[PATCH 10/17] driver core: Expose device trust authorization through sysfs

[Leon Romanovsky]

From: Leon Romanovsky <leonro@xxxxxxxxxx>

An initrd must authorize a disabled device after attestation even if a
built-in driver already attempted to probe it.

Expose requested trust_policy and read-only active trust. Allow explicit
disabled policy to advance once to adversary or full, fixing that decision
immediately. Reject later policy changes, including for a device claimed
before registration, while allowing same-value writes on live devices.
Keep initial unrestricted selection separate from explicit authorization
and bind-time commitment. Serialize writes with probing under the device
lock. Include the attributes in device ownership changes.

Signed-off-by: Leon Romanovsky <leonro@xxxxxxxxxx>
---
 Documentation/ABI/testing/sysfs-devices-trust |  51 +++++++++++++
 MAINTAINERS                                   |   1 +
 drivers/base/base.h                           |   6 ++
 drivers/base/core.c                           |  15 +++-
 drivers/base/trust.c                          | 102 ++++++++++++++++++++++++++
 5 files changed, 174 insertions(+), 1 deletion(-)

diff --git a/Documentation/ABI/testing/sysfs-devices-trust b/Documentation/ABI/testing/sysfs-devices-trust
new file mode 100644
index 0000000000000..97e07a174769c
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-devices-trust
@@ -0,0 +1,51 @@
+What:		/sys/devices/.../trust
+Date:		September 2026
+Contact:	driver-core@xxxxxxxxxxxxxxx
+Description:
+		(RO) The active trust level for the device. The values are
+		"disabled", "adversary", and "full". Driver core resolves
+		the level before DMA configuration and probe, and retains it
+		through removal and DMA cleanup. Outside a bind transaction
+		the level is "disabled". This does not imply that policy is
+		writable or that the device has never operated.
+
+What:		/sys/devices/.../trust_policy
+Date:		September 2026
+Contact:	driver-core@xxxxxxxxxxxxxxx
+Description:
+		(RW) The requested trust policy for the device. The initial
+		value may be selected by the bus before the device is
+		registered; otherwise it is "default". The policy is resolved
+		to an active trust level before the device is probed.
+
+		The values are "default", "disabled", "adversary", and
+		"full". "default" lets the bus select the active trust level.
+		"disabled" prevents a functional driver from binding.
+		"adversary" requests operation with the restrictions that the
+		bus and DMA layers can enforce for a potentially malicious
+		device; it does not isolate DMA by itself. "full" requests
+		normal operation.
+
+		Before an operating level is committed, policy may change
+		freely until "disabled" is selected. Explicit "disabled"
+		blocks binding while allowing a later write of "adversary" or
+		"full". Repeated rejected probes preserve this opportunity.
+		Authorization immediately fixes the requested policy.
+		Returning to "default" or "disabled", or selecting another
+		mode, fails.
+
+		Successful resolution to an allowed level commits policy
+		and operating level before driver or DMA setup. Both remain
+		fixed for the device object's lifetime, including through
+		failed or deferred probes, unbind, reset, and driver reload.
+		Later binds must resolve to the same level, including when
+		the policy is "default". A resolver error or disabled result
+		before commitment leaves the policy's writability unchanged.
+
+		For a live device, writing the existing value succeeds
+		without changing state. Other forbidden writes fail with
+		EBUSY; invalid strings fail with EINVAL. Removed devices
+		reject writes with ENODEV. Writes do not trigger probing or
+		attestation.
+		Queued probes may proceed immediately after authorization,
+		so userspace must complete verification before granting it.
diff --git a/MAINTAINERS b/MAINTAINERS
index b726344362b41..a145aaaca2ef3 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8150,6 +8150,7 @@ L:	driver-core@xxxxxxxxxxxxxxx
 S:	Supported
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/driver-core/driver-core.git
 F:	Documentation/ABI/testing/sysfs-bus-drivers-probe
+F:	Documentation/ABI/testing/sysfs-devices-trust
 F:	Documentation/core-api/kobject.rst
 F:	Documentation/driver-api/driver-model/
 F:	drivers/base/
diff --git a/drivers/base/base.h b/drivers/base/base.h
index 4683b9c2a8d83..c88b3302969f2 100644
--- a/drivers/base/base.h
+++ b/drivers/base/base.h
@@ -180,6 +180,12 @@ void bus_remove_driver(struct device_driver *drv);
 void device_release_driver_internal(struct device *dev, const struct device_driver *drv,
 				    struct device *parent);
 
+extern const struct attribute_group device_trust_attr_group;
+
+#if IS_ENABLED(CONFIG_KUNIT)
+int device_trust_set_policy(struct device *dev, enum device_trust_policy policy);
+#endif
+
 /* All active trust level transitions are serialized by the device lock. */
 void device_trust_init_state(struct device *dev);
 int device_trust_prepare(struct device *dev, const struct device_driver *drv);
diff --git a/drivers/base/core.c b/drivers/base/core.c
index a0ffe74993677..dca0179714c82 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -2989,10 +2989,14 @@ static int device_add_attrs(struct device *dev)
 	if (error)
 		goto err_remove_type_groups;
 
