inotify: fix lock ordering wrt do_page_fault's mmap_sem

Fix inotify lock order reversal with mmap_sem due to holding locks over
copy_to_user.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Reported-by: "Daniel J Blueman" <daniel.blueman@gmail.com>
Tested-by: "Daniel J Blueman" <daniel.blueman@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 20 insertions, 7 deletions

diff --git a/fs/inotify_user.c b/fs/inotify_user.c
index 60249429a253..d85c7d931cdf 100644
--- a/fs/inotify_user.c
+++ b/fs/inotify_user.c
@@ -323,7 +323,7 @@ out:
 }
 
 /*
- * remove_kevent - cleans up and ultimately frees the given kevent
+ * remove_kevent - cleans up the given kevent
  *
  * Caller must hold dev->ev_mutex.
  */
@@ -334,7 +334,13 @@ static void remove_kevent(struct inotify_device *dev,
 
         dev->event_count--;
         dev->queue_size -= sizeof(struct inotify_event) + kevent->event.len;
+}
 
+/*
+ * free_kevent - frees the given kevent.
+ */
+static void free_kevent(struct inotify_kernel_event *kevent)
+{
         kfree(kevent->name);
         kmem_cache_free(event_cachep, kevent);
 }
@@ -350,6 +356,7 @@ static void inotify_dev_event_dequeue(struct inotify_device *dev)
                 struct inotify_kernel_event *kevent;
                 kevent = inotify_dev_get_event(dev);
                 remove_kevent(dev, kevent);
+                free_kevent(kevent);
         }
 }
 
@@ -433,17 +440,15 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
         dev = file->private_data;
 
         while (1) {
-                int events;
 
                 prepare_to_wait(&dev->wq, &wait, TASK_INTERRUPTIBLE);
 
                 mutex_lock(&dev->ev_mutex);
-                events = !list_empty(&dev->events);
-                mutex_unlock(&dev->ev_mutex);
-                if (events) {
+                if (!list_empty(&dev->events)) {
                         ret = 0;
                         break;
                 }
+                mutex_unlock(&dev->ev_mutex);
 
                 if (file->f_flags & O_NONBLOCK) {
                         ret = -EAGAIN;
@@ -462,7 +467,6 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
         if (ret)
                 return ret;
 
-        mutex_lock(&dev->ev_mutex);
         while (1) {
                 struct inotify_kernel_event *kevent;
 
@@ -481,6 +485,13 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
                         }
                         break;
                 }
+                remove_kevent(dev, kevent);
+
+                /*
+                 * Must perform the copy_to_user outside the mutex in order
+                 * to avoid a lock order reversal with mmap_sem.
+                 */
+                mutex_unlock(&dev->ev_mutex);
 
                 if (copy_to_user(buf, &kevent->event, event_size)) {
                         ret = -EFAULT;
@@ -498,7 +509,9 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
                         count -= kevent->event.len;
                 }
 
-                remove_kevent(dev, kevent);
+                free_kevent(kevent);
+
+                mutex_lock(&dev->ev_mutex);
         }
         mutex_unlock(&dev->ev_mutex);