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 0C56EC2D0FD for ; Wed, 13 May 2020 10:02:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E0E1F206E5 for ; Wed, 13 May 2020 10:02:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589364127; bh=Ubz7VruHRIYMARfNxYnqVv5zsSbjW/xH3CYrn/L8SJg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:List-ID:From; b=HeVdejWPnugFVmVbmxcpsG5u/ioZLDQ0kU16qgcJMDwm0mCGL/wRW3tkogu5W2cqP X9IzU59C7rJSzRL5GRFXwgzeGkL/iZCmsLYtYoUCGvxy5NmfhsyMJdrBbc+e7h7egK 7ZYiHjj2c/756r7jAQXrBhwAdEPRYDMN1nJcicvE= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388631AbgEMKCG (ORCPT ); Wed, 13 May 2020 06:02:06 -0400 Received: from mail.kernel.org ([198.145.29.99]:50268 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387493AbgEMJu1 (ORCPT ); Wed, 13 May 2020 05:50:27 -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 42C8F20740; Wed, 13 May 2020 09:50:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1589363426; bh=Ubz7VruHRIYMARfNxYnqVv5zsSbjW/xH3CYrn/L8SJg=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=caogtro+jjim67ybypyOO0PW6Py1xZp//n5Dd5cgfpPIlV11d5Q9ZT8LRv+sk9vqp n6mP+bLHNX78Eq4vxXv5Hpv/H+dFBZi042Bo8pkk6dOCnBi7vsakhaLD2SghBTL29A LmHVGnvhyN7tCx38SO3FhxWRzy3iBM+vp6Spg1pQ= 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 5.4 68/90] batman-adv: fix batadv_nc_random_weight_tq Date: Wed, 13 May 2020 11:45:04 +0200 Message-Id: <20200513094416.485631562@linuxfoundation.org> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20200513094408.810028856@linuxfoundation.org> References: <20200513094408.810028856@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 @@ -1009,15 +1009,8 @@ 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;