If userspace reads "uevent" device attribute at the same time as another threads unbinds the device from its driver, change to dev->driver from a valid pointer to NULL may result in crash. Fix this by using READ_ONCE() when fetching the pointer, and take bus' drivers klist lock to make sure driver instance will not disappear while we access it.
Use WRITE_ONCE() when setting the driver pointer to ensure there is no tearing.
Cc: stable@vger.kernel.org Signed-off-by: Dmitry Torokhov dmitry.torokhov@gmail.com ---
v2: addressed Rafael's feedback by introducing device_set_driver() helper that does WRITE_ONCE() to prevent tearing.
I added Cc: stable however I do not think we need to worry too much about backporting it to [very] old kernels: the race window is very small, and in real life we do not unbind devices that often.
I believe there are more questionable places where we read dev->driver pointer, those need to be adjusted separately.
drivers/base/base.h | 18 ++++++++++++++++++ drivers/base/bus.c | 2 +- drivers/base/core.c | 34 +++++++++++++++++++++++++++++++--- drivers/base/dd.c | 7 +++---- 4 files changed, 53 insertions(+), 8 deletions(-)
diff --git a/drivers/base/base.h b/drivers/base/base.h index 8cf04a557bdb..ed2d7ccc7354 100644 --- a/drivers/base/base.h +++ b/drivers/base/base.h @@ -73,6 +73,7 @@ static inline void subsys_put(struct subsys_private *sp) kset_put(&sp->subsys); }
+struct subsys_private *bus_to_subsys(const struct bus_type *bus); struct subsys_private *class_to_subsys(const struct class *class);
struct driver_private { @@ -179,6 +180,23 @@ int driver_add_groups(const struct device_driver *drv, const struct attribute_gr void driver_remove_groups(const struct device_driver *drv, const struct attribute_group **groups); void device_driver_detach(struct device *dev);
+static inline void device_set_driver(struct device *dev, const struct device_driver *drv) +{ + + /* + * Majority (all?) read accesses to dev->driver happens either + * while holding device lock or in bus/driver code that is only + * invoked when the device is bound to a driver and there is no + * concern of the pointer being changed while it is being read. + * However when reading device's uevent file we read driver pointer + * without taking device lock (so we do not block there for + * arbitrary amount of time). We use WRITE_ONCE() here to prevent + * tearing so that READ_ONCE() can safely be used in uevent code. + */ + // FIXME - this cast should not be needed "soon" + WRITE_ONCE(dev->driver, (struct device_driver *)drv); +} + int devres_release_all(struct device *dev); void device_block_probing(void); void device_unblock_probing(void); diff --git a/drivers/base/bus.c b/drivers/base/bus.c index 6b9e65a42cd2..c8c7e0804024 100644 --- a/drivers/base/bus.c +++ b/drivers/base/bus.c @@ -57,7 +57,7 @@ static int __must_check bus_rescan_devices_helper(struct device *dev, * NULL. A call to subsys_put() must be done when finished with the pointer in * order for it to be properly freed. */ -static struct subsys_private *bus_to_subsys(const struct bus_type *bus) +struct subsys_private *bus_to_subsys(const struct bus_type *bus) { struct subsys_private *sp = NULL; struct kobject *kobj; diff --git a/drivers/base/core.c b/drivers/base/core.c index 9f4d4868e3b4..27fe69d06765 100644 --- a/drivers/base/core.c +++ b/drivers/base/core.c @@ -2623,6 +2623,34 @@ static const char *dev_uevent_name(const struct kobject *kobj) return NULL; }
+/* + * Try filling "DRIVER=<name>" uevent variable for a device. Because this + * function may race with binding and unbinding device from a driver we need to + * be careful. Binding is generally safe, at worst we miss the fact that device + * is already bound to a driver (but the driver information that is delivered + * through uevents is best-effort, it may become obsolete as soon as it is + * generated anyways). Unbinding is more risky as driver transitioning to NULL, + * so READ_ONCE() should be used to make sure we are dealing with the same + * pointer, and to ensure that driver structure is not going to disappear from + * under us we take bus' drivers klist lock. The assumption that only registered + * driver can be bound to a device, and to unregister a driver bus code will + * take the same lock. + */ +static void dev_driver_uevent(const struct device *dev, struct kobj_uevent_env *env) +{ + struct subsys_private *sp = bus_to_subsys(dev->bus); + + if (sp) { + scoped_guard(spinlock, &sp->klist_drivers.k_lock) { + struct device_driver *drv = READ_ONCE(dev->driver); + if (drv) + add_uevent_var(env, "DRIVER=%s", drv->name); + } + + subsys_put(sp); + } +} + static int dev_uevent(const struct kobject *kobj, struct kobj_uevent_env *env) { const struct device *dev = kobj_to_dev(kobj); @@ -2654,8 +2682,8 @@ static int dev_uevent(const struct kobject *kobj, struct kobj_uevent_env *env) if (dev->type && dev->type->name) add_uevent_var(env, "DEVTYPE=%s", dev->type->name);
- if (dev->driver) - add_uevent_var(env, "DRIVER=%s", dev->driver->name); + /* Add "DRIVER=%s" variable if the device is bound to a driver */ + dev_driver_uevent(dev, env);
/* Add common DT information about the device */ of_device_uevent(dev, env); @@ -3696,7 +3724,7 @@ int device_add(struct device *dev) device_pm_remove(dev); dpm_sysfs_remove(dev); DPMError: - dev->driver = NULL; + device_set_driver(dev, NULL); bus_remove_device(dev); BusError: device_remove_attrs(dev); diff --git a/drivers/base/dd.c b/drivers/base/dd.c index f0e4b4aba885..b526e0e0f52d 100644 --- a/drivers/base/dd.c +++ b/drivers/base/dd.c @@ -550,7 +550,7 @@ static void device_unbind_cleanup(struct device *dev) arch_teardown_dma_ops(dev); kfree(dev->dma_range_map); dev->dma_range_map = NULL; - dev->driver = NULL; + device_set_driver(dev, NULL); dev_set_drvdata(dev, NULL); if (dev->pm_domain && dev->pm_domain->dismiss) dev->pm_domain->dismiss(dev); @@ -629,8 +629,7 @@ static int really_probe(struct device *dev, const struct device_driver *drv) }
re_probe: - // FIXME - this cast should not be needed "soon" - dev->driver = (struct device_driver *)drv; + device_set_driver(dev, drv);
/* If using pinctrl, bind pins now before probing */ ret = pinctrl_bind_pins(dev); @@ -1014,7 +1013,7 @@ static int __device_attach(struct device *dev, bool allow_async) if (ret == 0) ret = 1; else { - dev->driver = NULL; + device_set_driver(dev, NULL); ret = 0; } } else {