[PATCH v2] ksmbd: deny access for an empty DACL instead of granting the request

[Aamir Ahmed]

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@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Aamir Ahmed <elb12345@xxxxxxxxxxxxx>
---

Notes:
    v2:
      - add the Assisted-by: LLM tag
    v1: https://lore.kernel.org/linux-cifs/AS8P251MB000152D9BF4B8E0601640661C8B32@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
    
    This supersedes both v1 postings; "ksmbd: fix empty DACL handling in
    smb_check_perm_dacl()" was the same patch sent by mistake and can be
    dropped.
    
    The bug was located by auditing smb_check_perm_dacl() for paths that
    return success (rc == 0) while leaving *pdaccess unmodified -- the same
    class as commit 08f41323f549 ("ksmbd: fix maximal access leak when
    object has no NT ACL"), which fixed one such path for the maximal-access
    caller. The empty-DACL and malformed-descriptor paths were not covered.
    
    Compiled (CC fs/smb/server/smbacl.o) and checkpatch --strict clean; the
    decision change was cross-checked with a small semantic model of the
    branch for owner/non-owner x requested-bits x trailing-bytes.

 fs/smb/server/smbacl.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c
index e752479..3611dc6 100644
--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -1494,21 +1494,27 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
 	if (pdacl_size > acl_size || pdacl_size < sizeof(struct smb_acl))
 		goto err_out;
 
-	if (!pdacl->num_aces) {
-		if (!(pdacl_size - sizeof(struct smb_acl)) &&
-		    *pdaccess & ~(FILE_READ_CONTROL_LE | FILE_WRITE_DAC_LE)) {
-			rc = -EACCES;
-			goto err_out;
-		}
-		goto err_out;
-	}
-
 	if (!uid)
 		sid_type = SIDUNIX_USER;
 	id_to_sid(uid, sid_type, &sid);
 	vfsuid = i_uid_into_vfsuid(idmap, d_inode(path->dentry));
 	is_owner = uid == from_kuid(&init_user_ns, vfsuid_into_kuid(vfsuid));
 
+	if (!pdacl->num_aces) {
+		/*
+		 * An empty (present, zero-ACE) DACL grants no access to
+		 * anyone except the object owner's implicit READ_CONTROL
+		 * and WRITE_DAC (MS-DTYP 2.4.5). Deny every other caller,
+		 * deny the owner any access beyond those two bits, and do
+		 * not let trailing bytes after a zero-ACE DACL become an
+		 * implicit grant.
+		 */
+		if (!is_owner ||
+		    (*pdaccess & ~(FILE_READ_CONTROL_LE | FILE_WRITE_DAC_LE)))
+			rc = -EACCES;
+		goto err_out;
+	}
+
 	if (*pdaccess & FILE_MAXIMAL_ACCESS_LE) {
 		ace = (struct smb_ace *)((char *)pdacl + sizeof(struct smb_acl));
 		aces_size = pdacl_size - sizeof(struct smb_acl);
-- 
2.53.0.windows.1