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.

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guaranteeing that the access is valid for the duration of its use, or it fails safely if the resource has already been revoked.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 uses a provider/consumer model built on Sleepable RCU (SRCU) to guarantee safe memory access:

- A resource provider, such as a driver for a hot-pluggable device, 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 acts as a handle containing 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_withdraw_access() to exit the SRCU critical section. The REVOCABLE_TRY_ACCESS_WITH() is a convenient helper for doing that.

- When the provider needs to remove the resource, it calls revocable_provider_revoke(). 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.

Acked-by: Danilo Krummrich dakr@kernel.org Acked-by: Simona Vetter simona.vetter@ffwll.ch Signed-off-by: Tzung-Bi Shih tzungbi@kernel.org --- v5: - No changes.

v4: https://lore.kernel.org/chrome-platform/20250923075302.591026-2-tzungbi@kern... - Rename: - revocable_provider_free() -> revocable_provider_revoke(). - REVOCABLE() -> REVOCABLE_TRY_ACCESS_WITH(). - revocable_release() -> revocable_withdraw_access(). - rcu_dereference() -> srcu_dereference() to fix a warning from lock debugging. - Move most docs to kernel-doc, include them in Documentation/, and modify the commit message accordingly. - Fix some doc errors. - Add Acked-by tags.

v3: https://lore.kernel.org/chrome-platform/20250912081718.3827390-2-tzungbi@ker... - 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...

Note (for my reference): - An optional .release() callback for revocable provider-managed resource hasn't added. - `make O=build SPHINXDIRS=driver-api/driver-model/ htmldocs` a way to verify the Documentation/.

.../driver-api/driver-model/index.rst | 1 + .../driver-api/driver-model/revocable.rst | 87 +++++++ MAINTAINERS | 7 + drivers/base/Makefile | 2 +- drivers/base/revocable.c | 233 ++++++++++++++++++ include/linux/revocable.h | 53 ++++ 6 files changed, 382 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..ce2d0eb1c8cb --- /dev/null +++ b/Documentation/driver-api/driver-model/revocable.rst @@ -0,0 +1,87 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================== +Revocable Resource Management +============================== + +Overview +======== + +.. kernel-doc:: drivers/base/revocable.c + :doc: Overview + +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 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 +---------------------- + +.. kernel-doc:: drivers/base/revocable.c + :identifiers: revocable_provider_alloc + +.. kernel-doc:: drivers/base/revocable.c + :identifiers: devm_revocable_provider_alloc + +.. kernel-doc:: drivers/base/revocable.c + :identifiers: revocable_provider_revoke + +For Resource Consumers +---------------------- + +.. kernel-doc:: drivers/base/revocable.c + :identifiers: revocable_alloc + +.. kernel-doc:: drivers/base/revocable.c + :identifiers: revocable_free + +.. kernel-doc:: drivers/base/revocable.c + :identifiers: revocable_try_access + +.. kernel-doc:: drivers/base/revocable.c + :identifiers: revocable_withdraw_access + +.. kernel-doc:: include/linux/revocable.h + :identifiers: REVOCABLE_TRY_ACCESS_WITH + +Example Usage +~~~~~~~~~~~~~ + +.. code-block:: c + + void consumer_use_resource(struct revocable *rev) + { + struct foo_resource *res; + + REVOCABLE_TRY_ACCESS_WITH(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_withdraw_access() is automatically called here. + } diff --git a/MAINTAINERS b/MAINTAINERS index 25ed1846d970..7d00ff431af9 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -22020,6 +22020,13 @@ 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..f8dd4363a87b --- /dev/null +++ b/drivers/base/revocable.c @@ -0,0 +1,233 @@ +// 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. + * + * 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 uses a provider/consumer model built on Sleepable + * RCU (SRCU) to guarantee safe memory access: + * + * - A resource provider, such as a driver for a hot-pluggable device, + * 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 acts as a handle containing 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_withdraw_access() to exit the SRCU critical section. The + * REVOCABLE_TRY_ACCESS_WITH() is a convenient helper for doing that. + * + * - When the provider needs to remove the resource, it calls + * revocable_provider_revoke(). 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_revoke() - Revoke the managed resource. + * @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_revoke(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_revoke); + +static void devm_revocable_provider_revoke(void *data) +{ + struct revocable_provider *rp = data; + + revocable_provider_revoke(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_revoke() 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_revoke, rp)) + return NULL; + + return rp; +} +EXPORT_SYMBOL_GPL(devm_revocable_provider_alloc); + +/** + * revocable_alloc() - Allocate struct revocable. + * @rp: The pointer of resource provider. + * + * This holds a refcount to the resource provider. + * + * Return: The pointer of struct revocable. 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 srcu_dereference(rp->res, &rp->srcu); +} +EXPORT_SYMBOL_GPL(revocable_try_access); + +/** + * revocable_withdraw_access() - 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_withdraw_access(struct revocable *rev) __releases(&rev->rp->srcu) +{ + struct revocable_provider *rp = rev->rp; + + srcu_read_unlock(&rp->srcu, rev->idx); +} +EXPORT_SYMBOL_GPL(revocable_withdraw_access); diff --git a/include/linux/revocable.h b/include/linux/revocable.h new file mode 100644 index 000000000000..7177bf045d9c --- /dev/null +++ b/include/linux/revocable.h @@ -0,0 +1,53 @@ +/* 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_revoke(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_withdraw_access(struct revocable *rev) __releases(&rev->rp->srcu); + +DEFINE_FREE(revocable, struct revocable *, if (_T) revocable_withdraw_access(_T)) + +#define _REVOCABLE_TRY_ACCESS_WITH(_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 + +/** + * REVOCABLE_TRY_ACCESS_WITH() - A helper for accessing revocable resource + * @_rev: The consumer's ``struct revocable *`` handle. + * @_res: A pointer variable that will be assigned the resource. + * + * The macro simplifies the access-release cycle for consumers, ensuring that + * revocable_withdraw_access() is always called, even in the case of an early + * exit. + * + * It 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_withdraw_access() function is automatically called when the scope + * of the loop is exited. + */ +#define REVOCABLE_TRY_ACCESS_WITH(_rev, _res) \ + _REVOCABLE_TRY_ACCESS_WITH(_rev, __UNIQUE_ID(label), _res) + +#endif /* __LINUX_REVOCABLE_H */

