From fd514089133a2a6cb98d4a615237015cd8075e12 Mon Sep 17 00:00:00 2001 From: Ian Abbott Date: Thu, 30 Jun 2016 19:58:32 +0100 Subject: [PATCH] staging: comedi: comedi_test: fix timer race conditions MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit commit 403fe7f34e3327ddac2e06a15e76a293d613381e upstream. Commit 73e0e4dfed4c ("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 240512474424 ("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: [] 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 0cf55bbef2f9 ("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: 73e0e4dfed4c ("staging: comedi: comedi_test: fix timer lock-up") Fixes: 0cf55bbef2f9 ("staging: comedi: comedi_test: implement commands on AO subdevice") Reported-by: Éric Piel Signed-off-by: Ian Abbott Cc: Éric Piel Signed-off-by: Greg Kroah-Hartman --- drivers/staging/comedi/drivers/comedi_test.c | 46 +++++--------------- 1 file changed, 12 insertions(+), 34 deletions(-) diff --git a/drivers/staging/comedi/drivers/comedi_test.c b/drivers/staging/comedi/drivers/comedi_test.c index 4ab186669f0c..ec5b9a23494d 100644 --- a/drivers/staging/comedi/drivers/comedi_test.c +++ b/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 */ - del_timer(&devpriv->ai_timer); + 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 */ - del_timer(&devpriv->ao_timer); + 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; } -- 2.20.1