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 9C120C6FA8E for ; Thu, 2 Mar 2023 17:00:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230172AbjCBRAb (ORCPT ); Thu, 2 Mar 2023 12:00:31 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40346 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229468AbjCBRA0 (ORCPT ); Thu, 2 Mar 2023 12:00:26 -0500 Received: from sin.source.kernel.org (sin.source.kernel.org [145.40.73.55]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CE4CD3BDB9 for ; Thu, 2 Mar 2023 09:00:24 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by sin.source.kernel.org (Postfix) with ESMTPS id 3F17CCE1F4D for ; Thu, 2 Mar 2023 17:00:23 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 06507C4339B; Thu, 2 Mar 2023 17:00:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1677776421; bh=5q3WaaN4P3Rr6DjavzewoZ/Xh3Ixiv4NNY7gLkhfqxs=; h=In-Reply-To:References:Date:From:To:Cc:Subject:From; b=ZdhlT7T8Q0qSIj0U1/izOsHMopvZdLxFpgjmjqHlN+lhvGz7ktuRx5r5ZTNKbvMyd bhyvSi4ivnZfFxVVuzOhsu06fWRFkENevn17eq6PaV1Co228AGxKPh6djfS0QPU3QM J3BMn6XRyyo0zxMn9vFNMhDiD8r9vncu5RJDJpV5+bhq5pN+c/qr9+4fndJapVjPGR 2AVxScTjI51XnM0LqUDH6RgnTUrDOPgZ16An31xkWsgCyRrLq2szd67h79DXEEgwZ7 1ZdPOiCwoDvOKVLLND8DNim1Rv3ser56Ypozyr/RmOwf+sw6poJZbvQJV/JwSJoia5 UBW09XBNPeU0A== Received: from compute3.internal (compute3.nyi.internal [10.202.2.43]) by mailauth.nyi.internal (Postfix) with ESMTP id E832D27C0054; Thu, 2 Mar 2023 12:00:19 -0500 (EST) Received: from imap48 ([10.202.2.98]) by compute3.internal (MEProxy); Thu, 02 Mar 2023 12:00:19 -0500 X-ME-Sender: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgedvhedrudeljedgleegucetufdoteggodetrfdotf fvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfqfgfvpdfurfetoffkrfgpnffqhgen uceurghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmne cujfgurhepofgfggfkjghffffhvfevufgtsehttdertderredtnecuhfhrohhmpedftehn ugihucfnuhhtohhmihhrshhkihdfuceolhhuthhosehkvghrnhgvlhdrohhrgheqnecugg ftrfgrthhtvghrnhepveffgfevhfeiteduueetgeevvdevudevteefveffudeiveefuddt leeitdeludfgnecuffhomhgrihhnpehkvghrnhgvlhdrohhrghenucevlhhushhtvghruf hiiigvpedtnecurfgrrhgrmhepmhgrihhlfhhrohhmpegrnhguhidomhgvshhmthhprghu thhhphgvrhhsohhnrghlihhthidqudduiedukeehieefvddqvdeifeduieeitdekqdhluh htoheppehkvghrnhgvlhdrohhrgheslhhinhhugidrlhhuthhordhush X-ME-Proxy: Feedback-ID: ieff94742:Fastmail Received: by mailuser.nyi.internal (Postfix, from userid 501) id 9575231A0063; Thu, 2 Mar 2023 12:00:19 -0500 (EST) X-Mailer: MessagingEngine.com Webmail Interface User-Agent: Cyrus-JMAP/3.9.0-alpha0-183-gbf7d00f500-fm-20230220.001-gbf7d00f5 Mime-Version: 1.0 Message-Id: <50938d7d-0a09-4a5b-8593-39bfe55c2dc0@app.fastmail.com> In-Reply-To: <7d344756-aec4-4df2-9427-da742ef9ce6b@app.fastmail.com> References: <20210601075354.5149-2-rppt@kernel.org> <162274330352.29796.17521974349959809425.tip-bot2@tip-bot2> <7d344756-aec4-4df2-9427-da742ef9ce6b@app.fastmail.com> Date: Thu, 02 Mar 2023 08:59:59 -0800 From: "Andy Lutomirski" To: "Linux Kernel Mailing List" Cc: "Mike Rapoport" , "Borislav Petkov" , "Hugh Dickins" , "the arch/x86 maintainers" Subject: Re: [tip: x86/urgent] x86/setup: Always reserve the first 1M of RAM Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 1, 2023, at 7:51 PM, Andy Lutomirski wrote: > On Thu, Jun 3, 2021, at 11:01 AM, tip-bot2 for Mike Rapoport wrote: >> The following commit has been merged into the x86/urgent branch of tip: >> >> Commit-ID: f1d4d47c5851b348b7713007e152bc68b94d728b >> Gitweb: >> https://git.kernel.org/tip/f1d4d47c5851b348b7713007e152bc68b94d728b >> Author: Mike Rapoport >> AuthorDate: Tue, 01 Jun 2021 10:53:52 +03:00 >> Committer: Borislav Petkov >> CommitterDate: Thu, 03 Jun 2021 19:57:55 +02:00 >> >> x86/setup: Always reserve the first 1M of RAM >> >> There are BIOSes that are known to corrupt the memory under 1M, or more >> precisely under 640K because the memory above 640K is anyway reserved >> for the EGA/VGA frame buffer and BIOS. >> >> To prevent usage of the memory that will be potentially clobbered by the >> kernel, the beginning of the memory is always reserved. The exact size >> of the reserved area is determined by CONFIG_X86_RESERVE_LOW build time >> and the "reservelow=" command line option. The reserved range may be >> from 4K to 640K with the default of 64K. There are also configurations >> that reserve the entire 1M range, like machines with SandyBridge graphic >> devices or systems that enable crash kernel. >> >> In addition to the potentially clobbered memory, EBDA of unknown size may >> be as low as 128K and the memory above that EBDA start is also reserved >> early. >> >> It would have been possible to reserve the entire range under 1M unless for >> the real mode trampoline that must reside in that area. >> >> To accommodate placement of the real mode trampoline and keep the memory >> safe from being clobbered by BIOS, reserve the first 64K of RAM before >> memory allocations are possible and then, after the real mode trampoline >> is allocated, reserve the entire range from 0 to 1M. >> >> Update trim_snb_memory() and reserve_real_mode() to avoid redundant >> reservations of the same memory range. >> >> Also make sure the memory under 1M is not getting freed by >> efi_free_boot_services(). > > This is quite broken. The comments in the patch seem to understand > that Linux tries twice to allocate the real mode trampoline, but the > code has some issues. > > > First, it actively breaks the logic here: > > > + /* > + * Don't free memory under 1M for two reasons: > + * - BIOS might clobber it > + * - Crash kernel needs it to be reserved > + */ > + if (start + size < SZ_1M) > + continue; > + if (start < SZ_1M) { > + size -= (SZ_1M - start); > + start = SZ_1M; > + } > + > > > The whole point is that, if we fail to allocate a trampoline, we free > boot services and try again. But if we can't free boot services below > 1M, then we can't allocate a trampoline in boot services memory. And > then it does: > > > + /* > + * Unconditionally reserve the entire fisrt 1M, see comment in > + * setup_arch(). > + */ > + memblock_reserve(0, SZ_1M); > My apologies, I misread this thing. The patch is *not* obviously buggy, but something is buggy. I'll keep investigating... --Andy