[PATCH 10/12] s390/ap: use generic driver_override infrastructure

Danilo Krummrich
2026-03-24T01:01:08Z

When the AP masks are updated via apmask_store() or aqmask_store(),
ap_bus_revise_bindings() is called after ap_attr_mutex has been
released.

This calls __ap_revise_reserved(), which accesses the driver_override
field without holding any lock, racing against a concurrent
driver_override_store() that may free the old string, resulting in a
potential UAF.

Fix this by using the driver-core driver_override infrastructure, which
protects all accesses with an internal spinlock.

Note that unlike most other buses, the AP bus does not check
driver_override in its match() callback; the override is checked in
ap_device_probe() and __ap_revise_reserved() instead.

Also note that we do not enable the driver_override feature of struct
bus_type, as AP - in contrast to most other buses - passes "" to
sysfs_emit() when the driver_override pointer is NULL. Thus, printing
"\n" instead of "(null)\n".

Additionally, AP has a custom counter that is modified in the
corresponding custom driver_override_store().

Fixes: d38a87d7c064 ("s390/ap: Support driver_override for AP queue devices")
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
drivers/s390/crypto/ap_bus.c | 34 +++++++++++++++++-----------------
drivers/s390/crypto/ap_bus.h | 1 -
drivers/s390/crypto/ap_queue.c | 24 ++++++------------------
3 files changed, 23 insertions(+), 36 deletions(-)

diff --git a/drivers/s390/crypto/ap_bus.c b/drivers/s390/crypto/ap_bus.c
index d652df96a507..f24e27add721 100644
--- a/drivers/s390/crypto/ap_bus.c
+++ b/drivers/s390/crypto/ap_bus.c
@@ -859,25 +859,24 @@ static int __ap_queue_devices_with_id_unregister(struct device *dev, void *data)

static int __ap_revise_reserved(struct device *dev, void *dummy)
{
- int rc, card, queue, devres, drvres;
+ int rc, card, queue, devres, drvres, ovrd;

if (is_queue_dev(dev)) {
struct ap_driver *ap_drv = to_ap_drv(dev->driver);
struct ap_queue *aq = to_ap_queue(dev);
- struct ap_device *ap_dev = &aq->ap_dev;

card = AP_QID_CARD(aq->qid);
queue = AP_QID_QUEUE(aq->qid);

- if (ap_dev->driver_override) {
- if (strcmp(ap_dev->driver_override,
- ap_drv->driver.name)) {
- pr_debug("reprobing queue=%02x.%04x\n", card, queue);
- rc = device_reprobe(dev);
- if (rc) {
- AP_DBF_WARN("%s reprobing queue=%02x.%04x failed\n",
- __func__, card, queue);
- }
+ ovrd = device_match_driver_override(dev, &ap_drv->driver);
+ if (ovrd > 0) {
+ /* override set and matches, nothing to do */
+ } else if (ovrd == 0) {
+ pr_debug("reprobing queue=%02x.%04x\n", card, queue);
+ rc = device_reprobe(dev);
+ if (rc) {
+ AP_DBF_WARN("%s reprobing queue=%02x.%04x failed\n",
+ __func__, card, queue);
}
} else {
mutex_lock(&ap_attr_mutex);
@@ -928,7 +927,7 @@ int ap_owned_by_def_drv(int card, int queue)
if (aq) {
const struct device_driver *drv = aq->ap_dev.device.driver;
const struct ap_driver *ap_drv = to_ap_drv(drv);
- bool override = !!aq->ap_dev.driver_override;
+ bool override = device_has_driver_override(&aq->ap_dev.device);

if (override && drv && ap_drv->flags & AP_DRIVER_FLAG_DEFAULT)
rc = 1;
@@ -977,7 +976,7 @@ static int ap_device_probe(struct device *dev)
{
struct ap_device *ap_dev = to_ap_dev(dev);
struct ap_driver *ap_drv = to_ap_drv(dev->driver);
- int card, queue, devres, drvres, rc = -ENODEV;
+ int card, queue, devres, drvres, rc = -ENODEV, ovrd;

if (!get_device(dev))
return rc;
@@ -991,10 +990,11 @@ static int ap_device_probe(struct device *dev)
*/
card = AP_QID_CARD(to_ap_queue(dev)->qid);
queue = AP_QID_QUEUE(to_ap_queue(dev)->qid);
- if (ap_dev->driver_override) {
- if (strcmp(ap_dev->driver_override,
- ap_drv->driver.name))
- goto out;
+ ovrd = device_match_driver_override(dev, &ap_drv->driver);
+ if (ovrd > 0) {
+ /* override set and matches, nothing to do */
+ } else if (ovrd == 0) {
+ goto out;
} else {
mutex_lock(&ap_attr_mutex);
devres = test_bit_inv(card, ap_perms.apm) &&
diff --git a/drivers/s390/crypto/ap_bus.h b/drivers/s390/crypto/ap_bus.h
index 51e08f27bd75..04ea256ecf91 100644
--- a/drivers/s390/crypto/ap_bus.h
+++ b/drivers/s390/crypto/ap_bus.h
@@ -166,7 +166,6 @@ void ap_driver_unregister(struct ap_driver *);
struct ap_device {
struct device device;
int device_type; /* AP device type. */
- const char *driver_override;
};

#define to_ap_dev(x) container_of((x), struct ap_device, device)
diff --git a/drivers/s390/crypto/ap_queue.c b/drivers/s390/crypto/ap_queue.c
index 3fe2e41c5c6b..ca9819e6f7e7 100644
--- a/drivers/s390/crypto/ap_queue.c
+++ b/drivers/s390/crypto/ap_queue.c
@@ -734,26 +734,14 @@ static ssize_t driver_override_show(struct device *dev,
struct device_attribute *attr,
char *buf)
{
- struct ap_queue *aq = to_ap_queue(dev);
- struct ap_device *ap_dev = &aq->ap_dev;
- int rc;
-
- device_lock(dev);
- if (ap_dev->driver_override)
- rc = sysfs_emit(buf, "%s\n", ap_dev->driver_override);
- else
- rc = sysfs_emit(buf, "\n");
- device_unlock(dev);
-
- return rc;
+ guard(spinlock)(&dev->driver_override.lock);
+ return sysfs_emit(buf, "%s\n", dev->driver_override.name ?: "");
}

static ssize_t driver_override_store(struct device *dev,
struct device_attribute *attr,
const char *buf, size_t count)
{
- struct ap_queue *aq = to_ap_queue(dev);
- struct ap_device *ap_dev = &aq->ap_dev;
int rc = -EINVAL;
bool old_value;

@@ -764,13 +752,13 @@ static ssize_t driver_override_store(struct device *dev,
if (ap_apmask_aqmask_in_use)
goto out;

- old_value = ap_dev->driver_override ? true : false;
- rc = driver_set_override(dev, &ap_dev->driver_override, buf, count);
+ old_value = device_has_driver_override(dev);
+ rc = __device_set_driver_override(dev, buf, count);
if (rc)
goto out;
- if (old_value && !ap_dev->driver_override)
+ if (old_value && !device_has_driver_override(dev))
--ap_driver_override_ctr;
- else if (!old_value && ap_dev->driver_override)
+ else if (!old_value && device_has_driver_override(dev))
++ap_driver_override_ctr;

rc = count;
--
2.53.0