[PATCH 3.14 09/33] staging: comedi: fix circular locking dependency in comedi_mmap()

Greg Kroah-Hartman
2014-04-24T22:41:11Z

From: Ian Abbott <abbotti@mev.co.uk>

commit b34aa86f12e8848ba453215602c8c50fa63c4cb3 upstream.

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+ #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:

-> #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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/staging/comedi/comedi_buf.c | 37 +++++++++++++++++++++++++++++--
drivers/staging/comedi/comedi_fops.c | 18 +++++++++++----
drivers/staging/comedi/comedi_internal.h | 2 +
3 files changed, 51 insertions(+), 6 deletions(-)

--- a/drivers/staging/comedi/comedi_buf.c
+++ b/drivers/staging/comedi/comedi_buf.c
@@ -61,6 +61,8 @@ static void __comedi_buf_free(struct com
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);
@@ -68,8 +70,11 @@ static void __comedi_buf_free(struct com
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 co
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 co
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 co

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
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;
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -1923,14 +1923,21 @@ static int comedi_mmap(struct file *file
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");
@@ -1970,7 +1977,9 @@ static int comedi_mmap(struct file *file
}

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;
@@ -1994,7 +2003,8 @@ static int comedi_mmap(struct file *file

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;
}

--- a/drivers/staging/comedi/comedi_internal.h
+++ b/drivers/staging/comedi/comedi_internal.h
@@ -19,6 +19,8 @@ void comedi_buf_reset(struct comedi_asyn
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);