Add kselftest cases for the revocable API.

The test consists of three parts: - A kernel module (revocable_test.ko) that creates a debugfs interface with `/provider` and `/consumer` files. - A user-space C program (revocable_test) that uses the kselftest harness to interact with the debugfs files. - An orchestrating shell script (test-revocable.sh) that loads the module, runs the C program, and unloads the module.

The test cases cover the following scenarios: - Basic: Verifies that a consumer can successfully access the resource provided via the provider. - Revocation: Verifies that after the provider revokes the resource, the consumer correctly receives a NULL pointer on a subsequent access. - Macro: Same as "Revocation" but uses the REVOCABLE().

Signed-off-by: Tzung-Bi Shih tzungbi@kernel.org --- v5: - No changes.

v4: https://lore.kernel.org/chrome-platform/20250923075302.591026-4-tzungbi@kern... - REVOCABLE() -> REVOCABLE_TRY_ACCESS_WITH(). - revocable_release() -> revocable_withdraw_access().

v3: https://lore.kernel.org/chrome-platform/20250912081718.3827390-4-tzungbi@ker... - No changes.

v2: https://lore.kernel.org/chrome-platform/20250820081645.847919-4-tzungbi@kern... - New in the series.

A way to run the kselftest (for my reference): - Update kernel to the DUT. - `mkdir build` and copy the kernel config to build/. - `make O=build LLVM=1 -j32` (for generated headers and built-in symbols). - `make O=build LLVM=1 KDIR=$(pwd) -C tools/testing/selftests/ TARGETS=drivers/base/revocable gen_tar`. - Copy build/kselftest/kselftest_install/kselftest-packages/kselftest.tar.gz to the DUT, extract, and execute the run_kselftest.sh.

MAINTAINERS | 1 + tools/testing/selftests/Makefile | 1 + .../selftests/drivers/base/revocable/Makefile | 7 + .../drivers/base/revocable/revocable_test.c | 116 +++++++++++ .../drivers/base/revocable/test-revocable.sh | 39 ++++ .../base/revocable/test_modules/Makefile | 10 + .../revocable/test_modules/revocable_test.c | 188 ++++++++++++++++++ 7 files changed, 362 insertions(+) create mode 100644 tools/testing/selftests/drivers/base/revocable/Makefile create mode 100644 tools/testing/selftests/drivers/base/revocable/revocable_test.c create mode 100755 tools/testing/selftests/drivers/base/revocable/test-revocable.sh create mode 100644 tools/testing/selftests/drivers/base/revocable/test_modules/Makefile create mode 100644 tools/testing/selftests/drivers/base/revocable/test_modules/revocable_test.c

diff --git a/MAINTAINERS b/MAINTAINERS index 7207538f31f4..235a1d6e3718 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -22027,6 +22027,7 @@ S: Maintained F: drivers/base/revocable.c F: drivers/base/revocable_test.c F: include/linux/revocable.h +F: tools/testing/selftests/drivers/base/revocable/

