From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-100.mta0.migadu.com [91.218.175.100]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 32B5340F749 for ; Fri, 28 Aug 2026 11:09:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.100 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787915398; cv=none; b=RQfHMGikBH5H0pz6c5Aj0b+6lq9or9zGXdpoxvJzNbmZo5H/Qv3ZmoCHUwx9cGgocj8ZN+5rCMbHTeBuCpM2v+w/tro0OSZcnNgMXeBAtW9ytIvSYrAar+GWXpiAnBjfHhucbUqzJjlKNGFLNAfGdUiOEJuQ5NNG2QXr65Bjpy8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787915398; c=relaxed/simple; bh=OoHGgjSfWYpB05+fSb88E0ID0/HP1IBQpVIbX01UjGk=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=B2ZuitdCGQyfFqqNPBiwAF/rXeoM+HuqIc22RhHfDA2px9muNLf6CGjWGmj1CCe5UXPAeG8QTpryp9KC34BKGthBhpNCYgO4Q+3KLZgIRuItlE/GEKrRtijPgZ/CAks9etjRps8tlWBSWjIIVJ/YaRSP78Rjp67c5xzMFJOEpNo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=RdniU1/D; arc=none smtp.client-ip=91.218.175.100 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="RdniU1/D" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=OoHGgjSfWYpB05+fSb88E0ID0/HP1IBQpVIbX01UjGk=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1787915388; v=1; x=1788520188; b=RdniU1/DbmPn0VtQ3oCjn/aSXaa7PAma4T34fwwGoPs1wOs9/MxtRHlPwLGmp3p7HTMZXCTB dZULfLBRB0RxyIlVbkElGGqIghNQtBlA0tLz/nSxP73dpOULj/+LTh7zOtWmcdbG1nm08/VATDj NeAv7aqxRVWu6e8lf73YbtNY= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id e93b957396df94b6; Fri, 28 Aug 2026 11:09:48 +0000 X-Mizu-Trace-ID: e93b957396df94b6 X-Migadu-Flow: FLOW_OUT From: Ridong Chen To: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Andrew Morton Cc: Muchun Song , Kairui Song , Qi Zheng , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , David Hildenbrand , Lorenzo Stoakes , Chris Down , Tejun Heo , Yu Zhao , cgroups@vger.kernel.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-mm@kvack.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-kernel@vger.kernel.org, Ridong Chen , Ridong Chen , stable@vger.kernel.org Subject: [PATCH v2 1/2] mm/page_counter: avoid integer overflow in effective_protection() Date: Fri, 28 Aug 2026 19:09:18 +0800 Message-Id: <20260828110919.1324028-2-ridong.chen@linux.dev> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260828110919.1324028-1-ridong.chen@linux.dev> References: <20260828110919.1324028-1-ridong.chen@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Ridong Chen effective_protection() scales a parent's protection by a ratio of page counts, e.g. for recursive protection: (parent_effective - siblings_protected) * (usage - protected) / (parent_usage - siblings_protected) The multiply is done at unsigned long width before dividing. On systems with >= 16TB RAM the product can exceed 2^64 and wrap, giving a bogus protection value and silently breaking memory.min/low enforcement. Use mul_u64_u64_div_u64() to multiply in a 128-bit intermediate. Because usage and parent_usage are not read atomically (a child is charged before its parent), usage - protected can briefly exceed the divisor, making the quotient overflow 64 bits and trap (#DE on x86). Cap it so the ratio stays <= 1. Reported by the sashiko review tool [1]. [1] https://sashiko.dev/#/patchset/20260826133054.88529-1-ridong.chen@linux.dev?part=1 Fixes: bc50bcc6e00b ("mm: memcontrol: clean up and document effective low/min calculations") Fixes: 8a931f801340 ("mm: memcontrol: recursive memory.low protection") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Ridong Chen --- mm/page_counter.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) diff --git a/mm/page_counter.c b/mm/page_counter.c index 661e0f2a5127..e8bd512069c5 100644 --- a/mm/page_counter.c +++ b/mm/page_counter.c @@ -8,6 +8,7 @@ #include #include #include +#include #include #include #include @@ -356,7 +357,8 @@ static unsigned long effective_protection(unsigned long usage, * otherwise get a smaller chunk than what they claimed. */ if (siblings_protected > parent_effective) - return protected * parent_effective / siblings_protected; + return mul_u64_u64_div_u64(protected, parent_effective, + siblings_protected); /* * Ok, utilized protection of all children is within what the @@ -397,13 +399,18 @@ static unsigned long effective_protection(unsigned long usage, if (parent_effective > siblings_protected && parent_usage > siblings_protected && usage > protected) { - unsigned long unclaimed; + unsigned long unclaimed = parent_effective - siblings_protected; + unsigned long unprotected = usage - protected; + unsigned long parent_unprotected = parent_usage - siblings_protected; - unclaimed = parent_effective - siblings_protected; - unclaimed *= usage - protected; - unclaimed /= parent_usage - siblings_protected; + /* + * The usages aren't read atomically, so a child can transiently + * appear to use more than its parent, making the ratio exceed 1 + * and the quotient overflow 64 bits (#DE on x86). Cap it. + */ + unprotected = min(unprotected, parent_unprotected); - ep += unclaimed; + ep += mul_u64_u64_div_u64(unclaimed, unprotected, parent_unprotected); } return ep; -- 2.34.1