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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9C84DC04A6A for ; Thu, 3 Aug 2023 09:36:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235095AbjHCJgu (ORCPT ); Thu, 3 Aug 2023 05:36:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33938 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232825AbjHCJf7 (ORCPT ); Thu, 3 Aug 2023 05:35:59 -0400 Received: from mgamail.intel.com (unknown [134.134.136.31]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C79463ABA; Thu, 3 Aug 2023 02:35:55 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1691055355; x=1722591355; h=from:to:cc:subject:in-reply-to:references:date: message-id:mime-version; bh=N5jGfZQ8A78DMSVEz0/KZZUx/pejQfPJtpi3xQENQBE=; b=ag5NWUCTxvo0KsTNEcexyZhuGyKd5Q9hczQDqb2kxMAT6m+GaMXttymq /pPCy1EShT896NHVGOooXdPoRuh6FrD1mtgpzefilE/+RODtjG8owBG0S xYecKTEq4VBhjKsLjkKyZb5n4q/cJPaMButY7Uh+7B/t60MrZGEFn4tXt flNj0rLiFg9hyd5qdmG2cM5uyhmjQctn48pxnci/hwi+JJK2AQjOCrfM0 wWFoun/S5TK2FdRQVODrwu+yl57m4JZpf46dv6ZhLCFdZwzjE+QA5KMER vJ9vhe8jat5a87PozeVlhezNIVUhfIBCEnRfOPFU9q7mrQs9LFCDWHenB w==; X-IronPort-AV: E=McAfee;i="6600,9927,10790"; a="433664741" X-IronPort-AV: E=Sophos;i="6.01,251,1684825200"; d="scan'208";a="433664741" Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga104.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 03 Aug 2023 02:35:55 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=McAfee;i="6600,9927,10790"; a="729502060" X-IronPort-AV: E=Sophos;i="6.01,251,1684825200"; d="scan'208";a="729502060" Received: from jnikula-mobl4.fi.intel.com (HELO localhost) ([10.237.66.162]) by orsmga002-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 03 Aug 2023 02:35:49 -0700 From: Jani Nikula To: Ziqi Zhao , syzbot+622bba18029bcde672e1@syzkaller.appspotmail.com, astrajoan@yahoo.com, airlied@gmail.com, daniel@ffwll.ch, dri-devel@lists.freedesktop.org, ivan.orlov0322@gmail.com, maarten.lankhorst@linux.intel.com, mripard@kernel.org, skhan@linuxfoundation.org, tzimmermann@suse.de Cc: davem@davemloft.net, dsahern@kernel.org, edumazet@google.com, jacob.e.keller@intel.com, jiri@nvidia.com, kuba@kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, pabeni@redhat.com, syzkaller-bugs@googlegroups.com Subject: Re: [PATCH v3] drm/modes: Fix division by zero due to overflow In-Reply-To: <20230802174746.2256-1-astrajoan@yahoo.com> Organization: Intel Finland Oy - BIC 0357606-4 - Westendinkatu 7, 02160 Espoo References: <00000000000034cf5d05fea52dd4@google.com> <20230802174746.2256-1-astrajoan@yahoo.com> Date: Thu, 03 Aug 2023 12:35:47 +0300 Message-ID: <87bkfov570.fsf@intel.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 02 Aug 2023, Ziqi Zhao wrote: > In the bug reported by Syzbot, the variable `den == (1 << 22)` and > `mode->vscan == (1 << 10)`, causing the multiplication to overflow and > accidentally make `den == 0`. To prevent any chance of overflow, we > replace `num` and `den` with 64-bit unsigned integers, and explicitly > check if the divisor `den` will overflow. If so, we employ full 64-bit > division with rounding; otherwise we keep the 64-bit to 32-bit division > that could potentially be better optimized. > > In order to minimize the performance overhead, the overflow check for > `den` is wrapped with an `unlikely` condition. Please let me know if > this usage is appropriate. > > Reported-by: syzbot+622bba18029bcde672e1@syzkaller.appspotmail.com > Signed-off-by: Ziqi Zhao Reviewed-by: Jani Nikula > --- > V1 -> V2: address style comments suggested by Jani Nikula > > V2 -> V3: change title to include context on overflow causing the > division by zero > > drivers/gpu/drm/drm_modes.c | 11 +++++++---- > 1 file changed, 7 insertions(+), 4 deletions(-) > > diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c > index ac9a406250c5..137101960690 100644 > --- a/drivers/gpu/drm/drm_modes.c > +++ b/drivers/gpu/drm/drm_modes.c > @@ -1285,13 +1285,13 @@ EXPORT_SYMBOL(drm_mode_set_name); > */ > int drm_mode_vrefresh(const struct drm_display_mode *mode) > { > - unsigned int num, den; > + u64 num, den; > > if (mode->htotal == 0 || mode->vtotal == 0) > return 0; > > - num = mode->clock; > - den = mode->htotal * mode->vtotal; > + num = mul_u32_u32(mode->clock, 1000); > + den = mul_u32_u32(mode->htotal, mode->vtotal); > > if (mode->flags & DRM_MODE_FLAG_INTERLACE) > num *= 2; > @@ -1300,7 +1300,10 @@ int drm_mode_vrefresh(const struct drm_display_mode *mode) > if (mode->vscan > 1) > den *= mode->vscan; > > - return DIV_ROUND_CLOSEST_ULL(mul_u32_u32(num, 1000), den); > + if (unlikely(den > UINT_MAX)) > + return DIV64_U64_ROUND_CLOSEST(num, den); > + > + return DIV_ROUND_CLOSEST_ULL(num, (u32) den); > } > EXPORT_SYMBOL(drm_mode_vrefresh); -- Jani Nikula, Intel Open Source Graphics Center