ext4: Fix double-free of blocks with EXT4_IOC_MOVE_EXT

ext4: Fix double-free of blocks with EXT4_IOC_MOVE_EXT

From: Akira Fujita <[email protected]>

ext4_move_extent() calls ext4_discard_preallocations() to discard inode PAs of orig and donor inodes in its beginning.
But the following case (1-4 steps) triggers the double-free of blocks, so move ext4_discard_preallocations() to the end of ext4_move_extents().

1. Discard inode PAs of orig and donor inodes with ext4_discard_preallocations() in ext4_move_extents().

orig : [ DATA1 ]
donor: [ DATA2 ]

2. While data blocks are exchanging between orig and donor inodes, new inode PAs is created to orig by other process's block allocation. (Since there are semaphore gaps in ext4_move_extents().) And new inode PAs is used partially (2-1).

2-1 Create new inode PAs to orig inode
orig : [ DATA1 | used PA1 | free PA1 ]
donor: [ DATA2 ]

3. Donor inode which has old orig inode's blocks is deleted after EXT4_IOC_MOVE_EXT finished (3-1, 3-2).
So the block bitmap corresponds to old orig inode's blocks are freed.

3-1 After EXT4_IOC_MOVE_EXT finished

   orig : [ DATA2 |  free PA1 ]
   donor: [ DATA1 |  used PA1 ]

3-2 Delete donor inode
orig : [ DATA2 | free PA1 ]
donor: [ FREE SPACE(DATA1) | FREE SPACE(used PA1) ]

4. The double-free of blocks is occurred, when close() is called to orig inode. Because ext4_discard_preallocations() for orig inode frees used PA1 and free PA1, though used PA1 is already freed in 3.

4-1 Double-free of blocks is occurred
orig : [ DATA2 | FREE SPACE(free PA1) ]
donor: [ FREE SPACE(DATA1) | DOUBLE FREE(used PA1) ]

---
 move_extent.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

index 5a106e0..3478889 100644
@@ -1289,10 +1289,6 @@ ext4_move_extents(struct file *o_filp, struct file *d_filp,
 			 ext4_ext_get_actual_len(ext_cur), block_end + 1) -
 		     max(le32_to_cpu(ext_cur->ee_block), block_start);
 
-	/* Discard preallocations of two inodes */
-	ext4_discard_preallocations(orig_inode);
-	ext4_discard_preallocations(donor_inode);

Thanks, applied to the ext4 patch queue.

- Ted