linux-kernel - [PATCH 29/74] staging: comedi: comedi_test: fix race when cancelling command

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Date:	Thu, 24 Jan 2013 01:26:41 -0200
From:	Herton Ronaldo Krzesinski <herton.krzesinski@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Ian Abbott <abbotti@....co.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Herton Ronaldo Krzesinski <herton.krzesinski@...onical.com>
Subject: [PATCH 29/74] staging: comedi: comedi_test: fix race when cancelling command

3.5.7.4 -stable review patch.  If anyone has any objections, please let me know.

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

From: Ian Abbott <abbotti@....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@....co.uk>
Reported-by: Éric Piel <piel@...mic.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
[ herton: adjust context ]
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@...onical.com>
---
 drivers/staging/comedi/drivers/comedi_test.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/staging/comedi/drivers/comedi_test.c b/drivers/staging/comedi/drivers/comedi_test.c
index 873e374..806c397 100644
--- a/drivers/staging/comedi/drivers/comedi_test.c
+++ b/drivers/staging/comedi/drivers/comedi_test.c
@@ -398,7 +398,7 @@ static int waveform_ai_cancel(struct comedi_device *dev,
 			      struct comedi_subdevice *s)
 {
 	devpriv->timer_running = 0;
-	del_timer(&devpriv->timer);
+	del_timer_sync(&devpriv->timer);
 	return 0;
 }

-- 
1.7.9.5

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