ocfs2: do not read the entry array of an empty xattr block

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.

Link: https://lore.kernel.org/20261024151012.1059683-7-joseph.qi@linux.alibaba.com
Fixes: 012255961c9e ("ocfs2: Enable xattr set in index btree")
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Heming Zhao <heming.zhao@suse.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions