staging: comedi: fix circular locking dependency in comedi_mmap()

Mmapping a comedi data buffer with lockdep checking enabled produced the
following kernel debug messages:

======================================================
[ INFO: possible circular locking dependency detected ]
3.5.0-rc3-ija1+ torvalds#9 Tainted: G         C
-------------------------------------------------------
comedi_test/4160 is trying to acquire lock:
 (&dev->mutex#2){+.+.+.}, at: [<ffffffffa00313f4>] comedi_mmap+0x57/0x1d9 [comedi]

but task is already holding lock:
 (&mm->mmap_sem){++++++}, at: [<ffffffff810c96fe>] vm_mmap_pgoff+0x41/0x76

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> alexander-zimmermann#1 (&mm->mmap_sem){++++++}:
       [<ffffffff8106d0e8>] lock_acquire+0x97/0x105
       [<ffffffff810ce3bc>] might_fault+0x6d/0x90
       [<ffffffffa0031ffb>] do_devinfo_ioctl.isra.7+0x11e/0x14c [comedi]
       [<ffffffffa003227f>] comedi_unlocked_ioctl+0x256/0xe48 [comedi]
       [<ffffffff810f7fcd>] vfs_ioctl+0x18/0x34
       [<ffffffff810f87fd>] do_vfs_ioctl+0x382/0x43c
       [<ffffffff810f88f9>] sys_ioctl+0x42/0x65
       [<ffffffff81415c62>] system_call_fastpath+0x16/0x1b

-> #0 (&dev->mutex#2){+.+.+.}:
       [<ffffffff8106c528>] __lock_acquire+0x101d/0x1591
       [<ffffffff8106d0e8>] lock_acquire+0x97/0x105
       [<ffffffff8140c894>] mutex_lock_nested+0x46/0x2a4
       [<ffffffffa00313f4>] comedi_mmap+0x57/0x1d9 [comedi]
       [<ffffffff810d5816>] mmap_region+0x281/0x492
       [<ffffffff810d5c92>] do_mmap_pgoff+0x26b/0x2a7
       [<ffffffff810c971a>] vm_mmap_pgoff+0x5d/0x76
       [<ffffffff810d493f>] sys_mmap_pgoff+0xc7/0x10d
       [<ffffffff81004d36>] sys_mmap+0x16/0x20
       [<ffffffff81415c62>] system_call_fastpath+0x16/0x1b

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&mm->mmap_sem);
                               lock(&dev->mutex#2);
                               lock(&mm->mmap_sem);
  lock(&dev->mutex#2);

 *** DEADLOCK ***

To avoid the circular dependency, just try to get the lock in
`comedi_mmap()` instead of blocking.  Since the comedi device's main mutex
is heavily used, do a down-read of its `attach_lock` rwsemaphore
instead.  Trying to down-read `attach_lock` should only fail if
some task has down-write locked it, and that is only done while the
comedi device is being attached to or detached from a low-level hardware
device.

Unfortunately, acquiring the `attach_lock` doesn't prevent another
task replacing the comedi data buffer we are trying to mmap.  The
details of the buffer are held in a `struct comedi_buf_map` and pointed
to by `s->async->buf_map` where `s` is the comedi subdevice whose buffer
we are trying to map.  The `struct comedi_buf_map` is already reference
counted with a `struct kref`, so we can stop it being freed prematurely.

Modify `comedi_mmap()` to call new function
`comedi_buf_map_from_subdev_get()` to read the subdevice's current
buffer map pointer and increment its reference instead of accessing
`async->buf_map` directly.  Call `comedi_buf_map_put()` to decrement the
reference once the buffer map structure has been dealt with.  (Note that
`comedi_buf_map_put()` does nothing if passed a NULL pointer.)

`comedi_buf_map_from_subdev_get()` checks the subdevice's buffer map
pointer has been set and the buffer map has been initialized enough for
`comedi_mmap()` to deal with it (specifically, check the `n_pages`
member has been set to a non-zero value).  If all is well, the buffer
map's reference is incremented and a pointer to it is returned.  The
comedi subdevice's spin-lock is used to protect the checks.  Also use
the spin-lock in `__comedi_buf_alloc()` and `__comedi_buf_free()` to
protect changes to the subdevice's buffer map structure pointer and the
buffer map structure's `n_pages` member.  (This checking of `n_pages` is
a bit clunky and I [Ian Abbott] plan to deal with it in the future.)