RFKILL M: Johannes Berg johannes@sipsolutions.net diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile index c46ebdb9b8ef..1136a8f12ef5 100644 --- a/tools/testing/selftests/Makefile +++ b/tools/testing/selftests/Makefile @@ -17,6 +17,7 @@ TARGETS += damon TARGETS += devices/error_logs TARGETS += devices/probe TARGETS += dmabuf-heaps +TARGETS += drivers/base/revocable TARGETS += drivers/dma-buf TARGETS += drivers/ntsync TARGETS += drivers/s390x/uvdevice diff --git a/tools/testing/selftests/drivers/base/revocable/Makefile b/tools/testing/selftests/drivers/base/revocable/Makefile new file mode 100644 index 000000000000..afa5ca0fa452 --- /dev/null +++ b/tools/testing/selftests/drivers/base/revocable/Makefile @@ -0,0 +1,7 @@ +# SPDX-License-Identifier: GPL-2.0 + +TEST_GEN_MODS_DIR := test_modules +TEST_GEN_PROGS_EXTENDED := revocable_test +TEST_PROGS := test-revocable.sh + +include ../../../lib.mk diff --git a/tools/testing/selftests/drivers/base/revocable/revocable_test.c b/tools/testing/selftests/drivers/base/revocable/revocable_test.c new file mode 100644 index 000000000000..3c0feb7a8944 --- /dev/null +++ b/tools/testing/selftests/drivers/base/revocable/revocable_test.c @@ -0,0 +1,116 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright 2025 Google LLC + * + * A selftest for the revocable API. + * + * The test cases cover the following scenarios: + * + * - Basic: Verifies that a consumer can successfully access the resource + * provided via the provider. + * + * - Revocation: Verifies that after the provider revokes the resource, + * the consumer correctly receives a NULL pointer on a subsequent access. + * + * - Macro: Same as "Revocation" but uses the REVOCABLE_TRY_ACCESS_WITH(). + */ + +#include <fcntl.h> +#include <unistd.h> + +#include "../../../kselftest_harness.h" + +#define DEBUGFS_PATH "/sys/kernel/debug/revocable_test" +#define TEST_CMD_RESOURCE_GONE "resource_gone" +#define TEST_DATA "12345678" +#define TEST_MAGIC_OFFSET 0x1234 + +FIXTURE(revocable_fixture) { + int pfd; + int cfd; +}; + +FIXTURE_SETUP(revocable_fixture) { + int ret; + + self->pfd = open(DEBUGFS_PATH "/provider", O_WRONLY); + ASSERT_NE(-1, self->pfd) + TH_LOG("failed to open provider fd"); + + ret = write(self->pfd, TEST_DATA, strlen(TEST_DATA)); + ASSERT_NE(-1, ret) { + close(self->pfd); + TH_LOG("failed to write test data"); + } + + self->cfd = open(DEBUGFS_PATH "/consumer", O_RDONLY); + ASSERT_NE(-1, self->cfd) + TH_LOG("failed to open consumer fd"); +} + +FIXTURE_TEARDOWN(revocable_fixture) { + close(self->cfd); + close(self->pfd); +} + +/* + * ASSERT_* is only available in TEST or TEST_F block. Use + * macro for the helper. + */ +#define READ_TEST_DATA(_fd, _offset, _data, _msg) \ + do { \ + int ret; \ + \ + ret = lseek(_fd, _offset, SEEK_SET); \ + ASSERT_NE(-1, ret) \ + TH_LOG("failed to lseek"); \ + \ + ret = read(_fd, _data, sizeof(_data) - 1); \ + ASSERT_NE(-1, ret) \ + TH_LOG(_msg); \ + data[ret] = '\0'; \ + } while (0) + +TEST_F(revocable_fixture, basic) { + char data[16]; + + READ_TEST_DATA(self->cfd, 0, data, "failed to read test data"); + EXPECT_STREQ(TEST_DATA, data); +} + +TEST_F(revocable_fixture, revocation) { + char data[16]; + int ret; + + READ_TEST_DATA(self->cfd, 0, data, "failed to read test data"); + EXPECT_STREQ(TEST_DATA, data); + + ret = write(self->pfd, TEST_CMD_RESOURCE_GONE, + strlen(TEST_CMD_RESOURCE_GONE)); + ASSERT_NE(-1, ret) + TH_LOG("failed to write resource gone