s390/ap: Introduce ap message buffer pool

There is a need for a do-not-allocate-memory path through the AP bus
layer. The pkey layer may be triggered via the in-kernel interface
from a protected key crypto algorithm (namely PAES) to convert a
secure key into a protected key. This happens in a workqueue context,
so sleeping is allowed but memory allocations causing IO operations
are not permitted.

To accomplish this, an AP message memory pool with pre-allocated space
is established. When ap_init_apmsg() with use_mempool set to true is
called, instead of kmalloc() the ap message buffer is allocated from
the ap_msg_pool. This pool only holds a limited amount of buffers:
ap_msg_pool_min_items with the item size AP_DEFAULT_MAX_MSG_SIZE and
exactly one of these items (if available) is returned if
ap_init_apmsg() with the use_mempool arg set to true is called. When
this pool is exhausted and use_mempool is set true, ap_init_apmsg()
returns -ENOMEM without any attempt to allocate memory and the caller
has to deal with that.

Default values for this mempool of ap messages is:
 * Each buffer is 12KB (that is the default AP bus size
   and all the urgent messages should fit into this space).
 * Minimum items held in the pool is 8. This value is adjustable
   via module parameter ap.msgpool_min_items.

The zcrypt layer may use this flag to indicate to the ap bus that the
processing path for this message should not allocate memory but should
use pre-allocated memory buffer instead.  This is to prevent deadlocks
with crypto and io for example with encrypted swap volumes.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Link: https://lore.kernel.org/r/20250424133619.16495-4-freude@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Author: hfreude

drivers/s390/crypto/ap_bus.c

@@ -41,6 +41,7 @@
 #include <linux/module.h>
 #include <asm/uv.h>
 #include <asm/chsc.h>
+#include <linux/mempool.h>
 
 #include "ap_bus.h"
 #include "ap_debug.h"
@@ -102,6 +103,27 @@ static struct ap_config_info *const ap_qci_info_old = &qci[1];
  */
 debug_info_t *ap_dbf_info;
 
+/*
+ * There is a need for a do-not-allocate-memory path through the AP bus
+ * layer. The pkey layer may be triggered via the in-kernel interface from
+ * a protected key crypto algorithm (namely PAES) to convert a secure key
+ * into a protected key. This happens in a workqueue context, so sleeping
+ * is allowed but memory allocations causing IO operations are not permitted.
+ * To accomplish this, an AP message memory pool with pre-allocated space
+ * is established. When ap_init_apmsg() with use_mempool set to true is
+ * called, instead of kmalloc() the ap message buffer is allocated from
+ * the ap_msg_pool. This pool only holds a limited amount of buffers:
+ * ap_msg_pool_min_items with the item size AP_DEFAULT_MAX_MSG_SIZE and
+ * exactly one of these items (if available) is returned if ap_init_apmsg()
+ * with the use_mempool arg set to true is called. When this pool is exhausted
+ * and use_mempool is set true, ap_init_apmsg() returns -ENOMEM without
+ * any attempt to allocate memory and the caller has to deal with that.
+ */
+static mempool_t *ap_msg_pool;
+static unsigned int ap_msg_pool_min_items = 8;
+module_param_named(msgpool_min_items, ap_msg_pool_min_items, uint, 0440);
+MODULE_PARM_DESC(msgpool_min_items, "AP message pool minimal items");
+
 /*
  * AP bus rescan related things.
  */
@@ -550,11 +572,22 @@ static void ap_poll_thread_stop(void)
 /*
  * ap_init_apmsg() - Initialize ap_message.
  */
-int ap_init_apmsg(struct ap_message *ap_msg)
+int ap_init_apmsg(struct ap_message *ap_msg, bool use_mempool)
 {
-	unsigned int maxmsgsize = atomic_read(&ap_max_msg_size);
+	unsigned int maxmsgsize;
 
 	memset(ap_msg, 0, sizeof(*ap_msg));
+
+	if (use_mempool) {
+		ap_msg->msg = mempool_alloc_preallocated(ap_msg_pool);
+		if (!ap_msg->msg)
+			return -ENOMEM;
+		ap_msg->bufsize = AP_DEFAULT_MAX_MSG_SIZE;
+		ap_msg->flags |= AP_MSG_FLAG_MEMPOOL;
+		return 0;
+	}
+
+	maxmsgsize = atomic_read(&ap_max_msg_size);
 	ap_msg->msg = kmalloc(maxmsgsize, GFP_KERNEL);
 	if (!ap_msg->msg)
 		return -ENOMEM;
@@ -569,7 +602,12 @@ EXPORT_SYMBOL(ap_init_apmsg);
  */
 void ap_release_apmsg(struct ap_message *ap_msg)
 {
-	kfree_sensitive(ap_msg->msg);
+	if (ap_msg->flags & AP_MSG_FLAG_MEMPOOL) {
+		memzero_explicit(ap_msg->msg, ap_msg->bufsize);
+		mempool_free(ap_msg->msg, ap_msg_pool);
+	} else {
+		kfree_sensitive(ap_msg->msg);
+	}
 }
 EXPORT_SYMBOL(ap_release_apmsg);
 
