Some resources can be removed asynchronously, for example, resources provided by a hot-pluggable device like USB. When holding a reference to such a resource, it's possible for the resource to be removed and its memory freed, leading to use-after-free errors on subsequent access.
Introduce the revocable to establish weak references to such resources. It allows a resource consumer to safely attempt to access a resource that might be freed at any time by the resource provider.
The implementation uses a provider/consumer model built on Sleepable RCU (SRCU) to guarantee safe memory access:
- A resource provider allocates a struct revocable_provider and initializes it with a pointer to the resource.
- A resource consumer that wants to access the resource allocates a struct revocable which holds a reference to the provider.
- To access the resource, the consumer uses revocable_try_access(). This function enters an SRCU read-side critical section and returns the pointer to the resource. If the provider has already freed the resource, it returns NULL. After use, the consumer calls revocable_release() to exit the SRCU critical section. The REVOCABLE() is a convenient helper for doing that.
- When the provider needs to remove the resource, it calls revocable_provider_free(). This function sets the internal resource pointer to NULL and then calls synchronize_srcu() to wait for all current readers to finish before the resource can be completely torn down.
Signed-off-by: Tzung-Bi Shih tzungbi@kernel.org --- v3: - No changes.
v2: https://lore.kernel.org/chrome-platform/20250820081645.847919-2-tzungbi@kern... - Rename "ref_proxy" -> "revocable". - Add introduction in kernel-doc format in revocable.c. - Add MAINTAINERS entry. - Add copyright. - Move from lib/ to drivers/base/. - EXPORT_SYMBOL() -> EXPORT_SYMBOL_GPL(). - Add Documentation/. - Rename _get() -> try_access(); _put() -> release(). - Fix a sparse warning by removing the redundant __rcu annotations. - Fix a sparse warning by adding __acquires() and __releases() annotations.
v1: https://lore.kernel.org/chrome-platform/20250814091020.1302888-2-tzungbi@ker...
.../driver-api/driver-model/index.rst | 1 + .../driver-api/driver-model/revocable.rst | 151 ++++++++++++ MAINTAINERS | 7 + drivers/base/Makefile | 2 +- drivers/base/revocable.c | 229 ++++++++++++++++++ include/linux/revocable.h | 37 +++ 6 files changed, 426 insertions(+), 1 deletion(-) create mode 100644 Documentation/driver-api/driver-model/revocable.rst create mode 100644 drivers/base/revocable.c create mode 100644 include/linux/revocable.h
diff --git a/Documentation/driver-api/driver-model/index.rst b/Documentation/driver-api/driver-model/index.rst index 4831bdd92e5c..8e1ee21185df 100644 --- a/Documentation/driver-api/driver-model/index.rst +++ b/Documentation/driver-api/driver-model/index.rst @@ -14,6 +14,7 @@ Driver Model overview platform porting + revocable
.. only:: subproject and html
diff --git a/Documentation/driver-api/driver-model/revocable.rst b/Documentation/driver-api/driver-model/revocable.rst new file mode 100644 index 000000000000..b9e2968ba9c1 --- /dev/null +++ b/Documentation/driver-api/driver-model/revocable.rst @@ -0,0 +1,151 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================== +Revocable Resource Management +============================== + +Overview +======== + +In a system with hot-pluggable devices, such as USB, resources provided by +these devices can be removed asynchronously. If a consumer holds a reference +to such a resource, the resource might be deallocated while the reference is +still held, leading to use-after-free errors upon subsequent access. + +The "revocable" mechanism addresses this by establishing a weak reference to a +resource that might be freed at any time. It allows a resource consumer to +safely attempt to access the resource, guaranteeing that the access is valid +for the duration of its use, or it fails safely if the resource has already +been revoked. + +The implementation is based on a provider/consumer model that uses Sleepable +RCU (SRCU) to ensure safe memory access without traditional locking. + +How It Works +============ + +1. **Provider**: A resource provider, such as a driver for a hot-pluggable + device, allocates a ``struct revocable_provider``. This structure is + initialized with a pointer to the actual resource it manages. + +2. **Consumer**: A consumer that needs to access the resource is given a + ``struct revocable``, which acts as a handle containing a reference to + the provider. + +3. **Accessing the Resource**: To access the resource, the consumer