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

From: Harald Freudenberger <[email-masked]>
Date: Tue, 24 Mar 2026 13:41:25 +0100

On 2026-03-24 01:59, Danilo Krummrich wrote:
> 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 <[email-masked]>
> ---
>  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;

Thanks Danilo
Reviewed-by: Harald Freudenberger <[email-masked]>