst
@@ -100,16 +100,17 @@ static void padata_parallel_worker(struc
  * Note: Every object which is parallelized by padata_do_parallel
  * must be seen by padata_do_serial.
  */
-int padata_do_parallel(struct padata_instance *pinst,
+int padata_do_parallel(struct padata_shell *ps,
 		       struct padata_priv *padata, int *cb_cpu)
 {
+	struct padata_instance *pinst = ps->pinst;
 	int i, cpu, cpu_index, target_cpu, err;
 	struct padata_parallel_queue *queue;
 	struct parallel_data *pd;
 
 	rcu_read_lock_bh();
 
-	pd = rcu_dereference_bh(pinst->pd);
+	pd = rcu_dereference_bh(ps->pd);
 
 	err = -EINVAL;
 	if (!(pinst->flags & PADATA_INIT) || pinst->flags & PADATA_INVALID)
@@ -212,10 +213,10 @@ static struct padata_priv *padata_find_n
 
 static void padata_reorder(struct parallel_data *pd)
 {
+	struct padata_instance *pinst = pd->ps->pinst;
 	int cb_cpu;
 	struct padata_priv *padata;
 	struct padata_serial_queue *squeue;
-	struct padata_instance *pinst = pd->pinst;
 	struct padata_parallel_queue *next_queue;
 
 	/*
@@ -349,36 +350,39 @@ void padata_do_serial(struct padata_priv
 }
 EXPORT_SYMBOL(padata_do_serial);
 
-static int padata_setup_cpumasks(struct parallel_data *pd,
-				 const struct cpumask *pcpumask,
-				 const struct cpumask *cbcpumask)
+static int padata_setup_cpumasks(struct padata_instance *pinst)
 {
 	struct workqueue_attrs *attrs;
+	int err;
+
+	attrs = alloc_workqueue_attrs();
+	if (!attrs)
+		return -ENOMEM;
+
+	/* Restrict parallel_wq workers to pd->cpumask.pcpu. */
+	cpumask_copy(attrs->cpumask, pinst->cpumask.pcpu);
+	err = apply_workqueue_attrs(pinst->parallel_wq, attrs);
+	free_workqueue_attrs(attrs);
+
+	return err;
+}
+
+static int pd_setup_cpumasks(struct parallel_data *pd,
+			     const struct cpumask *pcpumask,
+			     const struct cpumask *cbcpumask)
+{
 	int err = -ENOMEM;
 
 	if (!alloc_cpumask_var(&pd->cpumask.pcpu, GFP_KERNEL))
 		goto out;
-	cpumask_and(pd->cpumask.pcpu, pcpumask, cpu_online_mask);
-
 	if (!alloc_cpumask_var(&pd->cpumask.cbcpu, GFP_KERNEL))
 		goto free_pcpu_mask;
-	cpumask_and(pd->cpumask.cbcpu, cbcpumask, cpu_online_mask);
-
-	attrs = alloc_workqueue_attrs();
-	if (!attrs)
-		goto free_cbcpu_mask;
 
-	/* Restrict parallel_wq workers to pd->cpumask.pcpu. */
-	cpumask_copy(attrs->cpumask, pd->cpumask.pcpu);
-	err = apply_workqueue_attrs(pd->pinst->parallel_wq, attrs);
-	free_workqueue_attrs(attrs);
-	if (err < 0)
-		goto free_cbcpu_mask;
+	cpumask_copy(pd->cpumask.pcpu, pcpumask);
+	cpumask_copy(pd->cpumask.cbcpu, cbcpumask);
 
 	return 0;
 
-free_cbcpu_mask:
-	free_cpumask_var(pd->cpumask.cbcpu);
 free_pcpu_mask:
 	free_cpumask_var(pd->cpumask.pcpu);
 out:
@@ -422,12 +426,16 @@ static void padata_init_pqueues(struct p
 }
 
