[lvc-project] [PATCH] comedi: drivers: do not detach device if driv->attach() fails

[Nikita Zhandarovich]

Syzbot identified an issue [1] in comedi_device_attach() that occurs
when kernel attempts to detach a comedi device via
comedi_device_detach() even if a driver-specific attach() method
already failed. Attempts to follow through with detaching the
device and unregistering the driver trigger a warning.

Fix this by rearranging cleanup calls so that comedi_device_detach()
runs only if the device in question has been successfully attached.

Original idea for this patch belongs to Hillf Danton
<hdanton at sina.com>.

[1] Syzbot crash:
Unexpected driver unregister!
WARNING: CPU: 0 PID: 5970 at drivers/base/driver.c:273 driver_unregister drivers/base/driver.c:273 [inline]
WARNING: CPU: 0 PID: 5970 at drivers/base/driver.c:273 driver_unregister+0x90/0xb0 drivers/base/driver.c:270
...
Call Trace:
 <TASK>
 comedi_device_detach_locked+0x12f/0xa50 drivers/comedi/drivers.c:207
 comedi_device_detach+0x67/0xb0 drivers/comedi/drivers.c:215
 comedi_device_attach+0x43d/0x900 drivers/comedi/drivers.c:1011
 do_devconfig_ioctl+0x1b1/0x710 drivers/comedi/comedi_fops.c:872
 comedi_unlocked_ioctl+0x165d/0x2f00 drivers/comedi/comedi_fops.c:2178
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
...

Reported-by: syzbot+6616bba359cec7a1def1 at syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6616bba359cec7a1def1
Suggested-by: Hillf Danton <hdanton at sina.com>
Fixes: 74ece108f9e5 ("staging: comedi: move detach out of post-config")
Cc: stable at vger.kernel.org
Signed-off-by: Nikita Zhandarovich <n.zhandarovich at fintech.ru>
---
 drivers/comedi/drivers.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/comedi/drivers.c b/drivers/comedi/drivers.c
index c9ebaadc5e82..001083f96138 100644
--- a/drivers/comedi/drivers.c
+++ b/drivers/comedi/drivers.c
@@ -1005,10 +1005,13 @@ int comedi_device_attach(struct comedi_device *dev, struct comedi_devconfig *it)
 	dev->board_name = dev->board_ptr ? *(const char **)dev->board_ptr
 					 : dev->driver->driver_name;
 	ret = driv->attach(dev, it);
-	if (ret >= 0)
+	if (ret >= 0) {
 		ret = comedi_device_postconfig(dev);
-	if (ret < 0) {
-		comedi_device_detach(dev);
+		if (ret < 0) {
+			comedi_device_detach(dev);
+			module_put(driv->module);
+		}
+	} else {
 		module_put(driv->module);
 	}
 	/* On success, the driver module count has been incremented. */