staging: comedi: comedi_test: fix timer race conditions

Commit 73e0e4df ("staging: comedi: comedi_test: fix timer lock-up")
fixed a lock-up in the timer routine `waveform_ai_timer()` (which was
called `waveform_ai_interrupt()` at the time) caused by
commit 24051247 ("staging: comedi: comedi_test: use
comedi_handle_events()").  However, it introduced a race condition that
can result in the timer routine misbehaving, such as accessing freed
memory or dereferencing a NULL pointer.

73e0... changed the timer routine to do nothing unless a
`WAVEFORM_AI_RUNNING` flag was set, and changed `waveform_ai_cancel()`
to clear the flag and replace a call to `del_timer_sync()` with a call
to `del_timer()`.  `waveform_ai_cancel()` may be called from the timer
routine itself (via `comedi_handle_events()`), or from `do_cancel()`.
(`do_cancel()` is called as a result of a file operation (usually a
`COMEDI_CANCEL` ioctl command, or a release), or during device removal.)
When called from `do_cancel()`, the call to `waveform_ai_cancel()` is
followed by a call to `do_become_nonbusy()`, which frees up stuff for
the current asynchronous command under the assumption that it is now
safe to do so.  The race condition occurs when the timer routine
`waveform_ai_timer()` checks the `WAVEFORM_AI_RUNNING` flag just before
it is cleared by `waveform_ai_cancel()`, and is still running during the
call to `do_become_nonbusy()`.  In particular, it can lead to a NULL
pointer dereference:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffffc0c63add>] waveform_ai_timer+0x17d/0x290 [comedi_test]

That corresponds to this line in `waveform_ai_timer()`:

		unsigned int chanspec = cmd->chanlist[async->cur_chan];

but `do_become_nonbusy()` frees `cmd->chanlist` and sets it to `NULL`.

Fix the race by calling `del_timer_sync()` instead of `del_timer()` in
`waveform_ai_cancel()` when not in an interrupt context.  The only time
`waveform_ai_cancel()` is called in an interrupt context is when it is
called from the timer routine itself, via `comedi_handle_events()`.

There is no longer any need for the `WAVEFORM_AI_RUNNING` flag, so get
rid of it.

The bug was copied from the AI subdevice to the AO when support for
commands on the AO subdevice was added by commit 0cf55bbe ("staging:
comedi: comedi_test: implement commands on AO subdevice").  That
involves the timer routine `waveform_ao_timer()`, the comedi "cancel"
routine `waveform_ao_cancel()`, and the flag `WAVEFORM_AO_RUNNING`.  Fix
it in the same way as for the AI subdevice.

Fixes: 73e0e4df ("staging: comedi: comedi_test: fix timer lock-up")
Fixes: 0cf55bbe ("staging: comedi: comedi_test: implement commands
 on AO subdevice")
Reported-by: Éric Piel <piel@delmic.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: <stable@vger.kernel.org> # 4.4+
Cc: Éric Piel <piel@delmic.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/comedi_test.c

@@ -56,11 +56,6 @@
 
 #define N_CHANS 8
 
-enum waveform_state_bits {
-	WAVEFORM_AI_RUNNING,
-	WAVEFORM_AO_RUNNING
-};
-
 /* Data unique to this driver */
 struct waveform_private {
 	struct timer_list ai_timer;	/* timer for AI commands */
@@ -68,7 +63,6 @@ struct waveform_private {
 	unsigned int wf_amplitude;	/* waveform amplitude in microvolts */
 	unsigned int wf_period;		/* waveform period in microseconds */
 	unsigned int wf_current;	/* current time in waveform period */
-	unsigned long state_bits;
 	unsigned int ai_scan_period;	/* AI scan period in usec */
 	unsigned int ai_convert_period;	/* AI conversion period in usec */
 	struct timer_list ao_timer;	/* timer for AO commands */
@@ -191,10 +185,6 @@ static void waveform_ai_timer(unsigned long arg)
 	unsigned int nsamples;
 	unsigned int time_increment;
 
-	/* check command is still active */
-	if (!test_bit(WAVEFORM_AI_RUNNING, &devpriv->state_bits))
-		return;
-
 	now = ktime_to_us(ktime_get());
 	nsamples = comedi_nsamples_left(s, UINT_MAX);
 
@@ -386,11 +376,6 @@ static int waveform_ai_cmd(struct comedi_device *dev,
 	 */
 	devpriv->ai_timer.expires =
 		jiffies + usecs_to_jiffies(devpriv->ai_convert_period) + 1;
-
-	/* mark command as active */
-	smp_mb__before_atomic();
-	set_bit(WAVEFORM_AI_RUNNING, &devpriv->state_bits);
-	smp_mb__after_atomic();
 	add_timer(&devpriv->ai_timer);
 	return 0;
 }
@@ -400,11 +385,12 @@ static int waveform_ai_cancel(struct comedi_device *dev,
 {
 	struct waveform_private *devpriv = dev->private;
 
-	/* mark command as no longer active */
-	clear_bit(WAVEFORM_AI_RUNNING, &devpriv->state_bits);
-	smp_mb__after_atomic();
-	/* cannot call del_timer_sync() as may be called from timer routine */
+	if (in_softirq()) {
+		/* Assume we were called from the timer routine itself. */
 		del_timer(&devpriv->ai_timer);
+	} else {
+		del_timer_sync(&devpriv->ai_timer);
+	}
 	return 0;
 }
 
@@ -436,10 +422,6 @@ static void waveform_ao_timer(unsigned long arg)
 	u64 scans_since;
 	unsigned int scans_avail = 0;
 
-	/* check command is still active */
-	if (!test_bit(WAVEFORM_AO_RUNNING, &devpriv->state_bits))
-		return;
-
 	/* determine number of scan periods since last time */
 	now = ktime_to_us(ktime_get());
 	scans_since = now - devpriv->ao_last_scan_time;
@@ -518,11 +500,6 @@ static int waveform_ao_inttrig_start(struct comedi_device *dev,
 	devpriv->ao_last_scan_time = ktime_to_us(ktime_get());
 	devpriv->ao_timer.expires =
 		jiffies + usecs_to_jiffies(devpriv->ao_scan_period);
-
-	/* mark command as active */
-	smp_mb__before_atomic();
-	set_bit(WAVEFORM_AO_RUNNING, &devpriv->state_bits);
-	smp_mb__after_atomic();
 	add_timer(&devpriv->ao_timer);
 
 	return 1;
@@ -608,11 +585,12 @@ static int waveform_ao_cancel(struct comedi_device *dev,
 	struct waveform_private *devpriv = dev->private;
 
 	s->async->inttrig = NULL;
-	/* mark command as no longer active */
-	clear_bit(WAVEFORM_AO_RUNNING, &devpriv->state_bits);
-	smp_mb__after_atomic();
-	/* cannot call del_timer_sync() as may be called from timer routine */
+	if (in_softirq()) {
+		/* Assume we were called from the timer routine itself. */
 		del_timer(&devpriv->ao_timer);
+	} else {
+		del_timer_sync(&devpriv->ao_timer);
+	}
 	return 0;
 }