The "comedi_bond" driver allows a composite COMEDI device to be built up
from the subdevices of other COMEDI devices, although it currently only
supports digital I/O subdevices. Although it checks that it is not
trying to bind to itself, it is possible to end up with a cycle of
"comedi_bond" devices bound to each other. For example:
1. Configure /dev/comedi0 to use some COMEDI hardware device with
digital I/O subdevices, but not a "comedi_bond" device.
2. Configure /dev/comedi1 as a "comedi_bond" device bound to
/dev/comedi0.
3. Unconfigure /dev/comedi0 and reconfigure it as a "comedi_bond" device
bound to /dev/comedi1.
Now we have /dev/comedi0 and /dev/comedi1 bound in a cycle. When an
operation is performed on the digital I/O subdevice of /dev/comedi0 for
example, it will try and perform the operation on /dev/comedi1, which
will try and perform the operation on /dev/comedi0. The task will end
up deadlocked trying to lock /dev/comedi0's mutex which it has already
locked.
I discovered that possibility while investigating fix sysbot crash
https://syzkaller.appspot.com/bug?extid=4a6138c17a47937dcea1 ("possible
deadlock in comedi_do_insn"), but I think that report may be a false
positive.
To avoid that, replace the calls to `comedi_open()` and `comedi_close()`
in "kcomedilib" with calls to `comedi_open_from()` and
`comedi_close_from()`. These take an extra parameter that indicates the
COMEDI minor device number from which the open or close is being
performed. `comedi_open_from()` will refuse to open the device if doing
so would result in a cycle. The cycle detection depends on the extra
parameter having the correct value for this device and also for existing
devices in the chain.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
---
drivers/comedi/drivers/comedi_bond.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/comedi/drivers/comedi_bond.c b/drivers/comedi/drivers/comedi_bond.c
index 78c39fa84177..30650fa36fff 100644
--- a/drivers/comedi/drivers/comedi_bond.c
+++ b/drivers/comedi/drivers/comedi_bond.c
@@ -205,7 +205,7 @@ static int do_dev_config(struct comedi_device *dev, struct comedi_devconfig *it)
snprintf(file, sizeof(file), "/dev/comedi%d", minor);
file[sizeof(file) - 1] = 0;
- d = comedi_open(file);
+ d = comedi_open_from(file, dev->minor);
if (!d) {
dev_err(dev->class_dev,
@@ -326,7 +326,7 @@ static void bonding_detach(struct comedi_device *dev)
if (!bdev)
continue;
if (!test_and_set_bit(bdev->minor, devs_closed))
- comedi_close(bdev->dev);
+ comedi_close_from(bdev->dev, dev->minor);
kfree(bdev);
}
kfree(devpriv->devs);
--
2.51.0