[PATCH] driver core: use READ_ONCE() for dev->driver in dev_has_sync_state()

[Danilo Krummrich <dakr@kernel.org>]

dev_has_sync_state() reads dev->driver twice without holding
device_lock() -- once for the NULL check and once to dereference
->sync_state. Some callers only hold device_links_write_lock, which
doesn't prevent a concurrent unbind from clearing dev->driver via
device_unbind_cleanup().

Fix it by reading dev->driver exactly once with READ_ONCE(), pairing
with the WRITE_ONCE() in device_set_driver().

Link: https://lore.kernel.org/driver-core/DHW8QPU1VU1F.3P6PH69HLFBYC@kernel.org/
Fixes: ac338acf514e ("driver core: Add dev_has_sync_state()")
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 include/linux/device.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/device.h b/include/linux/device.h
index ac972e7bead4..4c1c9cb8570a 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -1018,9 +1018,12 @@ static inline void device_lock_assert(struct device *dev)
 
 static inline bool dev_has_sync_state(struct device *dev)
 {
+	struct device_driver *drv;
+
 	if (!dev)
 		return false;
-	if (dev->driver && dev->driver->sync_state)
+	drv = READ_ONCE(dev->driver);
+	if (drv && drv->sync_state)
 		return true;
 	if (dev->bus && dev->bus->sync_state)
 		return true;

base-commit: 5b484311507b5d403c1f7a45f6aa3778549e268b
-- 
2.53.0