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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5F6EDC433F5 for ; Thu, 20 Jan 2022 13:03:36 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id C0C7C6B0098; Thu, 20 Jan 2022 08:03:35 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id B94836B0099; Thu, 20 Jan 2022 08:03:35 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A5CD16B009A; Thu, 20 Jan 2022 08:03:35 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0250.hostedemail.com [216.40.44.250]) by kanga.kvack.org (Postfix) with ESMTP id 900CC6B0098 for ; Thu, 20 Jan 2022 08:03:35 -0500 (EST) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 55CD5180A30EC for ; Thu, 20 Jan 2022 13:03:35 +0000 (UTC) X-FDA: 79050681990.18.868CFE1 Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [78.32.30.218]) by imf05.hostedemail.com (Postfix) with ESMTP id 2963E100043 for ; Thu, 20 Jan 2022 13:03:33 +0000 (UTC) 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=sAMxEzPFNeLU4Z3zBK3VqMdaurVg22n/PAfNifA/IeM=; b=i/0yfFZdwPBfRpyJyxqG7OTf4G UOKt59GEwHciB3ZRrhCDdX828WaQxG43yeFXjiJbVbUOlu3BlKOKr6yAUYr6LFDLQJ0zWjg11ShZF Tce+9sg7ShWFGFg8PJ3S30n2oPdiFGmYV7yGQ0+YUSrvJSxjNDHtlBhcIclj6B5XDEhA1Ke0dJCJj ttzlo+fHyeOtxtrtsi6D2zN++LVwQuUYy0/Sd/2+3Nf3do6yrhRX6uvtQbjuTW/dzK5xKeX/nDnKw e8gRsi0DJqSIUVY7wVD+ngor0hQ/h1TlurgKP06zLbRqMe/G6zY8n/6I88PIoqvmO1I1WwnbKKDVY t9FSqCyQ==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:56798) 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 1nAX5z-0005eo-Pk; Thu, 20 Jan 2022 13:03:19 +0000 Received: from linux by shell.armlinux.org.uk with local (Exim 4.94.2) (envelope-from ) id 1nAX5m-0006Eq-Mh; Thu, 20 Jan 2022 13:03:06 +0000 Date: Thu, 20 Jan 2022 13:03:06 +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> <319b09bc-56a2-207f-6180-3cc7d8cd43d1@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <319b09bc-56a2-207f-6180-3cc7d8cd43d1@arm.com> X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 2963E100043 X-Stat-Signature: coy14n5j6dpsck6omkpk91qun6t8imoy Authentication-Results: imf05.hostedemail.com; dkim=pass header.d=armlinux.org.uk header.s=pandora-2019 header.b="i/0yfFZd"; spf=none (imf05.hostedemail.com: domain of "linux+linux-mm=kvack.org@armlinux.org.uk" has no SPF policy when checking 78.32.30.218) smtp.mailfrom="linux+linux-mm=kvack.org@armlinux.org.uk"; dmarc=pass (policy=none) header.from=armlinux.org.uk X-HE-Tag: 1642683813-816451 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Jan 20, 2022 at 12:22:35PM +0000, Robin Murphy wrote: > On 2022-01-19 19:12, Russell King (Oracle) wrote: > > 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(). > > Right, I emphasised "should" in an attempt to imply "in the absence of > serious bugs that have further-reaching consequences anyway". > > > 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. > > And doing so in one single place, on the justification of avoiding an > incredibly niche symptom, is even more so. Not to mention that an address > size fault is one of the best possible outcomes anyway, vs. the untold > damage that may stem from accesses actually going through to random parts of > the physical memory map. I don't see it as a "niche" symptom. If we start off with the struct page being invalid, then the result of page_to_pfn() can not be relied upon to produce something that is meaningful - which is exactly why the vmap() issue arises. With a pfn_valid() check, we at least know that the PFN points at memory. However, that memory could be _anything_ in the system - it could be the kernel image, and it could give userspace access to change kernel code. So, while it is useful to do a pfn_valid() check in vmap(), as I said to willy, this must _not_ be the primary check. It should IMHO use WARN_ON() to make it blatently obvious that it should be something we expect _not_ to trigger under normal circumstances, but is there to catch programming errors elsewhere. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!