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Cc: <stable@vger.kernel.org> # 3.14.x, 3.15.x
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ian-abbott

drivers/staging/comedi/comedi_buf.c

@@ -61,15 +61,20 @@ static void __comedi_buf_free(struct comedi_device *dev,
 			      struct comedi_subdevice *s)
 {
 	struct comedi_async *async = s->async;
+	struct comedi_buf_map *bm;
+	unsigned long flags;
 
 	if (async->prealloc_buf) {
 		vunmap(async->prealloc_buf);
 		async->prealloc_buf = NULL;
 		async->prealloc_bufsz = 0;
 	}
 
-	comedi_buf_map_put(async->buf_map);
+	spin_lock_irqsave(&s->spin_lock, flags);
+	bm = async->buf_map;
 	async->buf_map = NULL;
+	spin_unlock_irqrestore(&s->spin_lock, flags);
+	comedi_buf_map_put(bm);
 }
 
 static void __comedi_buf_alloc(struct comedi_device *dev,
@@ -80,6 +85,7 @@ static void __comedi_buf_alloc(struct comedi_device *dev,
 	struct page **pages = NULL;
 	struct comedi_buf_map *bm;
 	struct comedi_buf_page *buf;
+	unsigned long flags;
 	unsigned i;
 
 	if (!IS_ENABLED(CONFIG_HAS_DMA) && s->async_dma_dir != DMA_NONE) {
@@ -92,8 +98,10 @@ static void __comedi_buf_alloc(struct comedi_device *dev,
 	if (!bm)
 		return;
 
-	async->buf_map = bm;
 	kref_init(&bm->refcount);
+	spin_lock_irqsave(&s->spin_lock, flags);
+	async->buf_map = bm;
+	spin_unlock_irqrestore(&s->spin_lock, flags);
 	bm->dma_dir = s->async_dma_dir;
 	if (bm->dma_dir != DMA_NONE)
 		/* Need ref to hardware device to free buffer later. */
@@ -127,7 +135,9 @@ static void __comedi_buf_alloc(struct comedi_device *dev,
 
 		pages[i] = virt_to_page(buf->virt_addr);
 	}
+	spin_lock_irqsave(&s->spin_lock, flags);
 	bm->n_pages = i;
+	spin_unlock_irqrestore(&s->spin_lock, flags);
 
 	/* vmap the prealloc_buf if all the pages were allocated */
 	if (i == n_pages)
@@ -150,6 +160,29 @@ int comedi_buf_map_put(struct comedi_buf_map *bm)
 	return 1;
 }
 
+/* returns s->async->buf_map and increments its kref refcount */
+struct comedi_buf_map *
+comedi_buf_map_from_subdev_get(struct comedi_subdevice *s)
+{
+	struct comedi_async *async = s->async;
+	struct comedi_buf_map *bm = NULL;
+	unsigned long flags;
+
+	if (!async)
+		return NULL;
+
+	spin_lock_irqsave(&s->spin_lock, flags);
+	bm = async->buf_map;
+	/* only want it if buffer pages allocated */
+	if (bm && bm->n_pages)
+		comedi_buf_map_get(bm);
+	else
+		bm = NULL;
+	spin_unlock_irqrestore(&s->spin_lock, flags);
+
+	return bm;
+}
+
 bool comedi_buf_is_mmapped(struct comedi_async *async)
 {
 	struct comedi_buf_map *bm = async->buf_map;

drivers/staging/comedi/comedi_fops.c

@@ -1926,14 +1926,21 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
 	struct comedi_device *dev = file->private_data;
 	struct comedi_subdevice *s;
 	struct comedi_async *async;
-	struct comedi_buf_map *bm;
+	struct comedi_buf_map *bm = NULL;
 	unsigned long start = vma->vm_start;
 	unsigned long size;
 	int n_pages;
 	int i;
 	int retval;
 
-	mutex_lock(&dev->mutex);
+	/*
+	 * 'trylock' avoids circular dependency with current->mm->mmap_sem
+	 * and down-reading &dev->attach_lock should normally succeed without
+	 * contention unless the device is in the process of being attached
+	 * or detached.
+	 */
+	if (!down_read_trylock(&dev->attach_lock))
+		return -EAGAIN;
 
 	if (!dev->attached) {
 		dev_dbg(dev->class_dev, "no driver attached\n");
@@ -1973,7 +1980,9 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
 	}
 
 	n_pages = size >> PAGE_SHIFT;
-	bm = async->buf_map;
+
+	/* get reference to current buf map (if any) */
+	bm = comedi_buf_map_from_subdev_get(s);
 	if (!bm || n_pages > bm->n_pages) {
 		retval = -EINVAL;
 		goto done;
@@ -1997,7 +2006,8 @@ static int comedi_mmap(struct file *file, struct vm_area_struct *vma)
 
 	retval = 0;
 done:
-	mutex_unlock(&dev->mutex);
+	up_read(&dev->attach_lock);
+	comedi_buf_map_put(bm);	/* put reference to buf map - okay if NULL */
 	return retval;
 }
 

drivers/staging/comedi/comedi_internal.h

@@ -19,6 +19,8 @@ void comedi_buf_reset(struct comedi_async *async);
 bool comedi_buf_is_mmapped(struct comedi_async *async);
 void comedi_buf_map_get(struct comedi_buf_map *bm);
 int comedi_buf_map_put(struct comedi_buf_map *bm);
+struct comedi_buf_map *comedi_buf_map_from_subdev_get(
+		struct comedi_subdevice *s);
 unsigned int comedi_buf_write_n_allocated(struct comedi_async *async);
 void comedi_device_cancel_all(struct comedi_device *dev);