[PATCH] comedi: Reinit dev->spinlock between attachments to low-level drivers

[Ian Abbott]

`struct comedi_device` is the main controlling structure for a COMEDI
device created by the COMEDI subsystem.  It contains a member `spinlock`
containing a spin-lock that is initialized by the COMEDI subsystem, but
is reserved for use by a low-level driver attached to the COMEDI device
(at least since commit 25436dc9d84f ("Staging: comedi: remove RT
code")).

Some COMEDI devices (those created on initialization of the COMEDI
subsystem when the "comedi.comedi_num_legacy_minors" parameter is
non-zero) can be attached to different low-level drivers over their
lifetime using the `COMEDI_DEVCONFIG` ioctl command.  This can result in
inconsistent lock states being reported when there is a mismatch in the
spin-lock locking levels used by each low-level driver to which the
COMEDI device has been attached.  Fix it by reinitializing
`dev->spinlock` before calling the low-level driver's `attach` function
pointer if `CONFIG_LOCKDEP` is enabled.

Reported-by: syzbot+cc9f7f4a7df09f53c4a4@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=cc9f7f4a7df09f53c4a4
Fixes: ed9eccbe8970 ("Staging: add comedi core")
Signed-off-by: Ian Abbott <abbotti@xxxxxxxxx>
---
 drivers/comedi/drivers.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/comedi/drivers.c b/drivers/comedi/drivers.c
index db225a3bf012..5ab96b5eefd1 100644
--- a/drivers/comedi/drivers.c
+++ b/drivers/comedi/drivers.c
@@ -1063,6 +1063,14 @@ int comedi_device_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 		ret = -EIO;
 		goto out;
 	}
+	if (IS_ENABLED(CONFIG_LOCKDEP)) {
+		/*
+		 * dev->spinlock is for private use by the attached low-level
+		 * driver.  Reinitialize it to stop lock-dependency tracking
+		 * between attachments to different low-level drivers.
+		 */
+		spin_lock_init(&dev->spinlock);
+	}
 	dev->driver = driv;
 	dev->board_name = dev->board_ptr ? *(const char **)dev->board_ptr
 					 : dev->driver->driver_name;
-- 
2.51.0