 /* Allocate and initialize the internal cpumask dependend resources. */
-static struct parallel_data *padata_alloc_pd(struct padata_instance *pinst,
-					     const struct cpumask *pcpumask,
-					     const struct cpumask *cbcpumask)
+static struct parallel_data *padata_alloc_pd(struct padata_shell *ps)
 {
+	struct padata_instance *pinst = ps->pinst;
+	const struct cpumask *cbcpumask;
+	const struct cpumask *pcpumask;
 	struct parallel_data *pd;
 
+	cbcpumask = pinst->rcpumask.cbcpu;
+	pcpumask = pinst->rcpumask.pcpu;
+
 	pd = kzalloc(sizeof(struct parallel_data), GFP_KERNEL);
 	if (!pd)
 		goto err;
@@ -440,8 +448,8 @@ static struct parallel_data *padata_allo
 	if (!pd->squeue)
 		goto err_free_pqueue;
 
-	pd->pinst = pinst;
-	if (padata_setup_cpumasks(pd, pcpumask, cbcpumask) < 0)
+	pd->ps = ps;
+	if (pd_setup_cpumasks(pd, pcpumask, cbcpumask))
 		goto err_free_squeue;
 
 	padata_init_pqueues(pd);
@@ -490,32 +498,64 @@ static void __padata_stop(struct padata_
 }
 
 /* Replace the internal control structure with a new one. */
-static void padata_replace(struct padata_instance *pinst,
-			   struct parallel_data *pd_new)
+static int padata_replace_one(struct padata_shell *ps)
 {
-	struct parallel_data *pd_old = pinst->pd;
-	int notification_mask = 0;
+	struct parallel_data *pd_new;
 
-	pinst->flags |= PADATA_RESET;
+	pd_new = padata_alloc_pd(ps);
+	if (!pd_new)
+		return -ENOMEM;
 
-	rcu_assign_pointer(pinst->pd, pd_new);
+	ps->opd = rcu_dereference_protected(ps->pd, 1);
+	rcu_assign_pointer(ps->pd, pd_new);
 
-	synchronize_rcu();
+	return 0;
+}
+
+static int padata_replace(struct padata_instance *pinst, int cpu)
+{
+	int notification_mask = 0;
+	struct padata_shell *ps;
+	int err;
+
+	pinst->flags |= PADATA_RESET;
 
-	if (!cpumask_equal(pd_old->cpumask.pcpu, pd_new->cpumask.pcpu))
+	cpumask_copy(pinst->omask, pinst->rcpumask.pcpu);
+	cpumask_and(pinst->rcpumask.pcpu, pinst->cpumask.pcpu,
+		    cpu_online_mask);
+	if (cpu >= 0)
+		cpumask_clear_cpu(cpu, pinst->rcpumask.pcpu);
+	if (!cpumask_equal(pinst->omask, pinst->rcpumask.pcpu))
 		notification_mask |= PADATA_CPU_PARALLEL;
-	if (!cpumask_equal(pd_old->cpumask.cbcpu, pd_new->cpumask.cbcpu))
+
+	cpumask_copy(pinst->omask, pinst->rcpumask.cbcpu);
+	cpumask_and(pinst->rcpumask.cbcpu, pinst->cpumask.cbcpu,
+		    cpu_online_mask);
+	if (cpu >= 0)
+		cpumask_clear_cpu(cpu, pinst->rcpumask.cbcpu);
+	if (!cpumask_equal(pinst->omask, pinst->rcpumask.cbcpu))
 		notification_mask |= PADATA_CPU_SERIAL;
 
