Commit c0729eee authored by Ian Abbott's avatar Ian Abbott Committed by Greg Kroah-Hartman

staging: comedi: comedi_test: fix race when cancelling command

Éric Piel reported a kernel oops in the "comedi_test" module.  It was a
NULL pointer dereference within `waveform_ai_interrupt()` (actually a
timer function) that sometimes occurred when a running asynchronous
command is cancelled (either by the `COMEDI_CANCEL` ioctl or by closing
the device file).

This seems to be a race between the caller of `waveform_ai_cancel()`
which on return from that function goes and tears down the running
command, and the timer function which uses the command.  In particular,
`async->cmd.chanlist` gets freed (and the pointer set to NULL) by
`do_become_nonbusy()` in "comedi_fops.c" but a previously scheduled
`waveform_ai_interrupt()` timer function will dereference that pointer
regardless, leading to the oops.

Fix it by replacing the `del_timer()` call in `waveform_ai_cancel()`
with `del_timer_sync()`.
Signed-off-by: default avatarIan Abbott <abbotti@mev.co.uk>
Reported-by: default avatarÉric Piel <piel@delmic.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 34ffb33e
...@@ -345,7 +345,7 @@ static int waveform_ai_cancel(struct comedi_device *dev, ...@@ -345,7 +345,7 @@ static int waveform_ai_cancel(struct comedi_device *dev,
struct waveform_private *devpriv = dev->private; struct waveform_private *devpriv = dev->private;
devpriv->timer_running = 0; devpriv->timer_running = 0;
del_timer(&devpriv->timer); del_timer_sync(&devpriv->timer);
return 0; return 0;
} }
......
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