staging: comedi: comedi_test: fix timer race conditions

[ commit 403fe7f3 upstream.

  Patch is cut down a bit from upstream patch, as upstream patch fixed
  parts that don't exist in this version.  Also, some identifiers were
  renamed, so patch description has been edited accordingly
  -- Ian Abbott
]

Commit 73e0e4df ("staging: comedi: comedi_test: fix timer lock-up")
fixed a lock-up in the timer routine `waveform_ai_interrupt()` 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_interrupt()` 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 because `do_become_nonbusy()` frees
`async->cmd.chanlist` and sets it to `NULL`, but
`waveform_ai_interrupt()` dereferences it.

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.

Fixes: 73e0e4df ("staging: comedi: comedi_test: fix timer lock-up")
Reported-by: Éric Piel <piel@delmic.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: Éric Piel <piel@delmic.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>

drivers/staging/comedi/drivers/comedi_test.c

@@ -55,10 +55,6 @@ zero volts).
 
 #define N_CHANS 8
 
-enum waveform_state_bits {
-	WAVEFORM_AI_RUNNING = 0
-};
-
 /* Data unique to this driver */
 struct waveform_private {
 	struct timer_list timer;
@@ -67,7 +63,6 @@ struct waveform_private {
 	unsigned long usec_period;	/* waveform period in microseconds */
 	unsigned long usec_current;	/* current time (mod waveform period) */
 	unsigned long usec_remainder;	/* usec since last scan */
-	unsigned long state_bits;
 	unsigned int scan_period;	/* scan period in usec */
 	unsigned int convert_period;	/* conversion period in usec */
 	unsigned int ao_loopbacks[N_CHANS];
@@ -177,10 +172,6 @@ static void waveform_ai_interrupt(unsigned long arg)
 	unsigned int num_scans;
 	ktime_t now;
 
-	/* check command is still active */
-	if (!test_bit(WAVEFORM_AI_RUNNING, &devpriv->state_bits))
-		return;
-
 	now = ktime_get();
 
 	elapsed_time = ktime_to_us(ktime_sub(now, devpriv->last));
@@ -322,10 +313,6 @@ static int waveform_ai_cmd(struct comedi_device *dev,
 	devpriv->usec_remainder = 0;
 
 	devpriv->timer.expires = jiffies + 1;
-	/* mark command as active */
-	smp_mb__before_atomic();
-	set_bit(WAVEFORM_AI_RUNNING, &devpriv->state_bits);
-	smp_mb__after_atomic();
 	add_timer(&devpriv->timer);
 	return 0;
 }
@@ -335,11 +322,12 @@ 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->timer);
+	} else {
+		del_timer_sync(&devpriv->timer);
+	}
 	return 0;
 }