block: fix refcounting in BLKBSZSET

Adam Kovari and others reported that disconnecting an USB drive with
an ntfs-3g filesystem would cause "kernel BUG at fs/inode.c:1421!" to
be triggered.

The BUG could be traced back to ioctl(BLKBSZSET), which would
erroneously decrement the refcount on the bdev.  This is because
blkdev_get() expects the refcount to be already incremented and either
returns success or decrements the refcount and returns an error.

The bug was introduced by e525fd89 (block: make blkdev_get/put()
handle exclusive access), which didn't take into account this behavior
of blkdev_get().

This fixes
  https://bugzilla.kernel.org/show_bug.cgi?id=29202
(and likely 29792 too)

Reported-by: Adam Kovari <kovariadam@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 3 deletions

diff --git a/block/ioctl.c b/block/ioctl.c
index 9049d460fa89..1124cd297263 100644
--- a/block/ioctl.c
+++ b/block/ioctl.c
@@ -294,9 +294,11 @@ int blkdev_ioctl(struct block_device *bdev, fmode_t mode, unsigned cmd,
                         return -EINVAL;
                 if (get_user(n, (int __user *) arg))
                         return -EFAULT;
-                if (!(mode & FMODE_EXCL) &&
-                    blkdev_get(bdev, mode | FMODE_EXCL, &bdev) < 0)
-                        return -EBUSY;
+                if (!(mode & FMODE_EXCL)) {
+                        bdgrab(bdev);
+                        if (blkdev_get(bdev, mode | FMODE_EXCL, &bdev) < 0)
+                                return -EBUSY;
+                }
                 ret = set_blocksize(bdev, n);
                 if (!(mode & FMODE_EXCL))
                         blkdev_put(bdev, mode | FMODE_EXCL);