cmd"); + + READ_TEST_DATA(self->cfd, 0, data, + "failed to read test data after resource gone"); + EXPECT_STREQ("(null)", data); +} + +TEST_F(revocable_fixture, macro) { + char data[16]; + int ret; + + READ_TEST_DATA(self->cfd, TEST_MAGIC_OFFSET, data, + "failed to read test data"); + EXPECT_STREQ(TEST_DATA, data); + + ret = write(self->pfd, TEST_CMD_RESOURCE_GONE, + strlen(TEST_CMD_RESOURCE_GONE)); + ASSERT_NE(-1, ret) + TH_LOG("failed to write resource gone cmd"); + + READ_TEST_DATA(self->cfd, TEST_MAGIC_OFFSET, data, + "failed to read test data after resource gone"); + EXPECT_STREQ("(null)", data); +} + +TEST_HARNESS_MAIN diff --git a/tools/testing/selftests/drivers/base/revocable/test-revocable.sh b/tools/testing/selftests/drivers/base/revocable/test-revocable.sh new file mode 100755 index 000000000000..3a34be28001a --- /dev/null +++ b/tools/testing/selftests/drivers/base/revocable/test-revocable.sh @@ -0,0 +1,39 @@ +#!/bin/bash +# SPDX-License-Identifier: GPL-2.0 + +mod_name="revocable_test" +ksft_fail=1 +ksft_skip=4 + +if [ "$(id -u)" -ne 0 ]; then + echo "$0: Must be run as root" + exit "$ksft_skip" +fi + +if ! which insmod > /dev/null 2>&1; then + echo "$0: Need insmod" + exit "$ksft_skip" +fi + +if ! which rmmod > /dev/null 2>&1; then + echo "$0: Need rmmod" + exit "$ksft_skip" +fi + +insmod test_modules/"${mod_name}".ko + +if [ ! -d /sys/kernel/debug/revocable_test/ ]; then + mount -t debugfs none /sys/kernel/debug/ + + if [ ! -d /sys/kernel/debug/revocable_test/ ]; then + echo "$0: Error mounting debugfs" + exit "$ksft_fail" + fi +fi + +./revocable_test +ret=$? + +rmmod "${mod_name}" + +exit "${ret}" diff --git a/tools/testing/selftests/drivers/base/revocable/test_modules/Makefile b/tools/testing/selftests/drivers/base/revocable/test_modules/Makefile new file mode 100644 index 000000000000..f29e4f909402 --- /dev/null +++ b/tools/testing/selftests/drivers/base/revocable/test_modules/Makefile @@ -0,0 +1,10 @@ +TESTMODS_DIR := $(realpath $(dir $(abspath $(lastword $(MAKEFILE_LIST))))) +KDIR ?= /lib/modules/$(shell uname -r)/build + +obj-m += revocable_test.o + +all: + $(Q)$(MAKE) -C $(KDIR) M=$(TESTMODS_DIR) + +clean: + $(Q)$(MAKE) -C $(KDIR) M=$(TESTMODS_DIR) clean diff --git a/tools/testing/selftests/drivers/base/revocable/test_modules/revocable_test.c b/tools/testing/selftests/drivers/base/revocable/test_modules/revocable_test.c new file mode 100644 index 000000000000..c90ec150a6c8 --- /dev/null +++ b/tools/testing/selftests/drivers/base/revocable/test_modules/revocable_test.c @@ -0,0 +1,188 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright 2025 Google LLC + * + * A kernel module for testing the revocable API. + */ + +#include <linux/debugfs.h> +#include <linux/module.h> +#include <linux/revocable.h> +#include <linux/slab.h> + +#define TEST_CMD_RESOURCE_GONE "resource_gone" +#define TEST_MAGIC_OFFSET 0x1234 + +static struct dentry *debugfs_dir; + +struct revocable_test_provider_priv { + struct revocable_provider *rp; + struct dentry *dentry; + char res[16]; +}; + +static int revocable_test_consumer_open(struct inode *inode, struct file *filp) +{ + struct revocable *rev; + struct revocable_provider *rp = inode->i_private; + + rev = revocable_alloc(rp); + if (!rev) + return -ENOMEM; + filp->private_data = rev; + + return 0; +} + +static int revocable_test_consumer_release(struct inode *inode, + struct file *filp) +{ + struct revocable *rev = filp->private_data; + + revocable_free(rev); + return 