-	if (atomic_dec_and_test(&pd_old->refcnt))
-		padata_free_pd(pd_old);
+	list_for_each_entry(ps, &pinst->pslist, list) {
+		err = padata_replace_one(ps);
+		if (err)
+			break;
+	}
+
+	synchronize_rcu();
+
+	list_for_each_entry_continue_reverse(ps, &pinst->pslist, list)
+		if (atomic_dec_and_test(&ps->opd->refcnt))
+			padata_free_pd(ps->opd);
 
 	if (notification_mask)
 		blocking_notifier_call_chain(&pinst->cpumask_change_notifier,
 					     notification_mask,
-					     &pd_new->cpumask);
+					     &pinst->cpumask);
 
 	pinst->flags &= ~PADATA_RESET;
+
+	return err;
 }
 
 /**
@@ -568,7 +608,7 @@ static int __padata_set_cpumasks(struct
 				 cpumask_var_t cbcpumask)
 {
 	int valid;
-	struct parallel_data *pd;
+	int err;
 
 	valid = padata_validate_cpumask(pinst, pcpumask);
 	if (!valid) {
@@ -581,19 +621,15 @@ static int __padata_set_cpumasks(struct
 		__padata_stop(pinst);
 
 out_replace:
-	pd = padata_alloc_pd(pinst, pcpumask, cbcpumask);
-	if (!pd)
-		return -ENOMEM;
-
 	cpumask_copy(pinst->cpumask.pcpu, pcpumask);
 	cpumask_copy(pinst->cpumask.cbcpu, cbcpumask);
 
-	padata_replace(pinst, pd);
+	err = padata_setup_cpumasks(pinst) ?: padata_replace(pinst, -1);
 
 	if (valid)
 		__padata_start(pinst);
 
-	return 0;
+	return err;
 }
 
 /**
@@ -676,46 +712,32 @@ EXPORT_SYMBOL(padata_stop);
 
 static int __padata_add_cpu(struct padata_instance *pinst, int cpu)
 {
-	struct parallel_data *pd;
+	int err = 0;
 
 	if (cpumask_test_cpu(cpu, cpu_online_mask)) {
-		pd = padata_alloc_pd(pinst, pinst->cpumask.pcpu,
-				     pinst->cpumask.cbcpu);
-		if (!pd)
-			return -ENOMEM;
-
-		padata_replace(pinst, pd);
+		err = padata_replace(pinst, -1);
 
 		if (padata_validate_cpumask(pinst, pinst->cpumask.pcpu) &&
 		    padata_validate_cpumask(pinst, pinst->cpumask.cbcpu))
 			__padata_start(pinst);
 	}
 
-	return 0;
+	return err;
 }
 
 static int __padata_remove_cpu(struct padata_instance *pinst, int cpu)
 {
-	struct parallel_data *pd = NULL;
+	int err = 0;
 
 	if (cpumask_test_cpu(cpu, cpu_online_mask)) {
-
 		if (!padata_validate_cpumask(pinst, pinst->cpumask.pcpu) ||
 		    !padata_validate_cpumask(pinst, pinst->cpumask.cbcpu))
 			__padata_stop(pinst);
 
-		pd = padata_alloc_pd(pinst, pinst->cpumask.pcpu,
-				     pinst->cpumask.cbcpu);
-		if (!pd)
-			return -ENOMEM;
-
-		padata_replace(pinst, pd);
-
-		cpumask_clear_cpu(cpu, pd->cpumask.cbcpu);
-		cpumask_clear_cpu(cpu, pd->cpumask.pcpu);
+		err = padata_replace(pinst, cpu);
 	}
 
-	return 0;
+	return err;
 }
 
  /**
@@ -798,8 +820,12 @@ static void __padata_free(struct padata_
 	cpuhp_state_remove_instance_nocalls(hp_online, &pinst->node);
 #endif
 
+	WARN_ON(!list_empty(&pinst->pslist));
+
 	padata_stop(pinst);
-	padata_free_pd(pinst->pd);
+	free_cpumask_var(pinst->omask);
+	free_cpumask_var(pinst->rcpumask.cbcpu);
+	free_cpumask_var(pinst->rcpumask.pcpu);
 	free_cpumask_var(pinst->cpumask.pcpu);
 	free_cpumask_var(pinst->cpumask.cbcpu);
 	destroy_workqueue(pinst->serial_wq);
@@ -946,7 +972,6 @@ static struct padata_instance *padata_al
 					    const struct cpumask *cbcpumask)
 {
 	struct padata_instance *pinst;
-	struct parallel_data *pd = NULL;
 
 	pinst = kzalloc(sizeof(struct padata_instance), GFP_KERNEL);
 	if (!pinst)
@@ -974,14 +999,22 @@ static struct padata_instance *padata_al
 	    !padata_validate_cpumask(pinst, cbcpumask))
 		goto err_free_masks;
 
