[PATCH v2 0/2] ksmbd: fix explicit DACL checks and parent-derived rights

[Ian Bridges]

For explicit access requests, smb_check_perm_dacl() uses one ACE: the
first for the caller or the last Everyone or Authenticated Users ACE.
When that ACE is a deny ACE, the function allows the complement of the
ACE's mask, so the deny grants every right it does not name. Deny ACEs
before that ACE are ignored. Guest and anonymous sessions also match
Authenticated Users. On acl_xattr shares, callers can therefore read
or write files that the DACL denies, and guest and anonymous sessions
can read files granted only to Authenticated Users.

The function also never checks the parent directory, whose
FILE_DELETE_CHILD grants DELETE on a file in it and whose
FILE_LIST_DIRECTORY grants FILE_READ_ATTRIBUTES. ksmbd can therefore
refuse the opens a caller needs to rename a file, set its delete
disposition or read its attributes, even when the parent grants those
rights.

- Patch 1 evaluates explicit requests in DACL order and fixes the
  Authenticated Users match.
- Patch 2 lets the parent's DACL grant DELETE and
  FILE_READ_ATTRIBUTES.

Patch 2 passes the authenticated argument that patch 1 adds to
smb_check_perm_dacl() on to the parent check. It also restores
parent-derived rights that patch 1 can remove from guests. Backport
both patches in order.

Older stable trees first need commit cc2f133e80eb ("ksmbd: fix maximum
allowed access checks") and the earlier DACL fixes through "ksmbd:
grant a caller what its groups and OWNER RIGHTS ACEs allow".

Changes since v1:
- Patch 2: in the parent check, ACEs for the caller's groups and OWNER
  RIGHTS now decide before the parent's POSIX ACL. Sashiko's review of
  v1 found that a POSIX ACL entry for the caller stopped the parent from
  granting DELETE through them.
- Patch 2: a comment on the DACL walk now says which MAXIMUM_ALLOWED
  requests skip group ACEs.

v1: https://lore.kernel.org/linux-cifs/20261107062233.1579198-1-icb@xxxxxxxxxxxx/

Each patch was tested with KASAN on ksmbd-for-next 6f42553e493d.
smbtorture gives the same results as on the base.

This series was developed with LLM assistance.

Ian Bridges (2):
  ksmbd: evaluate the DACL in order for explicit access requests
  ksmbd: let the parent's DACL grant DELETE and FILE_READ_ATTRIBUTES

 fs/smb/server/mgmt/user_config.h   |   5 ++
 fs/smb/server/smb2pdu.c            |  11 ++-
 fs/smb/server/smbacl.c             | 127 +++++++++++++++++++++++++----
 fs/smb/server/smbacl.h             |   2 +-
 fs/smb/server/tests/smbacl_kunit.c |   4 +-
 5 files changed, 126 insertions(+), 23 deletions(-)


base-commit: 6f42553e493d3f25059a215748ae241e525c865f
-- 
2.47.3