From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 533EBCA90AF for ; Wed, 13 May 2020 09:47:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2CD2020740 for ; Wed, 13 May 2020 09:47:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589363220; bh=7+ERR58qtP0Taf+nilPY80hBVVqMfzujDkJgWpGVnnk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=HJYsVspfW0INg1CozMyQjtaLHdE098RTF8U2xpEbZdkkul37HrQAdtV3Tye0tiJyY j4kltY49o/Oba+031O8KDAn8QwNLa1PIrJPhTFOLoiaBJUANghyXcvC86llgbkjcIQ JI1FKGeuARApxhkEFbiuBKgqiCNAghASAIkB+t/0= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732761AbgEMJq7 (ORCPT ); Wed, 13 May 2020 05:46:59 -0400 Received: from mail.kernel.org ([198.145.29.99]:44476 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732730AbgEMJq4 (ORCPT ); Wed, 13 May 2020 05:46:56 -0400 Received: from localhost (83-86-89-107.cable.dynamic.v4.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 3215420740; Wed, 13 May 2020 09:46:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589363215; bh=7+ERR58qtP0Taf+nilPY80hBVVqMfzujDkJgWpGVnnk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=DsITpYvqnbVYe6fpqAvx3VeXkyMJuLMtrfz8CDTNDSgeZQlc1sbxmrgCbdP3BKqXU NTC5hbuMSnWl93FKYgxmHNNx0GmjYLUI4y1dgfoPP9tDaJkfCX4o8SDDYXRzGGGhAD 7xcgbLALrzG9yLi+ifPBZzo5Syebp1En7Z04Rwjs= From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, George Spelvin , Sven Eckelmann , Simon Wunderlich Subject: [PATCH 4.19 33/48] batman-adv: fix batadv_nc_random_weight_tq Date: Wed, 13 May 2020 11:44:59 +0200 Message-Id: <20200513094400.031666955@linuxfoundation.org> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20200513094351.100352960@linuxfoundation.org> References: <20200513094351.100352960@linuxfoundation.org> User-Agent: quilt/0.66 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: George Spelvin commit fd0c42c4dea54335967c5a86f15fc064235a2797 upstream. and change to pseudorandom numbers, as this is a traffic dithering operation that doesn't need crypto-grade. The previous code operated in 4 steps: 1. Generate a random byte 0 <= rand_tq <= 255 2. Multiply it by BATADV_TQ_MAX_VALUE - tq 3. Divide by 255 (= BATADV_TQ_MAX_VALUE) 4. Return BATADV_TQ_MAX_VALUE - rand_tq This would apperar to scale (BATADV_TQ_MAX_VALUE - tq) by a random value between 0/255 and 255/255. But! The intermediate value between steps 3 and 4 is stored in a u8 variable. So it's truncated, and most of the time, is less than 255, after which the division produces 0. Specifically, if tq is odd, the product is always even, and can never be 255. If tq is even, there's exactly one random byte value that will produce a product byte of 255. Thus, the return value is 255 (511/512 of the time) or 254 (1/512 of the time). If we assume that the truncation is a bug, and the code is meant to scale the input, a simpler way of looking at it is that it's returning a random value between tq and BATADV_TQ_MAX_VALUE, inclusive. Well, we have an optimized function for doing just that. Fixes: 3c12de9a5c75 ("batman-adv: network coding - code and transmit packets if possible") Signed-off-by: George Spelvin Signed-off-by: Sven Eckelmann Signed-off-by: Simon Wunderlich Signed-off-by: Greg Kroah-Hartman --- net/batman-adv/network-coding.c | 9 +-------- 1 file changed, 1 insertion(+), 8 deletions(-) --- a/net/batman-adv/network-coding.c +++ b/net/batman-adv/network-coding.c @@ -1021,15 +1021,8 @@ static struct batadv_nc_path *batadv_nc_ */ static u8 batadv_nc_random_weight_tq(u8 tq) { - u8 rand_val, rand_tq; - - get_random_bytes(&rand_val, sizeof(rand_val)); - /* randomize the estimated packet loss (max TQ - estimated TQ) */ - rand_tq = rand_val * (BATADV_TQ_MAX_VALUE - tq); - - /* normalize the randomized packet loss */ - rand_tq /= BATADV_TQ_MAX_VALUE; + u8 rand_tq = prandom_u32_max(BATADV_TQ_MAX_VALUE + 1 - tq); /* convert to (randomized) estimated tq again */ return BATADV_TQ_MAX_VALUE - rand_tq;