6.6-stable review patch. If anyone has any objections, please let me know.
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From: Ian Abbott abbotti@mev.co.uk
commit 35b6fc51c666fc96355be5cd633ed0fe4ccf68b2 upstream.
syzbot reports a use-after-free in comedi in the below link, which is due to comedi gladly removing the allocated async area even though poll requests are still active on the wait_queue_head inside of it. This can cause a use-after-free when the poll entries are later triggered or removed, as the memory for the wait_queue_head has been freed. We need to check there are no tasks queued on any of the subdevices' wait queues before allowing the device to be detached by the `COMEDI_DEVCONFIG` ioctl.
Tasks will read-lock `dev->attach_lock` before adding themselves to the subdevice wait queue, so fix the problem in the `COMEDI_DEVCONFIG` ioctl handler by write-locking `dev->attach_lock` before checking that all of the subdevices are safe to be deleted. This includes testing for any sleepers on the subdevices' wait queues. It remains locked until the device has been detached. This requires the `comedi_device_detach()` function to be refactored slightly, moving the bulk of it into new function `comedi_device_detach_locked()`.
Note that the refactor of `comedi_device_detach()` results in `comedi_device_cancel_all()` now being called while `dev->attach_lock` is write-locked, which wasn't the case previously, but that does not matter.
Thanks to Jens Axboe for diagnosing the problem and co-developing this patch.
Cc: stable stable@kernel.org Fixes: 2f3fdcd7ce93 ("staging: comedi: add rw_semaphore to protect against device detachment") Link: https://lore.kernel.org/all/687bd5fe.a70a0220.693ce.0091.GAE@google.com/ Reported-by: syzbot+01523a0ae5600aef5895@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=01523a0ae5600aef5895 Co-developed-by: Jens Axboe axboe@kernel.dk Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Ian Abbott abbotti@mev.co.uk Tested-by: Jens Axboe axboe@kernel.dk Link: https://lore.kernel.org/r/20250722155316.27432-1-abbotti@mev.co.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/comedi/comedi_fops.c | 33 +++++++++++++++++++++++++-------- drivers/comedi/comedi_internal.h | 1 + drivers/comedi/drivers.c | 13 ++++++++++--- 3 files changed, 36 insertions(+), 11 deletions(-)
--- a/drivers/comedi/comedi_fops.c +++ b/drivers/comedi/comedi_fops.c @@ -787,6 +787,7 @@ static int is_device_busy(struct comedi_ struct comedi_subdevice *s; int i;
+ lockdep_assert_held_write(&dev->attach_lock); lockdep_assert_held(&dev->mutex); if (!dev->attached) return 0; @@ -795,7 +796,16 @@ static int is_device_busy(struct comedi_ s = &dev->subdevices[i]; if (s->busy) return 1; - if (s->async && comedi_buf_is_mmapped(s)) + if (!s->async) + continue; + if (comedi_buf_is_mmapped(s)) + return 1; + /* + * There may be tasks still waiting on the subdevice's wait + * queue, although they should already be about to be removed + * from it since the subdevice has no active async command. + */ + if (wq_has_sleeper(&s->async->wait_head)) return 1; }
@@ -825,15 +835,22 @@ static int do_devconfig_ioctl(struct com return -EPERM;
if (!arg) { - if (is_device_busy(dev)) - return -EBUSY; - if (dev->attached) { - struct module *driver_module = dev->driver->module; + int rc = 0;
- comedi_device_detach(dev); - module_put(driver_module); + if (dev->attached) { + down_write(&dev->attach_lock); + if (is_device_busy(dev)) { + rc = -EBUSY; + } else { + struct module *driver_module = + dev->driver->module; + + comedi_device_detach_locked(dev); + module_put(driver_module); + } + up_write(&dev->attach_lock); } - return 0; + return rc; }
if (copy_from_user(&it, arg, sizeof(it))) --- a/drivers/comedi/comedi_internal.h +++ b/drivers/comedi/comedi_internal.h @@ -50,6 +50,7 @@ extern struct mutex comedi_drivers_list_ int insn_inval(struct comedi_device *dev, struct comedi_subdevice *s, struct comedi_insn *insn, unsigned int *data);
+void comedi_device_detach_locked(struct comedi_device *dev); void comedi_device_detach(struct comedi_device *dev); int comedi_device_attach(struct comedi_device *dev, struct comedi_devconfig *it); --- a/drivers/comedi/drivers.c +++ b/drivers/comedi/drivers.c @@ -158,7 +158,7 @@ static void comedi_device_detach_cleanup int i; struct comedi_subdevice *s;
- lockdep_assert_held(&dev->attach_lock); + lockdep_assert_held_write(&dev->attach_lock); lockdep_assert_held(&dev->mutex); if (dev->subdevices) { for (i = 0; i < dev->n_subdevices; i++) { @@ -195,16 +195,23 @@ static void comedi_device_detach_cleanup comedi_clear_hw_dev(dev); }
-void comedi_device_detach(struct comedi_device *dev) +void comedi_device_detach_locked(struct comedi_device *dev) { + lockdep_assert_held_write(&dev->attach_lock); lockdep_assert_held(&dev->mutex); comedi_device_cancel_all(dev); - down_write(&dev->attach_lock); dev->attached = false; dev->detach_count++; if (dev->driver) dev->driver->detach(dev); comedi_device_detach_cleanup(dev); +} + +void comedi_device_detach(struct comedi_device *dev) +{ + lockdep_assert_held(&dev->mutex); + down_write(&dev->attach_lock); + comedi_device_detach_locked(dev); up_write(&dev->attach_lock); }