[PATCH v6 0/4] ntfs: serialize attribute-list access

ntfs_external_attr_find() walks base_ni->attr_list using raw
attr_list_entry pointers stored in ctx->al_entry. Because other paths
can replace base_ni->attr_list and free the old buffer while a lookup
still holds one of those cursors, a concurrent attribute-list mutation
can turn a saved cursor into a stale or freed pointer.

Reported in:
  https://lore.kernel.org/all/[email protected]/

This series fixes that hazard by replacing the raw ctx->al_entry
pointer with restartable, generation-checked locators, and by adding
locks that serialize attribute-list buffer replacement and on-disk
persistence so concurrent mutations cannot corrupt state or double-free
memory.

The four patches are:

  1. ntfs: replace raw ctx->al_entry with restartable attr-list locators
  2. ntfs: protect attribute-list buffer replacement with
     attr_list_persist_lock
  3. ntfs: protect attribute-list creation/teardown with
     attr_list_persist_lock
  4. ntfs: protect mapping-pairs attr-list updates with
     attr_list_persist_lock

What changes
------------

Instead of a raw struct attr_list_entry *, the search context now
carries three value-only locators:

  - al_cursor  -- where ntfs_external_attr_find() resumes enumeration;
                  invalidated on generation mismatch.
  - al_insert  -- insertion point captured on -ENOENT lookups.
  - al_exact   -- immutable identity snapshot (type, lowest_vcn,
                  mft_reference, instance, name length) used by writers
                  to re-locate the same ALE after a buffer replacement.

The inode gains attr_list_lock, an rwsem that protects the live buffer,
its size and generation, and in-place ALE changes. Readers hold it
only while dereferencing the buffer; writers hold it while replacing the
buffer or changing an ALE, then increment the generation. Pointers into
the buffer never survive an unlock, and the rwsem is dropped before
persistence or an operation that can take runlist.lock or extent_lock.

attr_list_persist_lock is a non-recursive mutex that serializes a
mutation through persistence and rollback, including list creation and
teardown. ntfs_attrlist_update_locked() writes the live buffer without
attr_list_lock, so the caller holds this mutex from mutation through the
write. It also stabilizes the buffer while attaching extents. When both
locks are needed, take attr_list_persist_lock first. Creation owns the
mutex itself, so callers must create first and then restart; re-entrant
teardown uses persist_locked instead of recursively acquiring it.

---
* Changes in v6:
  * Patch 2: initialize new_al before an early error path can reach its
    cleanup, fixing an x86_64 allmodconfig Clang build warning.
* Changes in v5:
  * Patch 3: keep each in-place ALE update and its persist in one
    transaction. This fixes the Patch 2 review: a record move could
    update an ALE while persistence copied the same buffer.
  * Patch 3: serialize attribute-list creation, teardown, and extent
    attachment.
  * Patch 4: acquire the mutex unconditionally for mapping-pairs updates to
    close the attribute-list creation TOCTOU; drop it before creating a
    missing list and restart afterward to avoid self-deadlock.
* Changes in v4:
  * Patch 1: fix attr-list lock/unlock and uninitialized locator on
    the AT_END path
  * Patch 2: hold attr_list_persist_lock around all
    ntfs_attrlist_update() callers and recheck the attribute-list
    state after locking
  * Patch 4: avoid self-deadlock rebuilding $ATTRIBUTE_LIST's own
    mapping pairs
* Changes in v3:
  * Patch 1: fix $ATTRIBUTE_LIST extent double-delivery during
    mapping-pairs rebuild (xfstests generic/040)
  * Patch 3: fix self-deadlock on attr_list_persist_lock during
    attr-list rebuild (xfstests generic/040)
* Changes in v2:
  * add Fixes tags
  * add Reported-by and Tested-by tags

---
Hyunchul Lee (4):
      ntfs: replace raw ctx->al_entry with restartable attr-list locators
      ntfs: protect attribute-list buffer replacement with attr_list_persist_lock
      ntfs: protect attribute-list creation/teardown with attr_list_persist_lock
      ntfs: protect mapping-pairs attr-list updates with attr_list_persist_lock

 fs/ntfs/attrib.c   | 567 ++++++++++++++++++++++++++++++++++++++++++++---------
 fs/ntfs/attrib.h   |  48 ++++-
 fs/ntfs/attrlist.c | 181 +++++++++++++----
 fs/ntfs/attrlist.h |   3 +-
 fs/ntfs/index.c    |  26 ++-
 fs/ntfs/inode.c    | 110 +++++++++--
 fs/ntfs/inode.h    |  10 +
 fs/ntfs/namei.c    |   2 +-
 fs/ntfs/super.c    |   2 +-
 9 files changed, 793 insertions(+), 156 deletions(-)
---
base-commit: 5c244a0e9ca95468f4c2087b2c9269825429a13f
change-id: 20260727-topic-attr-list-lock-v6-f1a909e3c859

Best regards,
-- 
Thanks,
Hyunchul