ksmbd: deny access for an empty DACL instead of granting the request

smb_check_perm_dacl() returns the access mask it computes to the SMB2
CREATE handler, which uses it as the granted access for the open. For a
security descriptor whose DACL is present but contains no ACEs, the
function is supposed to deny access: an empty (non-NULL) DACL grants no
access to anyone except the object owner's implicit READ_CONTROL and
WRITE_DAC (MS-DTYP 2.4.5).

The empty-DACL branch only denies when both

  - the DACL has no trailing bytes (pdacl_size == sizeof(struct smb_acl)),
    and
  - the request contains bits beyond READ_CONTROL / WRITE_DAC.

In every other case it takes the shared "goto err_out" with rc still 0,
which returns success without modifying *pdaccess, so the caller grants
the full requested access. Three problems follow:

  - The READ_CONTROL / WRITE_DAC carve-out models the owner's implicit
    rights, but the check runs before is_owner is computed, so those
    rights are handed to *every* caller, not just the owner. For
    READ_CONTROL the SMB2 CREATE path does not fall back to
    inode_permission(), so any user can read the security descriptor of
    an object whose empty DACL is meant to deny them.

  - A request of exactly READ_CONTROL and/or WRITE_DAC is granted to any
    caller. WRITE_DAC lets the handle rewrite the DACL.

  - A DACL that declares zero ACEs but carries trailing bytes
    (pdacl_size > sizeof(struct smb_acl)) skips the deny entirely and is
    granted whatever was requested.

On shares where the NT ACL is the authoritative access control -- e.g.
"force user" shares, where every client maps to one POSIX identity so
POSIX cannot distinguish users -- this is an access-control bypass.

Compute is_owner before the empty-DACL branch and deny access from an
empty DACL unless the caller is the owner requesting no more than
READ_CONTROL / WRITE_DAC.

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Aamir Ahmed <elb12345@hotmail.co.uk>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

0 files changed, 0 insertions, 0 deletions