uses + ``revocable_try_access()``. This function enters an SRCU read-side + critical section and returns a pointer to the resource. If the provider + has already revoked the resource, this function returns ``NULL``. The + consumer must check for this ``NULL`` return. + +4. **Releasing the Resource**: After the consumer has finished using the + resource, it must call ``revocable_release()`` to exit the SRCU critical + section. This signals that the consumer no longer requires access. The + ``REVOCABLE()`` macro is provided as a convenient and safe way to manage + the access-release cycle. + +5. **Revoking the Resource**: When the provider needs to remove the resource + (e.g., the device is unplugged), it calls ``revocable_provider_free()``. + This function first sets the internal resource pointer to ``NULL``, + preventing any new consumers from accessing it. It then calls + ``synchronize_srcu()``, which waits for all existing consumers currently + in the SRCU critical section to finish their work. Once all consumers + have released their access, the resource can be safely deallocated. + +Revocable vs. Device-Managed (devm) Resources +============================================= + +It's important to understand the distinction between a standard +device-managed (devm) resource and a resource managed by a +``revocable_provider``. + +The key difference is their lifetime: + +* A **devm resource** is tied to the lifetime of the device. It is + automatically freed when the device is unbound. +* A **revocable_provider** persists as long as there are active references + to it from ``revocable`` consumer handles. + +This means that a ``revocable_provider`` can outlive the device that created +it. This is a deliberate design feature, allowing consumers to hold a +reference to a resource even after the underlying device has been removed, +without causing a fault. When the consumer attempts to access the resource, +it will simply be informed that the resource is no longer available. + +API and Usage +============= + +For Resource Providers +---------------------- + +``struct revocable_provider *revocable_provider_alloc(void *res);`` + Allocates a provider handle for the given resource ``res``. It returns a + pointer to the ``revocable_provider`` on success, or ``NULL`` on failure. + +``struct revocable_provider *devm_revocable_provider_alloc(struct device *dev, void *res);`` + A device-managed version of ``revocable_provider_alloc``. It is + convenient to allocate providers via this function if the ``res`` is also + tied to the lifetime of the ``dev``. ``revocable_provider_free`` will be + called automatically when the device is unbound. + +``void revocable_provider_free(struct revocable_provider *rp);`` + Revokes the resource. This function marks the resource as unavailable and + waits for all current consumers to finish before the underlying memory + can be freed. + +For Resource Consumers +---------------------- + +``struct revocable *revocable_alloc(struct revocable_provider *rp);`` + Allocates a consumer handle for a given provider ``rp``. + +``void revocable_free(struct revocable *rev);`` + Frees a consumer handle. + +``void *revocable_try_access(struct revocable *rev);`` + Attempts to gain access to the resource. Returns a pointer to the + resource on success or ``NULL`` if it has been revoked. + +``void revocable_release(struct revocable *rev);`` + Releases access to the resource, exiting the SRCU critical section. + +The ``REVOCABLE()`` Macro +========================= + +The ``REVOCABLE()`` macro simplifies the access-release cycle for consumers, +ensuring that ``revocable_release()`` is always called, even in the case of +an early exit. + +``REVOCABLE(rev, res)`` + * ``rev``: The consumer's ``struct revocable *`` handle. + * ``res``: A pointer variable that will be assigned the resource. + +The macro creates a ``for`` loop that executes exactly once. Inside the loop, +``res`` is populated with the result of ``revocable_try_access()``. The +consumer code **must** check if ``res`` is ``NULL`` before using it. The +``revocable_release()`` function is automatically called when the scope of +the loop is exited. + +Example Usage +------------- + +.. code-block:: c + + void consumer_use_resource(struct revocable *rev) + { + struct foo_resource *res; + + REVOCABLE(rev, res) { + // Always check if the resource is valid. + if (!res) { + pr_warn("Resource is not available\n"); + return; + } + + // At this point, 'res' is guaranteed to be valid until + // this block exits. + do_something_with(res); + } + + // revocable_release() is automatically called here. + } diff --git a/MAINTAINERS b/MAINTAINERS index fa7f80bd7b2f..5d11aeeb546e 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -21877,6 +21877,13 @@ F: include/uapi/linux/rseq.h F: kernel/rseq.c F: tools/testing/selftests/rseq/
+REVOCABLE RESOURCE MANAGEMENT +M: Tzung-Bi Shih tzungbi@kernel.org +L: linux-kernel@vger.kernel.org +S: Maintained +F: drivers/base/revocable.c +F: include/linux/revocable.h + RFKILL M: Johannes Berg johannes@sipsolutions.net L: linux-wireless@vger.kernel.org diff --git a/drivers/base/Makefile b/drivers/base/Makefile index 8074a10183dc..bdf854694e39 100644 --- a/drivers/base/Makefile +++ b/drivers/base/Makefile @@ -6,7 +6,7 @@ obj-y := component.o core.o bus.o dd.o syscore.o \ cpu.o firmware.o init.o map.o devres.o \ attribute_container.o transport_class.o \ topology.o container.o property.o cacheinfo.o \ - swnode.o faux.o + swnode.o faux.o revocable.o obj-$(CONFIG_AUXILIARY_BUS) += auxiliary.o obj-$(CONFIG_DEVTMPFS) += devtmpfs.o obj-y += power/ diff --git a/drivers/base/revocable.c b/drivers/base/revocable.c new file mode 100644 index 000000000000..80a48896b241 --- /dev/null +++ b/drivers/base/revocable.c @@ -0,0 +1,229 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright 2025 Google LLC + * + * Revocable resource management + */ + +#include <linux/device.h> +#include <linux/kref.h> +#include <linux/revocable.h> +#include <linux/slab.h> +#include <linux/srcu.h> + +/** + * DOC: Overview + * + * Some resources can be removed asynchronously, for example, resources + * provided by a hot-pluggable device like USB. When holding a reference + * to such a resource, it's possible for the resource to be removed and + * its memory freed, leading to use-after-free errors on subsequent access. + * + * Introduce the revocable to establish weak references to such resources. + * It allows a resource consumer to safely attempt to access a resource + * that might be freed at any time by the resource provider. + * + * The implementation uses a provider/consumer model built on Sleepable + * RCU (SRCU) to guarantee safe memory access: + * + * - A resource provider allocates a struct revocable_provider and + * initializes it with a pointer to the resource. + * + * - A resource consumer that wants to access the resource allocates a + * struct revocable which holds a reference to the provider. + * + * - To access the resource, the consumer uses revocable_try_access(). + * This function enters an SRCU read-side critical section and returns + * the pointer to the resource. If the provider has already freed the + * resource, it returns NULL. After use, the consumer calls + * revocable_release() to exit the SRCU critical section. The + * REVOCABLE() is a convenient helper for doing that. + * + * - When the provider needs to remove the resource, it calls + * revocable_provider_free(). This function sets the internal resource + * pointer to NULL and then calls synchronize_srcu() to wait for all + * current readers to finish before the resource can be completely torn + * down. + */ + +/** + * struct revocable_provider - A handle for resource provider. + * @srcu: The SRCU to protect the resource. + * @res: The pointer of resource. It can point to anything. + * @kref: The refcount for this handle. + */ +struct revocable_provider { + struct srcu_struct srcu; + void __rcu *res; + struct kref kref; +}; + +/** + * struct revocable - A handle for resource consumer. + * @rp: The pointer of resource provider. + * @idx: The index for the RCU critical section. + */ +struct revocable { + struct revocable_provider *rp; + int idx; +}; + +/** + * revocable_provider_alloc() - Allocate struct revocable_provider. + * @res: The pointer of resource. + * + * This holds an initial refcount to the struct. + * + * Return: The pointer of struct revocable_provider. NULL on errors. + */ +struct revocable_provider *revocable_provider_alloc(void *res) +{ + struct revocable_provider *rp; + + rp = kzalloc(sizeof(*rp), GFP_KERNEL); + if (!rp) + return NULL; + + init_srcu_struct(&rp->srcu); + rcu_assign_pointer(rp->res, res); + synchronize_srcu(&rp->srcu); + kref_init(&rp->kref); + + return rp; +} +EXPORT_SYMBOL_GPL(revocable_provider_alloc); + +static void revocable_provider_release(struct kref *kref) +{ + struct revocable_provider *rp = container_of(kref, + struct revocable_provider, kref); + + cleanup_srcu_struct(&rp->srcu); + kfree(rp); +} + +/** + * revocable_provider_free() - Free struct revocable_provider. + * @rp: The pointer of resource provider. + * + * This sets the resource `(struct revocable_provider *)->res` to NULL to + * indicate the