0; +} + +static ssize_t revocable_test_consumer_read(struct file *filp, + char __user *buf, + size_t count, loff_t *offset) +{ + char *res; + char data[16]; + size_t len; + struct revocable *rev = filp->private_data; + + switch (*offset) { + case 0: + res = revocable_try_access(rev); + snprintf(data, sizeof(data), "%s", res ?: "(null)"); + revocable_withdraw_access(rev); + break; + case TEST_MAGIC_OFFSET: + REVOCABLE_TRY_ACCESS_WITH(rev, res) + snprintf(data, sizeof(data), "%s", res ?: "(null)"); + break; + default: + return 0; + } + + len = min_t(size_t, strlen(data), count); + if (copy_to_user(buf, data, len)) + return -EFAULT; + + *offset = len; + return len; +} + +static const struct file_operations revocable_test_consumer_fops = { + .open = revocable_test_consumer_open, + .release = revocable_test_consumer_release, + .read = revocable_test_consumer_read, + .llseek = default_llseek, +}; + +static int revocable_test_provider_open(struct inode *inode, struct file *filp) +{ + struct revocable_test_provider_priv *priv; + + priv = kzalloc(sizeof(*priv), GFP_KERNEL); + if (!priv) + return -ENOMEM; + filp->private_data = priv; + + return 0; +} + +static int revocable_test_provider_release(struct inode *inode, + struct file *filp) +{ + struct revocable_test_provider_priv *priv = filp->private_data; + + debugfs_remove(priv->dentry); + if (priv->rp) + revocable_provider_revoke(priv->rp); + kfree(priv); + + return 0; +} + +static ssize_t revocable_test_provider_write(struct file *filp, + const char __user *buf, + size_t count, loff_t *offset) +{ + size_t copied; + char data[64]; + struct revocable_test_provider_priv *priv = filp->private_data; + + copied = strncpy_from_user(data, buf, sizeof(data)); + if (copied < 0) + return copied; + if (copied == sizeof(data)) + data[sizeof(data) - 1] = '\0'; + + /* + * Note: The test can't just close the FD for signaling the + * resource gone. Subsequent file operations on the opening + * FD of debugfs return -EIO after calling debugfs_remove(). + * See also debugfs_file_get(). + * + * Here is a side command channel for signaling the resource + * gone. + */ + if (!strcmp(data, TEST_CMD_RESOURCE_GONE)) { + revocable_provider_revoke(priv->rp); + priv->rp = NULL; + } else { + if (priv->res[0] != '\0') + return 0; + + strscpy(priv->res, data); + + priv->rp = revocable_provider_alloc(&priv->res); + if (!priv->rp) + return -ENOMEM; + + priv->dentry = debugfs_create_file("consumer", 0400, + debugfs_dir, priv->rp, + &revocable_test_consumer_fops); + if (!priv->dentry) { + revocable_provider_revoke(priv->rp); + return -ENOMEM; + } + } + + return copied; +} + +static const struct file_operations revocable_test_provider_fops = { + .open = revocable_test_provider_open, + .release = revocable_test_provider_release, + .write = revocable_test_provider_write, +}; + +static int __init revocable_test_init(void) +{ + debugfs_dir = debugfs_create_dir("revocable_test", NULL); + if (!debugfs_dir) + return -ENOMEM; + + if (!debugfs_create_file("provider", 0200, debugfs_dir, NULL, + &revocable_test_provider_fops)) { + debugfs_remove_recursive(debugfs_dir); + return -ENOMEM; + } + + return 0; +} + +static void __exit revocable_test_exit(void) +{ + debugfs_remove_recursive(debugfs_dir); +} + +module_init(revocable_test_init); +module_exit(revocable_test_exit); + +MODULE_LICENSE("GPL"); +MODULE_AUTHOR("Tzung-Bi Shih tzungbi@kernel.org"); +MODULE_DESCRIPTION("Revocable Kselftest");

