[PATCH 6/6] ocfs2: do not read the entry array of an empty xattr block

[Joseph Qi <joseph.qi@linux.alibaba.com>]

ocfs2_cp_xattr_block_to_bucket() sizes the name and value area it has to
carry over from the block's last entry, so an empty block makes it read
xh_entries[-1].  That lands on the header, where xe_name_offset overlaps
xh_name_value_len, and block storage never maintains that field, so it
reads as 0 and the copy runs from the header to the end of the block.

The bucket that comes out has xh_count 0 but a name_value_len of
blocksize less the header, and a matching xh_free_start that claims
those bytes are in use -- nearly the whole bucket with the common 4K
block size -- while the last block of the bucket holds a copy of the old
block's bytes.  Nothing but a trace event reads xh_name_value_len, the
copied bytes came from the same inode and were wiped before the entry
was removed, and ocfs2_defrag_xattr_bucket() recomputes xh_free_start,
so this corrupts the header rather than leaking anything.

Leave the bucket empty instead, carrying over the hash the block was
keyed by, since ocfs2_add_new_xattr_bucket() and
ocfs2_xattr_bucket_find() both read xh_entries[0] of an empty bucket.
That read is the exception ocfs2_xa_remove_entry() documents rather than
a new out of bounds access.

A block reaches the conversion empty when truncating an external value
back fails partway through: the "reusing" and "overwriting" callers of
ocfs2_xa_cleanup_value_truncate() remove the entry without setting
ctxt->set_abort, so ocfs2_xattr_block_set() sees a plain -ENOSPC and
converts.  The "growing" caller does set it, and the block form makes a
single attempt with no defragment and retry in between, so that is the
only route here.

Fixes: 012255961c9e ("ocfs2: Enable xattr set in index btree")
Cc: <stable@vger.kernel.org>
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
---
 fs/ocfs2/xattr.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/ocfs2/xattr.c b/fs/ocfs2/xattr.c
index 822b1e9c3ba2..ac41653e59f5 100644
--- a/fs/ocfs2/xattr.c
+++ b/fs/ocfs2/xattr.c
@@ -4704,6 +4704,21 @@ static void ocfs2_cp_xattr_block_to_bucket(struct inode *inode,
 	for (i = 0; i < blks; i++)
 		memset(bucket_block(bucket, i), 0, blocksize);
 
+	/*
+	 * An empty block has no names or values to carry over, and the
+	 * xh_entries[count - 1] below would be xh_entries[-1].  Leave the
+	 * bucket empty, but carry over the hash ocfs2_xa_remove_entry() left
+	 * in the block's xh_entries[0] so that the bucket still holds the key
+	 * it is looked up by.
+	 */
+	if (!count) {
+		xh->xh_num_buckets = cpu_to_le16(1);
+		xh->xh_free_start = cpu_to_le16(OCFS2_XATTR_BUCKET_SIZE);
+		xh->xh_entries[0].xe_name_hash =
+			xb_xh->xh_entries[0].xe_name_hash;
+		return;
+	}
+
 	/*
 	 * Since the xe_name_offset is based on ocfs2_xattr_header,
 	 * there is a offset change corresponding to the change of
-- 
2.39.3