[PATCH 0/6] ocfs2: fix stale xattr entry pointers and empty header reads

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

Five defects in the ocfs2 extended attribute code, plus one robustness
measure that belongs with them, all on the paths that move entries
between a block, a bucket and an index tree.  They fall into two groups.

Stale entry pointers (patches 1 to 3)

ocfs2_xa_set() writes through a pointer into xh_entries[] that two of
its callers invalidate first.  Defragmenting a bucket ends in a sort()
on xe_name_hash, and lib/sort.c is an unstable heapsort, so entries that
share a hash can trade slots.  Copying a block into a bucket changes the
order outright, since a block holds its entries in insertion order while
a bucket holds them by hash, yet ocfs2_xattr_update_xattr_search()
relocates the search by the index the entry had in the block.  Neither
path compares the entry's name with the one being set, so setxattr() can
replace a neighbouring attribute's value and still report success, or
trip ocfs2_xa_reuse_entry()'s BUG_ON.  Patch 1 takes the hash before the
sort; patches 2 and 3 look the entry up again afterwards.

Reads of an empty entry array (patches 4 to 6)

A block and a bucket may both hold no entries: ocfs2_xa_remove_entry()
keeps the last slot when it removes the only one, so an empty bucket
still carries the hash it is keyed by.  Three sites index xh_entries[]
without checking xh_count and so read xh_entries[-1], which lands on the
header and comes back as xh_count | xh_free_start << 16.  Nothing
faults, but the value is acted on: the collision check reports no
collision for an empty bucket, the divide commits a sentinel hash that
hides part of the key space from ocfs2_xattr_bucket_find(), and the
block to bucket copy claims storage the bucket does not hold.  Patches 4
to 6 guard each site and keep the one read of xh_entries[0] that
ocfs2_xa_remove_entry() documents as intentional.

Joseph Qi (6):
  ocfs2: read the xattr entry hash before defragmenting the bucket
  ocfs2: find the xattr entry again after defragmenting a bucket
  ocfs2: find the xattr entry again after converting a block to a bucket
  ocfs2: do not read the entry array of an empty xattr bucket
  ocfs2: do not index before the entry array when dividing an empty
    bucket
  ocfs2: do not read the entry array of an empty xattr block

 fs/ocfs2/xattr.c | 146 +++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 123 insertions(+), 23 deletions(-)

-- 
2.39.3