Introduce fs_revocable_replace() to simplify the use of the revocable API with file_operations.

The function, should be called from a driver's ->open(), replaces the fops with a wrapper that automatically handles the `try_access` and `withdraw_access`.

When the file is closed, the wrapper's ->release() restores the original fops and cleanups. This centralizes the revocable logic, making drivers cleaner and easier to maintain.

Signed-off-by: Tzung-Bi Shih tzungbi@kernel.org --- PoC patch.

Known issues: - All file operations call revocable_try_access() for guaranteeing the resource even if the resource may be unused in the fops.

v5: - Rename to "fs_revocable". - Move the replacement context to struct file. - Support multiple revocable providers.

v4: https://lore.kernel.org/chrome-platform/20250923075302.591026-6-tzungbi@kern... - New in the series.

fs/Makefile | 2 +- fs/fs_revocable.c | 154 +++++++++++++++++++++++++++++++++++ include/linux/fs.h | 2 + include/linux/fs_revocable.h | 21 +++++ 4 files changed, 178 insertions(+), 1 deletion(-) create mode 100644 fs/fs_revocable.c create mode 100644 include/linux/fs_revocable.h

diff --git a/fs/Makefile b/fs/Makefile index e3523ab2e587..ec2b21385deb 100644 --- a/fs/Makefile +++ b/fs/Makefile @@ -16,7 +16,7 @@ obj-y := open.o read_write.o file_table.o super.o \ stack.o fs_struct.o statfs.o fs_pin.o nsfs.o \ fs_types.o fs_context.o fs_parser.o fsopen.o init.o \ kernel_read_file.o mnt_idmapping.o remap_range.o pidfs.o \ - file_attr.o + file_attr.o fs_revocable.o

obj-$(CONFIG_BUFFER_HEAD) += buffer.o mpage.o obj-$(CONFIG_PROC_FS) += proc_namespace.o diff --git a/fs/fs_revocable.c b/fs/fs_revocable.c new file mode 100644 index 000000000000..4b6d755abfed --- /dev/null +++ b/fs/fs_revocable.c @@ -0,0 +1,154 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright 2025 Google LLC + * + * File operation replacement with Revocable + */ + +#include <linux/cleanup.h> +#include <linux/fs_revocable.h> +#include <linux/poll.h> +#include <linux/revocable.h> + +struct fs_revocable_replacement { + const struct fs_revocable_operations *frops; + const struct file_operations *orig_fops; + struct file_operations fops; + struct revocable **revs; + size_t num_revs; +}; + +static int fs_revocable_try_access(struct file *filp) +{ + struct fs_revocable_replacement *rr = filp->f_rr; + + return rr->frops->try_access(rr->revs, rr->num_revs, + filp->private_data); +} + +static void fs_revocable_withdraw_access(struct fs_revocable_replacement *rr) +{ + for (size_t i = 0; i < rr->num_revs; ++i) + revocable_withdraw_access(rr->revs[i]); +} + +DEFINE_FREE(fs_revocable_replacement, struct fs_revocable_replacement *, + if (_T) fs_revocable_withdraw_access(_T)) + +static ssize_t fs_revocable_read(struct file *filp, char __user *buffer, + size_t length, loff_t *offset) +{ + struct fs_revocable_replacement *rr + __free(fs_revocable_replacement) = filp->f_rr; + int ret; + + ret = fs_revocable_try_access(filp); + if (ret) + return ret; + return rr->orig_fops->read(filp, buffer, length, offset); +} + +static __poll_t fs_revocable_poll(struct file *filp, poll_table *wait) +{ + struct fs_revocable_replacement *rr + __free(fs_revocable_replacement) = filp->f_rr; + int ret; + + ret = fs_revocable_try_access(filp); + if (ret) + return ret; + return rr->orig_fops->poll(filp, wait); +} + +static long fs_revocable_unlocked_ioctl(struct file *filp, unsigned int cmd, + unsigned long arg) +{ + struct fs_revocable_replacement *rr + __free(fs_revocable_replacement) = filp->f_rr; + int ret; + + ret = fs_revocable_try_access(filp); + if (ret) + return ret; + return rr->orig_fops->unlocked_ioctl(filp, cmd, arg); +} + +static int fs_revocable_release(struct inode *inode, struct file *filp) +{ + int ret = 0; + struct fs_revocable_replacement *rr = filp->f_rr; + + if (!rr->orig_fops->release) + goto recover; + + ret = fs_revocable_try_access(filp); + if (ret) + goto recover; + + ret = rr->orig_fops->release(inode, filp); + + fs_revocable_withdraw_access(rr); +recover: + filp->f_op = rr->orig_fops; + filp->f_rr = NULL; + + for (size_t i = 0; i < rr->num_revs; ++i) + revocable_free(rr->revs[i]); + kfree(rr->revs); + kfree(rr); + + return ret; +} + +/** + * fs_revocable_replace() - Replace the file operations to be revocable-aware. + * + * Should be used only from ->open() instances. + */ +int fs_revocable_replace(struct file *filp, + const struct fs_revocable_operations *frops, + struct revocable_provider **rps, size_t num_rps) +{ + struct fs_revocable_replacement *rr; + size_t i; + + rr = kzalloc(sizeof(*rr), GFP_KERNEL); + if (!rr) + return -ENOMEM; + filp->f_rr = rr; + + rr->frops = frops; + rr->revs = kcalloc(num_rps, sizeof(*rr->revs), GFP_KERNEL); + if (!rr->revs) + goto free_rr; + for (i = 0; i < num_rps; ++i) { + rr->revs[i] = revocable_alloc(rps[i]); + if (!rr->revs[i]) + goto free_revs; + } + rr->num_revs = num_rps; + rr->orig_fops = filp->f_op; + + memcpy(&rr->fops, filp->f_op, sizeof(rr->fops)); + rr->fops.release = fs_revocable_release; + + if (rr->fops.read) + rr->fops.read = fs_revocable_read; + if (rr->fops.poll) + rr->fops.poll = fs_revocable_poll; + if (rr->fops.unlocked_ioctl) + rr->fops.unlocked_ioctl = fs_revocable_unlocked_ioctl; + + filp->f_op = &rr->fops; + return 0; +free_revs: + if (i) { + while (--i) + revocable_free(rr->revs[i]); + } + kfree(rr->revs); +free_rr: + kfree(rr); + return -ENOMEM; +} +EXPORT_SYMBOL_GPL(fs_revocable_replace); diff --git a/include/linux/fs.h b/include/linux/fs.h index 68c4a59ec8fb..a03dd7f343a9 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -80,6 +80,7 @@ struct fs_context; struct fs_parameter_spec; struct file_kattr; struct iomap_ops; +struct fs_revocable_replacement;

