driver core: get_dev_from_fwnode(): document potential race

Commit 9a4681a485ee ("driver core: Export get_dev_from_fwnode()") made
get_dev_from_fwnode() publicly available, but didn't document the
guarantees a caller must uphold:

get_dev_from_fwnode() obtains a reference count from the device pointer
stored in a struct fwnode_handle. While having its own reference count,
struct fwnode_handle does not keep a reference count of the device it
has a pointer to.

Consequently, a caller must guarantee that it is impossible that the
last device reference is dropped and the device is released concurrently
while calling get_dev_from_fwnode(), otherwise this is a potential UAF
and hence a bug.

Thus, document this potential race condition for get_dev_from_fwnode().

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Saravana Kannan <saravanak@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250829205911.33142-1-dakr@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/drivers/base/core.c b/drivers/base/core.c
index d22d6b23e758..cd806be435cd 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -5278,6 +5278,25 @@ void device_set_node(struct device *dev, struct fwnode_handle *fwnode)
 }
 EXPORT_SYMBOL_GPL(device_set_node);
 
+/**
+ * get_dev_from_fwnode - Obtain a reference count of the struct device the
+ * struct fwnode_handle is associated with.
+ * @fwnode: The pointer to the struct fwnode_handle to obtain the struct device
+ * reference count of.
+ *
+ * This function obtains a reference count of the device the device pointer
+ * embedded in the struct fwnode_handle points to.
+ *
+ * Note that the struct device pointer embedded in struct fwnode_handle does
+ * *not* have a reference count of the struct device itself.
+ *
+ * Hence, it is a UAF (and thus a bug) to call this function if the caller can't
+ * guarantee that the last reference count of the corresponding struct device is
+ * not dropped concurrently.
+ *
+ * This is possible since struct fwnode_handle has its own reference count and
+ * hence can out-live the struct device it is associated with.
+ */
 struct device *get_dev_from_fwnode(struct fwnode_handle *fwnode)
 {
 	return get_device((fwnode)->dev);