+	error = device_add_group(dev, &device_trust_attr_group);
+	if (error)
+		goto err_remove_dev_groups;
+
 	if (device_supports_offline(dev) && !dev_offline_disabled(dev)) {
 		error = device_create_file(dev, &dev_attr_online);
 		if (error)
-			goto err_remove_dev_groups;
+			goto err_remove_dev_trust_group;
 	}
 
 	if (fw_devlink_flags && !fw_devlink_is_permissive() && dev->fwnode) {
@@ -3022,6 +3026,8 @@ static int device_add_attrs(struct device *dev)
 	device_remove_file(dev, &dev_attr_waiting_for_supplier);
  err_remove_dev_online:
 	device_remove_file(dev, &dev_attr_online);
+ err_remove_dev_trust_group:
+	device_remove_group(dev, &device_trust_attr_group);
  err_remove_dev_groups:
 	device_remove_groups(dev, dev->groups);
  err_remove_type_groups:
@@ -3047,6 +3053,7 @@ static void device_remove_attrs(struct device *dev)
 	device_remove_file(dev, &dev_attr_removable);
 	device_remove_file(dev, &dev_attr_waiting_for_supplier);
 	device_remove_file(dev, &dev_attr_online);
+	device_remove_group(dev, &device_trust_attr_group);
 	device_remove_groups(dev, dev->groups);
 
 	if (type)
@@ -4790,6 +4797,12 @@ static int device_attrs_change_owner(struct device *dev, kuid_t kuid,
 	if (error)
 		return error;
 
+	/* Change the trust attributes of @dev to @kuid/@kgid. */
+	error = sysfs_group_change_owner(kobj, &device_trust_attr_group, kuid,
+					 kgid);
+	if (error)
+		return error;
+
 	if (device_supports_offline(dev) && !dev_offline_disabled(dev)) {
 		/* Change online device attributes of @dev to @kuid/@kgid. */
 		error = sysfs_file_change_owner(kobj, dev_attr_online.attr.name,
diff --git a/drivers/base/trust.c b/drivers/base/trust.c
index 10a135b8bf399..884e7c2dfbbb6 100644
--- a/drivers/base/trust.c
+++ b/drivers/base/trust.c
@@ -3,6 +3,7 @@
 #include <linux/device.h>
 #include <linux/errno.h>
 #include <linux/export.h>
+#include <linux/sysfs.h>
 #include <kunit/visibility.h>
 
 #include "base.h"
@@ -122,3 +123,104 @@ bool device_is_adversarial(const struct device *dev)
 	return device_get_trust_level(dev) == DEVICE_TRUST_ADVERSARY;
 }
 EXPORT_SYMBOL_GPL(device_is_adversarial);
+
+static const char * const device_trust_policy_names[] = {
+	[DEVICE_TRUST_POLICY_DEFAULT] = "default",
+	[DEVICE_TRUST_POLICY_DISABLED] = "disabled",
+	[DEVICE_TRUST_POLICY_ADVERSARY] = "adversary",
+	[DEVICE_TRUST_POLICY_FULL] = "full",
+};
+
+static const char * const device_trust_level_names[] = {
+	[DEVICE_TRUST_DISABLED] = "disabled",
+	[DEVICE_TRUST_ADVERSARY] = "adversary",
+	[DEVICE_TRUST_FULL] = "full",
+};
+
+static ssize_t trust_show(struct device *dev,
+			  const struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%s\n",
+			  device_trust_level_names[device_get_trust_level(dev)]);
+}
+static DEVICE_ATTR_RO(trust);
+
+static ssize_t trust_policy_show(struct device *dev,
+				 const struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%s\n",
+			  device_trust_policy_names[READ_ONCE(dev->trust_policy)]);
+}
+
+VISIBLE_IF_KUNIT int device_trust_set_policy(struct device *dev,
+					   enum device_trust_policy policy)
+{
+	int ret = 0;
+
+	if (policy < DEVICE_TRUST_POLICY_DEFAULT ||
+	    policy > DEVICE_TRUST_POLICY_FULL)
+		return -EINVAL;
+
+	device_lock(dev);
+	if (dev->p->dead) {
+		ret = -ENODEV;
+		goto unlock;
+	}
+	if (policy == dev->trust_policy)
+		goto unlock;
+
+	/* A driver that claimed the device before registration was not admitted. */
+	if (READ_ONCE(dev->driver)) {
+		ret = -EBUSY;
+		goto unlock;
+	}
+
+	switch (dev->p->trust_state) {
+	case DEVICE_TRUST_SELECTABLE:
+		if (policy == DEVICE_TRUST_POLICY_DISABLED)
+			dev->p->trust_state = DEVICE_TRUST_WAITING;
+		break;
+	case DEVICE_TRUST_WAITING:
+		if (policy != DEVICE_TRUST_POLICY_ADVERSARY &&
+		    policy != DEVICE_TRUST_POLICY_FULL) {
+			ret = -EBUSY;
+			goto unlock;
+		}
+		dev->p->trust_state = DEVICE_TRUST_AUTHORIZED;
+		break;
+	default:
+		ret = -EBUSY;
+		goto unlock;
+	}
+	WRITE_ONCE(dev->trust_policy, policy);
+unlock:
+	device_unlock(dev);
+	return ret;
+}
+EXPORT_SYMBOL_IF_KUNIT(device_trust_set_policy);
+
+static ssize_t trust_policy_store(struct device *dev,
+				  const struct device_attribute *attr,
+				  const char *buf, size_t count)
+{
+	int policy, ret;
+
+	policy = sysfs_match_string(device_trust_policy_names, buf);
+	if (policy < 0)
+		return policy;
+
+	ret = device_trust_set_policy(dev, policy);
+
+	return ret ? ret : count;
+}
+static DEVICE_ATTR_RW(trust_policy);
+
+static struct attribute *device_trust_attrs[] = {
+	&dev_attr_trust.attr,
+	&dev_attr_trust_policy.attr,
+	NULL,
+};
+
+const struct attribute_group device_trust_attr_group = {
+	.attrs = device_trust_attrs,
+};

-- 
2.56.0