ksmbd: fix wrong UserName check in session_user

The offset of UserName is related to the address of security
buffer. To ensure the validaty of UserName, we need to compare name_off
+ name_len with secbuf_len instead of auth_msg_len.

[   27.096243] ==================================================================
[   27.096890] BUG: KASAN: slab-out-of-bounds in smb_strndup_from_utf16+0x188/0x350
[   27.097609] Read of size 2 at addr ffff888005e3b542 by task kworker/0:0/7
...
[   27.099950] Call Trace:
[   27.100194]  <TASK>
[   27.100397]  dump_stack_lvl+0x33/0x50
[   27.100752]  print_report+0xcc/0x620
[   27.102305]  kasan_report+0xae/0xe0
[   27.103072]  kasan_check_range+0x35/0x1b0
[   27.103757]  smb_strndup_from_utf16+0x188/0x350
[   27.105474]  smb2_sess_setup+0xaf8/0x19c0
[   27.107935]  handle_ksmbd_work+0x274/0x810
[   27.108315]  process_one_work+0x419/0x760
[   27.108689]  worker_thread+0x2a2/0x6f0
[   27.109385]  kthread+0x160/0x190
[   27.110129]  ret_from_fork+0x1f/0x30
[   27.110454]  </TASK>

Cc: stable@vger.kernel.org
Signed-off-by: Chih-Yen Chang <cc85nod@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: u1f383

fs/ksmbd/smb2pdu.c

@@ -1356,7 +1356,7 @@ static struct ksmbd_user *session_user(struct ksmbd_conn *conn,
 	struct authenticate_message *authblob;
 	struct ksmbd_user *user;
 	char *name;
-	unsigned int auth_msg_len, name_off, name_len, secbuf_len;
+	unsigned int name_off, name_len, secbuf_len;
 
 	secbuf_len = le16_to_cpu(req->SecurityBufferLength);
 	if (secbuf_len < sizeof(struct authenticate_message)) {
@@ -1366,9 +1366,8 @@ static struct ksmbd_user *session_user(struct ksmbd_conn *conn,
 	authblob = user_authblob(conn, req);
 	name_off = le32_to_cpu(authblob->UserName.BufferOffset);
 	name_len = le16_to_cpu(authblob->UserName.Length);
-	auth_msg_len = le16_to_cpu(req->SecurityBufferOffset) + secbuf_len;
 
-	if (auth_msg_len < (u64)name_off + name_len)
+	if (secbuf_len < (u64)name_off + name_len)
 		return NULL;
 
 	name = smb_strndup_from_utf16((const char *)authblob + name_off,