[PATCH] Fix a race condition between ->i_mapping and iput()

This race became a cause of oops, and can reproduce by the following.

    while true; do
	dd if=/dev/zero of=/dev/.static/dev/hdg1 bs=512 count=1000 & sync
    done

This race condition was between __sync_single_inode() and iput().

          cpu0 (fs's inode)                 cpu1 (bdev's inode)
          -----------------                 -------------------
                                       close("/dev/hda2")
                                       [...]
__sync_single_inode()
   /* copy the bdev's ->i_mapping */
   mapping = inode->i_mapping;

                                       generic_forget_inode()
                                          bdev_clear_inode()
					     /* restre the fs's ->i_mapping */
				             inode->i_mapping = &inode->i_data;
				          /* bdev's inode was freed */
                                          destroy_inode(inode);

   if (wait) {
      /* dereference a freed bdev's mapping->host */
      filemap_fdatawait(mapping);  /* Oops */

Since __sync_single_inode() is only taking a ref-count of fs's inode, the
another process can be close() and freeing the bdev's inode while writing
fs's inode.  So, __sync_signle_inode() accesses the freed ->i_mapping,
oops.

This patch takes a ref-count on the bdev's inode for the fs's inode before
setting a ->i_mapping, and the clear_inode() of the fs's inode does iput() on
the bdev's inode.  So if the fs's inode is still living, bdev's inode
shouldn't be freed.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 25 insertions, 7 deletions

diff --git a/fs/block_dev.c b/fs/block_dev.c
index f5958f413bd1..44aaba202f78 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -414,21 +414,31 @@ EXPORT_SYMBOL(bdput);
 static struct block_device *bd_acquire(struct inode *inode)
 {
         struct block_device *bdev;
+
         spin_lock(&bdev_lock);
         bdev = inode->i_bdev;
-        if (bdev && igrab(bdev->bd_inode)) {
+        if (bdev) {
+                atomic_inc(&bdev->bd_inode->i_count);
                 spin_unlock(&bdev_lock);
                 return bdev;
         }
         spin_unlock(&bdev_lock);
+
         bdev = bdget(inode->i_rdev);
         if (bdev) {
                 spin_lock(&bdev_lock);
-                if (inode->i_bdev)
-                        __bd_forget(inode);
-                inode->i_bdev = bdev;
-                inode->i_mapping = bdev->bd_inode->i_mapping;
-                list_add(&inode->i_devices, &bdev->bd_inodes);
+                if (!inode->i_bdev) {
+                        /*
+                         * We take an additional bd_inode->i_count for inode,
+                         * and it's released in clear_inode() of inode.
+                         * So, we can access it via ->i_mapping always
+                         * without igrab().
+                         */
+                        atomic_inc(&bdev->bd_inode->i_count);
+                        inode->i_bdev = bdev;
+                        inode->i_mapping = bdev->bd_inode->i_mapping;
+                        list_add(&inode->i_devices, &bdev->bd_inodes);
+                }
                 spin_unlock(&bdev_lock);
         }
         return bdev;
@@ -438,10 +448,18 @@ static struct block_device *bd_acquire(struct inode *inode)
 
 void bd_forget(struct inode *inode)
 {
+        struct block_device *bdev = NULL;
+
         spin_lock(&bdev_lock);
-        if (inode->i_bdev)
+        if (inode->i_bdev) {
+                if (inode->i_sb != blockdev_superblock)
+                        bdev = inode->i_bdev;
                 __bd_forget(inode);
+        }
         spin_unlock(&bdev_lock);
+
+        if (bdev)
+                iput(bdev->bd_inode);
 }
 
 int bd_claim(struct block_device *bdev, void *holder)