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driver core: fix potential NULL pointer dereference in dev_uevent()

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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.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/r/20250311052417.1846985-3-dmitry.torokhov@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Dmitry Torokhov 2025-03-10 22:24:16 -07:00 • committed by Greg Kroah-Hartman
parent 04d3e5461c
commit 18daa52418
3 changed files with 44 additions and 4 deletions
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13
drivers/base/base.h
View file

@ -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 {
@ -182,8 +183,18 @@ 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"
dev->driver = (struct device_driver *)drv;
WRITE_ONCE(dev->driver, (struct device_driver *)drv);
}
int devres_release_all(struct device *dev);

2
drivers/base/bus.c
View file

@ -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;

33
drivers/base/core.c
View file

@ -2624,6 +2624,35 @@ 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 the device from a driver,
* we need to be careful. Binding is generally safe, at worst we miss the
* fact that the 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 pointer is 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);
@ -2655,8 +2684,8 @@ 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);