resource has gone. + * + * This drops the refcount to the resource provider. If it is the final + * reference, revocable_provider_release() will be called to free the struct. + */ +void revocable_provider_free(struct revocable_provider *rp) +{ + rcu_assign_pointer(rp->res, NULL); + synchronize_srcu(&rp->srcu); + kref_put(&rp->kref, revocable_provider_release); +} +EXPORT_SYMBOL_GPL(revocable_provider_free); + +static void devm_revocable_provider_free(void *data) +{ + struct revocable_provider *rp = data; + + revocable_provider_free(rp); +} + +/** + * devm_revocable_provider_alloc() - Dev-managed revocable_provider_alloc(). + * @dev: The device. + * @res: The pointer of resource. + * + * It is convenient to allocate providers via this function if the @res is + * also tied to the lifetime of the @dev. revocable_provider_free() will + * be called automatically when the device is unbound. + * + * This holds an initial refcount to the struct. + * + * Return: The pointer of struct revocable_provider. NULL on errors. + */ +struct revocable_provider *devm_revocable_provider_alloc(struct device *dev, + void *res) +{ + struct revocable_provider *rp; + + rp = revocable_provider_alloc(res); + if (!rp) + return NULL; + + if (devm_add_action_or_reset(dev, devm_revocable_provider_free, rp)) + return NULL; + + return rp; +} +EXPORT_SYMBOL_GPL(devm_revocable_provider_alloc); + +/** + * revocable_alloc() - Allocate struct revocable_provider. + * @rp: The pointer of resource provider. + * + * This holds a refcount to the resource provider. + * + * Return: The pointer of struct revocable_provider. NULL on errors. + */ +struct revocable *revocable_alloc(struct revocable_provider *rp) +{ + struct revocable *rev; + + rev = kzalloc(sizeof(*rev), GFP_KERNEL); + if (!rev) + return NULL; + + rev->rp = rp; + kref_get(&rp->kref); + + return rev; +} +EXPORT_SYMBOL_GPL(revocable_alloc); + +/** + * revocable_free() - Free struct revocable. + * @rev: The pointer of struct revocable. + * + * This drops a refcount to the resource provider. If it is the final + * reference, revocable_provider_release() will be called to free the struct. + */ +void revocable_free(struct revocable *rev) +{ + struct revocable_provider *rp = rev->rp; + + kref_put(&rp->kref, revocable_provider_release); + kfree(rev); +} +EXPORT_SYMBOL_GPL(revocable_free); + +/** + * revocable_try_access() - Try to access the resource. + * @rev: The pointer of struct revocable. + * + * This tries to de-reference to the resource and enters a RCU critical + * section. + * + * Return: The pointer to the resource. NULL if the resource has gone. + */ +void *revocable_try_access(struct revocable *rev) __acquires(&rev->rp->srcu) +{ + struct revocable_provider *rp = rev->rp; + + rev->idx = srcu_read_lock(&rp->srcu); + return rcu_dereference(rp->res); +} +EXPORT_SYMBOL_GPL(revocable_try_access); + +/** + * revocable_release() - Stop accessing to the resource. + * @rev: The pointer of struct revocable. + * + * Call this function to indicate the resource is no longer used. It exits + * the RCU critical section. + */ +void revocable_release(struct revocable *rev) __releases(&rev->rp->srcu) +{ + struct revocable_provider *rp = rev->rp; + + srcu_read_unlock(&rp->srcu, rev->idx); +} +EXPORT_SYMBOL_GPL(revocable_release); diff --git a/include/linux/revocable.h b/include/linux/revocable.h new file mode 100644 index 000000000000..17d9b7ce633d --- /dev/null +++ b/include/linux/revocable.h @@ -0,0 +1,37 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* + * Copyright 2025 Google LLC + */ + +#ifndef __LINUX_REVOCABLE_H +#define __LINUX_REVOCABLE_H + +#include <linux/cleanup.h> + +struct device; +struct revocable; +struct revocable_provider; + +struct revocable_provider *revocable_provider_alloc(void *res); +void revocable_provider_free(struct revocable_provider *rp); +struct revocable_provider *devm_revocable_provider_alloc(struct device *dev, + void *res); + +struct revocable *revocable_alloc(struct revocable_provider *rp); +void revocable_free(struct revocable *rev); +void *revocable_try_access(struct revocable *rev) __acquires(&rev->rp->srcu); +void revocable_release(struct revocable *rev) __releases(&rev->rp->srcu); + +DEFINE_FREE(revocable, struct revocable *, if (_T) revocable_release(_T)) + +#define _REVOCABLE(_rev, _label, _res) \ + for (struct revocable *__UNIQUE_ID(name) __free(revocable) = _rev; \ + (_res = revocable_try_access(_rev)) || true; ({ goto _label; })) \ + if (0) { \ +_label: \ + break; \ + } else + +#define REVOCABLE(_rev, _res) _REVOCABLE(_rev, __UNIQUE_ID(label), _res) + +#endif /* __LINUX_REVOCABLE_H */