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Subject[ 024/128] staging: comedi: comedi_test: fix race when cancelling command
3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ian Abbott <abbotti@mev.co.uk>

commit c0729eeefdcd76db338f635162bf0739fd2c5f6f upstream.

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: Ian Abbott <abbotti@mev.co.uk>
Reported-by: Éric Piel <piel@delmic.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/staging/comedi/drivers/comedi_test.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/staging/comedi/drivers/comedi_test.c
+++ b/drivers/staging/comedi/drivers/comedi_test.c
@@ -461,7 +461,7 @@ static int waveform_ai_cancel(struct com
struct comedi_subdevice *s)
{
devpriv->timer_running = 0;
- del_timer(&devpriv->timer);
+ del_timer_sync(&devpriv->timer);
return 0;
}


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