[PATCH] ext4: fix slab-use-after-free in do_split()

[Shuai Liu <847807867@qq.com>]

do_split() allocates a new directory block with ext4_append() to hold
the second half of the entries of the leaf block being split, then
dx_move_dirents() copies entries from the old block (*bh) into the new
one (bh2).  Nothing verifies that the newly allocated block differs
from the block being split.

On a filesystem whose block bitmap is inconsistent with the directory
tree, a block already referenced as a directory leaf can still be
marked free, so ext4_append() can return that same block.  With
concurrent operations on such a filesystem (as exercised by the
syzbot reproducer), the block can even be freed between allocations,
resulting in bh2 aliasing *bh.  dx_move_dirents() then copies the
block onto itself, corrupting directory entries mid-copy: a rec_len
field can be overwritten with name bytes, so the subsequent wipe of
the stale entry memset()s a bogus length, causing the slab-use-
after-free reported by syzbot.

Fix this by checking for bh2 == *bh right after ext4_append() and
bailing out with -EFSCORRUPTED before any directory block contents
are modified, reporting the corruption via ext4_error_inode_block().

Reported-by: syzbot+09bec78ee77613a3efdd@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=09bec78ee77613a3efdd
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Shuai Liu <847807867@qq.com>
---
 fs/ext4/namei.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 354fc2d22df1..4ef7983d46c6 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1928,6 +1928,13 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
 		*bh = NULL;
 		return ERR_CAST(bh2);
 	}
+	if (bh2 == *bh) {
+		ext4_error_inode_block(dir, newblock, -EFSCORRUPTED,
+				       "cannot split aliased directory block");
+		brelse(bh2);
+		*bh = NULL;
+		return ERR_PTR(-EFSCORRUPTED);
+	}
 
 	BUFFER_TRACE(*bh, "get_write_access");
 	err = ext4_journal_get_write_access(handle, dir->i_sb, *bh,
-- 
2.43.0