-	pd = padata_alloc_pd(pinst, pcpumask, cbcpumask);
-	if (!pd)
+	if (!alloc_cpumask_var(&pinst->rcpumask.pcpu, GFP_KERNEL))
 		goto err_free_masks;
+	if (!alloc_cpumask_var(&pinst->rcpumask.cbcpu, GFP_KERNEL))
+		goto err_free_rcpumask_pcpu;
+	if (!alloc_cpumask_var(&pinst->omask, GFP_KERNEL))
+		goto err_free_rcpumask_cbcpu;
 
-	rcu_assign_pointer(pinst->pd, pd);
+	INIT_LIST_HEAD(&pinst->pslist);
 
 	cpumask_copy(pinst->cpumask.pcpu, pcpumask);
 	cpumask_copy(pinst->cpumask.cbcpu, cbcpumask);
+	cpumask_and(pinst->rcpumask.pcpu, pcpumask, cpu_online_mask);
+	cpumask_and(pinst->rcpumask.cbcpu, cbcpumask, cpu_online_mask);
+
+	if (padata_setup_cpumasks(pinst))
+		goto err_free_omask;
 
 	pinst->flags = 0;
 
@@ -997,6 +1030,12 @@ static struct padata_instance *padata_al
 
 	return pinst;
 
+err_free_omask:
+	free_cpumask_var(pinst->omask);
+err_free_rcpumask_cbcpu:
+	free_cpumask_var(pinst->rcpumask.cbcpu);
+err_free_rcpumask_pcpu:
+	free_cpumask_var(pinst->rcpumask.pcpu);
 err_free_masks:
 	free_cpumask_var(pinst->cpumask.pcpu);
 	free_cpumask_var(pinst->cpumask.cbcpu);
@@ -1035,6 +1074,61 @@ void padata_free(struct padata_instance
 }
 EXPORT_SYMBOL(padata_free);
 
+/**
+ * padata_alloc_shell - Allocate and initialize padata shell.
+ *
+ * @pinst: Parent padata_instance object.
+ */
+struct padata_shell *padata_alloc_shell(struct padata_instance *pinst)
+{
+	struct parallel_data *pd;
+	struct padata_shell *ps;
+
+	ps = kzalloc(sizeof(*ps), GFP_KERNEL);
+	if (!ps)
+		goto out;
+
+	ps->pinst = pinst;
+
+	get_online_cpus();
+	pd = padata_alloc_pd(ps);
+	put_online_cpus();
+
+	if (!pd)
+		goto out_free_ps;
+
+	mutex_lock(&pinst->lock);
+	RCU_INIT_POINTER(ps->pd, pd);
+	list_add(&ps->list, &pinst->pslist);
+	mutex_unlock(&pinst->lock);
+
+	return ps;
+
+out_free_ps:
+	kfree(ps);
+out:
+	return NULL;
+}
+EXPORT_SYMBOL(padata_alloc_shell);
+
+/**
+ * padata_free_shell - free a padata shell
+ *
+ * @ps: padata shell to free
+ */
+void padata_free_shell(struct padata_shell *ps)
+{
+	struct padata_instance *pinst = ps->pinst;
+
+	mutex_lock(&pinst->lock);
+	list_del(&ps->list);
+	padata_free_pd(rcu_dereference_protected(ps->pd, 1));
+	mutex_unlock(&pinst->lock);
+
+	kfree(ps);
+}
+EXPORT_SYMBOL(padata_free_shell);
+
 #ifdef CONFIG_HOTPLUG_CPU
 
 static __init int padata_driver_init(void)