extern void __init inode_init(void); extern void __init inode_init_early(void); @@ -1248,6 +1249,7 @@ struct file { }; file_ref_t f_ref; /* --- cacheline 3 boundary (192 bytes) --- */ + struct fs_revocable_replacement *f_rr; } __randomize_layout __attribute__((aligned(4))); /* lest something weird decides that 2 is OK */

diff --git a/include/linux/fs_revocable.h b/include/linux/fs_revocable.h new file mode 100644 index 000000000000..bb066392d232 --- /dev/null +++ b/include/linux/fs_revocable.h @@ -0,0 +1,21 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* + * Copyright 2025 Google LLC + */ + +#ifndef __LINUX_FS_REVOCABLE_H +#define __LINUX_FS_REVOCABLE_H + +#include <linux/fs.h> +#include <linux/revocable.h> + +struct fs_revocable_operations { + int (*try_access)(struct revocable **revs, size_t num_revs, void *data); +}; + +int fs_revocable_replace(struct file *filp, + const struct fs_revocable_operations *frops, + struct revocable_provider **rps, size_t num_rps); + +#endif /* __LINUX_FS_REVOCABLE_H */

Signed-off-by: Tzung-Bi Shih tzungbi@kernel.org --- PoC patch.

v5: - No primary changes but modify the API usage accordingly to support multiple revocable providers.

v4: https://lore.kernel.org/chrome-platform/20250923075302.591026-7-tzungbi@kern... - New in the series.

drivers/char/misc.c | 8 ++++++++ include/linux/miscdevice.h | 4 ++++ 2 files changed, 12 insertions(+)

diff --git a/drivers/char/misc.c b/drivers/char/misc.c index 726516fb0a3b..5b412e18b8a6 100644 --- a/drivers/char/misc.c +++ b/drivers/char/misc.c @@ -50,6 +50,7 @@ #include <linux/tty.h> #include <linux/kmod.h> #include <linux/gfp.h> +#include <linux/fs_revocable.h>

/* * Head entry for the doubly linked miscdevice list @@ -159,6 +160,13 @@ static int misc_open(struct inode *inode, struct file *file)

err = 0; replace_fops(file, new_fops); + + if (c->frops && c->rps) { + err = fs_revocable_replace(file, c->frops, c->rps, c->num_rps); + if (err) + goto fail; + } + if (file->f_op->open) err = file->f_op->open(inode, file); fail: diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h index 7d0aa718499c..b7024a0cebb9 100644 --- a/include/linux/miscdevice.h +++ b/include/linux/miscdevice.h @@ -5,6 +5,7 @@ #include <linux/list.h> #include <linux/types.h> #include <linux/device.h> +#include <linux/revocable.h>

/* * These allocations are managed by device@lanana.org. If you need @@ -92,6 +93,9 @@ struct miscdevice { const struct attribute_group **groups; const char *nodename; umode_t mode; + struct revocable_provider **rps; + size_t num_rps; + const struct fs_revocable_operations *frops; };

extern int misc_register(struct miscdevice *misc);

On Thu, Dec 11, 2025 at 02:46:08PM +0000, Tzung-Bi Shih wrote:

On Thu, Dec 11, 2025 at 10:43:06PM +0900, Laurent Pinchart wrote:

...

On Thu, Dec 11, 2025 at 05:36:57PM +0900, Wolfram Sang wrote:

...

...

...

Isn't there even prototype code from Dan Williams?