@@ -2457,6 +2495,14 @@ static int __init ap_module_init(void)
 	/* init ap_queue hashtable */
 	hash_init(ap_queues);
 
+	/* create ap msg buffer memory pool */
+	ap_msg_pool = mempool_create_kmalloc_pool(ap_msg_pool_min_items,
+						  AP_DEFAULT_MAX_MSG_SIZE);
+	if (!ap_msg_pool) {
+		rc = -ENOMEM;
+		goto out;
+	}
+
 	/* set up the AP permissions (ioctls, ap and aq masks) */
 	ap_perms_init();
 
@@ -2503,6 +2549,7 @@ static int __init ap_module_init(void)
 out_bus:
 	bus_unregister(&ap_bus_type);
 out:
+	mempool_destroy(ap_msg_pool);
 	ap_debug_exit();
 	return rc;
 }
@@ -2513,6 +2560,7 @@ static void __exit ap_module_exit(void)
 	ap_irq_exit();
 	root_device_unregister(ap_root_device);
 	bus_unregister(&ap_bus_type);
+	mempool_destroy(ap_msg_pool);
 	ap_debug_exit();
 }
 

drivers/s390/crypto/ap_bus.h

@@ -236,8 +236,9 @@ struct ap_message {
 #define AP_MSG_FLAG_SPECIAL  0x0001	/* flag msg as 'special' with NQAP */
 #define AP_MSG_FLAG_USAGE    0x0002	/* CCA, EP11: usage (no admin) msg */
 #define AP_MSG_FLAG_ADMIN    0x0004	/* CCA, EP11: admin (=control) msg */
+#define AP_MSG_FLAG_MEMPOOL  0x0008 /* ap msg buffer allocated via mempool */
 
-int ap_init_apmsg(struct ap_message *ap_msg);
+int ap_init_apmsg(struct ap_message *ap_msg, bool use_mempool);
 void ap_release_apmsg(struct ap_message *ap_msg);
 
 enum ap_sm_wait ap_sm_event(struct ap_queue *aq, enum ap_sm_event event);

drivers/s390/crypto/zcrypt_api.c

@@ -648,7 +648,7 @@ static long zcrypt_rsa_modexpo(struct ap_perms *perms,
 
 	trace_s390_zcrypt_req(mex, TP_ICARSAMODEXPO);
 
-	rc = ap_init_apmsg(&ap_msg);
+	rc = ap_init_apmsg(&ap_msg, false);
 	if (rc)
 		goto out;
 
@@ -753,7 +753,7 @@ static long zcrypt_rsa_crt(struct ap_perms *perms,
 
 	trace_s390_zcrypt_req(crt, TP_ICARSACRT);
 
-	rc = ap_init_apmsg(&ap_msg);
+	rc = ap_init_apmsg(&ap_msg, false);
 	if (rc)
 		goto out;
 
@@ -861,7 +861,7 @@ static long _zcrypt_send_cprb(bool userspace, struct ap_perms *perms,
 
 	xcrb->status = 0;
 
-	rc = ap_init_apmsg(&ap_msg);
+	rc = ap_init_apmsg(&ap_msg, false);
 	if (rc)
 		goto out;
 
@@ -1045,7 +1045,7 @@ static long _zcrypt_send_ep11_cprb(bool userspace, struct ap_perms *perms,
 
 	trace_s390_zcrypt_req(xcrb, TP_ZSENDEP11CPRB);
 
-	rc = ap_init_apmsg(&ap_msg);
+	rc = ap_init_apmsg(&ap_msg, false);
 	if (rc)
 		goto out;
 
@@ -1219,7 +1219,7 @@ static long zcrypt_rng(char *buffer)
 
 	trace_s390_zcrypt_req(buffer, TP_HWRNGCPRB);
 
-	rc = ap_init_apmsg(&ap_msg);
+	rc = ap_init_apmsg(&ap_msg, false);
 	if (rc)
 		goto out;
 	rc = prep_rng_ap_msg(&ap_msg, &func_code, &domain);