[ 3.5.y.z extended stable ] Patch "staging: comedi: comedi_test: fix race when cancelling" has been added to staging queue

[Herton Ronaldo Krzesinski]

This is a note to let you know that I have just added a patch titled

    staging: comedi: comedi_test: fix race when cancelling

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Herton

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>From ccf267832d9fac3ad7dfae552ad7b757c6ab88dc Mon Sep 17 00:00:00 2001
From: Ian Abbott <abbotti at mev.co.uk>
Date: Fri, 4 Jan 2013 11:33:21 +0000
Subject: [PATCH] staging: comedi: comedi_test: fix race when cancelling
 command
MIME-Version: 1.0
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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 at mev.co.uk>
Reported-by: Éric Piel <piel at delmic.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
[ herton: adjust context ]
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.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