ksmbd: validate zero num_subauth before sub_auth is accessed

Access psid->sub_auth[psid->num_subauth - 1] without checking
if num_subauth is non-zero leads to an out-of-bounds read.
This patch adds a validation step to ensure num_subauth != 0
before sub_auth is accessed.

Cc: stable@vger.kernel.org
Signed-off-by: Norbert Szetei <norbert@doyensec.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/smbacl.c

@@ -270,6 +270,11 @@ static int sid_to_id(struct mnt_idmap *idmap,
 		return -EIO;
 	}
 
+	if (psid->num_subauth == 0) {
+		pr_err("%s: zero subauthorities!\n", __func__);
+		return -EIO;
+	}
+
 	if (sidtype == SIDOWNER) {
 		kuid_t uid;
 		uid_t id;