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 B8C22C433F5 for ; Wed, 19 Jan 2022 19:12:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236948AbiASTMg (ORCPT ); Wed, 19 Jan 2022 14:12:36 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54194 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232902AbiASTMe (ORCPT ); Wed, 19 Jan 2022 14:12:34 -0500 Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [IPv6:2001:4d48:ad52:32c8:5054:ff:fe00:142]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E8BA2C061574 for ; Wed, 19 Jan 2022 11:12:33 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=kfq6BtDbp9OpFo3P4pVC1BirR13ne2CthW+xXJdID+M=; b=GvHU3PaKQOlo0Hv93WyifT/5Kl zS75z1K4qcrri5b+Jhl3HcS808J0atfkJlEi/6xfaFQOCUvy1hALu+SnQn1Z6Ex1Ha0kdAvi69rSA 4jT0j7ioTEza909AvkMWjk64fQQVUJf85mn2cC5DispMk7H9RRCa+Gmyc+vTgPNmmcQmf8bcCBe7E G1H6y/JlOf7lwx7XjwdTUV5bRVUhWz6mdC55FFYzCm6OKXzvUrkkyCkRLEcyIJQo+oNKkHzwDyTwQ Oins7PgQ0dDjLnr2o709Se/iEg/ZLEHKYp4Z0mQ2RhvMM08WXbe+xplRPA1VSb7BimqiNyQ4x1xoa dvcnXonA==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:56788) by pandora.armlinux.org.uk with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1nAGNc-00053a-Ko; Wed, 19 Jan 2022 19:12:24 +0000 Received: from linux by shell.armlinux.org.uk with local (Exim 4.94.2) (envelope-from ) id 1nAGNa-0005VR-9Z; Wed, 19 Jan 2022 19:12:22 +0000 Date: Wed, 19 Jan 2022 19:12:22 +0000 From: "Russell King (Oracle)" To: Robin Murphy Cc: Matthew Wilcox , Yury Norov , Catalin Marinas , Will Deacon , Andrew Morton , Nicholas Piggin , Ding Tianhong , Anshuman Khandual , Alexey Klimov , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] vmap(): don't allow invalid pages Message-ID: References: <20220118235244.540103-1-yury.norov@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: Russell King (Oracle) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 19, 2022 at 06:43:10PM +0000, Robin Murphy wrote: > Indeed, my impression is that the only legitimate way to get hold of a page > pointer without assumed provenance is via pfn_to_page(), which is where > pfn_valid() comes in. Thus pfn_valid(page_to_pfn()) really *should* be a > tautology. That can only be true if pfn == page_to_pfn(pfn_to_page(pfn)) for all values of pfn. Given how pfn_to_page() is defined in the sparsemem case: #define __pfn_to_page(pfn) \ ({ unsigned long __pfn = (pfn); \ struct mem_section *__sec = __pfn_to_section(__pfn); \ __section_mem_map_addr(__sec) + __pfn; \ }) #define page_to_pfn __page_to_pfn that isn't the case, especially when looking at page_to_pfn(): #define __page_to_pfn(pg) \ ({ const struct page *__pg = (pg); \ int __sec = page_to_section(__pg); \ (unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec))); \ }) Where: static inline unsigned long page_to_section(const struct page *page) { return (page->flags >> SECTIONS_PGSHIFT) & SECTIONS_MASK; } So if page_to_section() returns something that is, e.g. zero for an invalid page in a non-zero section, you're not going to end up with the right pfn from page_to_pfn(). As I've said now a couple of times, trying to determine of a struct page pointer is valid is the wrong question to be asking. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!