"[PATCH 1/3] cdev: Finish the cdev api with queued mode support"

https://lkml.org/lkml/2021/1/20/997

I mentioned that in my LPC talk in 2022 :-) I think we should merge that (or a rebased, possibly improved version of it). I've meant to try plumbing that series in V4L2 but couldn't find the time so far.

Yes, you mentioned it in 2022 but maybe not everyone in this thread is right now aware of it ;) The patch above got changes requested. I talked to Dan very briefly about it at Maintainers Summit 2023 and he was also open (back then) to pick it up again.

After discussing with Tzung-Bi today after his presentation (thank you Tzung-Bi for your time, it helped me understand the problem you're facing better), I wonder if this series is fixing the issue in the right place.

Thank you for your time too for providing me some more context.

...

At the core of the problem is a devm_kzalloc() call to allocate driver-specific data. That data structure is then referenced from a cdev, which can dereference is after it gets freed. It seems that reference-counting the data structure instead of using devm_kzalloc() could be a better solution.

After discussing with you, I recalled this was one of my previous attempts. See the series [1] and Greg's feedback [2].

I want to provide some more context about the cdev level solution. I failed to do so for misc device [3] mainly because all misc devices share a same cdev [4]. If one of the misc device drivers "revoke" the cdev, all other drivers stop working.

I'm not saying we shouldn't seek for cdev level solution. But at least it doesn't work for misc device. Still need some other ways for misc devices.

[1] https://lore.kernel.org/chrome-platform/20250721044456.2736300-8-tzungbi@ker... [2] https://lore.kernel.org/chrome-platform/2025072114-unifier-screen-1594@gregk... [3] https://lore.kernel.org/chrome-platform/aQ1xfHuyg1y8eJQ_@google.com/ [4] https://elixir.bootlin.com/linux/v6.17/source/drivers/char/misc.c#L299

Continuing the context, the subsystem level solution for misc device without revocable could be more or less like the following patch. Observed 2 main issues of it:

1. Because it tries to synchronize the misc device and open files, it has a big lock between them. misc_deregister() needs to wait for all open files. I think this is a common issue shared by "replacing file operations" approaches. All file operations are considered as critical sections.

2. It doesn't stop existing open files. UAF still happens when the dangling FD tries to access the miscdevice (which should have been freed).

diff --git a/drivers/char/misc.c b/drivers/char/misc.c index 726516fb0a3b..0ce415da10c2 100644 --- a/drivers/char/misc.c +++ b/drivers/char/misc.c @@ -115,6 +116,89 @@ static const struct seq_operations misc_seq_ops = { }; #endif

+static struct miscdevice *find_miscdevice(int minor) +{ + struct miscdevice *c; + + list_for_each_entry(c, &misc_list, list) + if (c->minor == minor) + return c; + return NULL; +} + +static __poll_t misc_some_poll(struct file *filp, poll_table *wait) +{ + struct miscdevice *c; + + c = find_miscdevice(iminor(filp->f_inode)); + if (!c) + return -ENODEV; + if (!c->fops->poll) + return 0; + + guard(mutex)(&c->some_lock); + if (!c->registered) + return -ENODEV; + return c->fops->poll(filp, wait); +} + +static const struct file_operations misc_some_fops = { + .poll = misc_some_poll, + .read = misc_some_read, + .unlocked_ioctl = misc_some_ioctl, + .release = misc_some_release, +};

@@ -161,6 +245,7 @@ static int misc_open(struct inode *inode, struct file *file) replace_fops(file, new_fops); if (file->f_op->open) err = file->f_op->open(inode, file); + file->f_op = &misc_some_fops; fail: mutex_unlock(&misc_mtx); return err; @@ -262,6 +347,8 @@ int misc_register(struct miscdevice *misc) goto out; }

+ mutex_init(&misc->some_lock); + misc->registered = true; /* * Add it to the front, so that later devices can "override" * earlier defaults @@ -283,6 +370,9 @@ EXPORT_SYMBOL(misc_register);

void misc_deregister(struct miscdevice *misc) { + scoped_guard(mutex, &misc->some_lock) + misc->registered = false; + mutex_lock(&misc_mtx); list_del_init(&misc->list); device_destroy(&misc_class, MKDEV(MISC_MAJOR, misc->minor));

diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h index 7d0aa718499c..3b42cf273f97 100644 --- a/include/linux/miscdevice.h +++ b/include/linux/miscdevice.h @@ -92,6 +92,8 @@ struct miscdevice { const struct attribute_group **groups; const char *nodename; umode_t mode; + struct mutex some_lock; + bool registered; };