ksmbd: avoid out of bounds access in decode_preauth_ctxt()

Confirm that the accessed pneg_ctxt->HashAlgorithms address sits within
the SMB request boundary; deassemble_neg_contexts() only checks that the
eight byte smb2_neg_context header + (client controlled) DataLength are
within the packet boundary, which is insufficient.

Checking for sizeof(struct smb2_preauth_neg_context) is overkill given
that the type currently assumes SMB311_SALT_SIZE bytes of trailing Salt.

Signed-off-by: David Disseldorp <[e-mail]>
Acked-by: Namjae Jeon <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/ksmbd/smb2pdu.c

···
 }
 
 static __le32 decode_preauth_ctxt(struct ksmbd_conn *conn,
-				  struct smb2_preauth_neg_context *pneg_ctxt)
+				  struct smb2_preauth_neg_context *pneg_ctxt,
+				  int len_of_ctxts)
 {
-	__le32 err = STATUS_NO_PREAUTH_INTEGRITY_HASH_OVERLAP;
+	/*
+	 * sizeof(smb2_preauth_neg_context) assumes SMB311_SALT_SIZE Salt,
+	 * which may not be present. Only check for used HashAlgorithms[1].
+	 */
+	if (len_of_ctxts < MIN_PREAUTH_CTXT_DATA_LEN)
+		return STATUS_INVALID_PARAMETER;
 
-	if (pneg_ctxt->HashAlgorithms == SMB2_PREAUTH_INTEGRITY_SHA512) {
-		conn->preauth_info->Preauth_HashId =
-			SMB2_PREAUTH_INTEGRITY_SHA512;
-		err = STATUS_SUCCESS;
-	}
+	if (pneg_ctxt->HashAlgorithms != SMB2_PREAUTH_INTEGRITY_SHA512)
+		return STATUS_NO_PREAUTH_INTEGRITY_HASH_OVERLAP;
 
-	return err;
+	conn->preauth_info->Preauth_HashId = SMB2_PREAUTH_INTEGRITY_SHA512;
+	return STATUS_SUCCESS;
 }
 
 static void decode_encrypt_ctxt(struct ksmbd_conn *conn,
···
 				break;
 
 			status = decode_preauth_ctxt(conn,
-						     (struct smb2_preauth_neg_context *)pctx);
+						     (struct smb2_preauth_neg_context *)pctx,
+						     len_of_ctxts);
 			if (status != STATUS_SUCCESS)
 				break;
 		} else if (pctx->ContextType == SMB2_ENCRYPTION_CAPABILITIES) {