[PATCH v2] ext4: don't append a directory block already mapped in the inode

[Adriano Cordova]

ext4_append() grows a directory by one block.  It checks that the target
logical block is a hole, but a corrupt block bitmap can still make the
allocator hand back a physical block that is already in use by this
inode.  The in-memory copy of a block is keyed by its physical block
number, so the "new" block and that existing one are the same memory;
callers that split a directory (make_indexed_dir()/do_split()) then move
entries between two aliased buffers and corrupt the directory, until a
bogus rec_len read from the middle of a name runs the wipe out of bounds:

  BUG: KASAN: slab-use-after-free in dx_move_dirents [inline]
  Write of size 90458 ...

Reject the block and report the corrupt bitmap instead of corrupting
memory.

Reported-by: syzbot+09bec78ee77613a3efdd@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=09bec78ee77613a3efdd
Tested-by: syzbot+09bec78ee77613a3efdd@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Adriano Cordova <adrianox@xxxxxxxxx>
---
Changes in v2:
- Rework after review of v1 ("ext4: wipe moved dirents with their real
  length"). dx_make_map() already validates each entry, so the bad
  rec_len does not come from disk. The problem is that the block
  allocator was handing back a block already mapped by the inode.

 fs/ext4/namei.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 3b9740c1c16d..ff6013306b74 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -83,6 +83,25 @@ static struct buffer_head *ext4_append(handle_t *handle,
 	bh = ext4_bread(handle, inode, *block, EXT4_GET_BLOCKS_CREATE);
 	if (IS_ERR(bh))
 		return bh;
+
+	for (map.m_lblk = 0; map.m_lblk < *block; map.m_lblk += map.m_len) {
+		map.m_len = *block - map.m_lblk;
+		err = ext4_map_blocks(NULL, inode, &map, 0);
+		if (err < 0)
+			goto out;
+		if (err == 0) {
+			map.m_len = 1;
+			continue;
+		}
+		if (unlikely(map.m_pblk == bh->b_blocknr)) {
+			EXT4_ERROR_INODE(inode,
+					 "new block %llu already mapped",
+					 (unsigned long long)bh->b_blocknr);
+			err = -EFSCORRUPTED;
+			goto out;
+		}
+	}
+
 	inode->i_size += inode->i_sb->s_blocksize;
 	EXT4_I(inode)->i_disksize = inode->i_size;
 	err = ext4_mark_inode_dirty(handle, inode);
-- 
2.51.0