From patchwork Fri Aug 23 13:10:29 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Andrew Cooper X-Patchwork-Id: 13775172 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 lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4A8A9C52D7C for ; Fri, 23 Aug 2024 13:10:53 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.782343.1191821 (Exim 4.92) (envelope-from ) id 1shU3q-0007gS-Ns; Fri, 23 Aug 2024 13:10:38 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 782343.1191821; Fri, 23 Aug 2024 13:10:38 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1shU3q-0007gL-LF; Fri, 23 Aug 2024 13:10:38 +0000 Received: by outflank-mailman (input) for mailman id 782343; Fri, 23 Aug 2024 13:10:37 +0000 Received: from se1-gles-flk1-in.inumbo.com ([94.247.172.50] helo=se1-gles-flk1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1shU3p-0007gF-Cc for xen-devel@lists.xenproject.org; Fri, 23 Aug 2024 13:10:37 +0000 Received: from mail-ej1-x62f.google.com (mail-ej1-x62f.google.com [2a00:1450:4864:20::62f]) by se1-gles-flk1.inumbo.com (Halon) with ESMTPS id 1481e9f8-6151-11ef-8776-851b0ebba9a2; Fri, 23 Aug 2024 15:10:35 +0200 (CEST) Received: by mail-ej1-x62f.google.com with SMTP id a640c23a62f3a-a7aa086b077so227325366b.0 for ; Fri, 23 Aug 2024 06:10:33 -0700 (PDT) Received: from andrewcoop.eng.citrite.net ([185.25.67.249]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a868f436322sm259382566b.117.2024.08.23.06.10.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Aug 2024 06:10:31 -0700 (PDT) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 1481e9f8-6151-11ef-8776-851b0ebba9a2 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=citrix.com; s=google; t=1724418633; x=1725023433; darn=lists.xenproject.org; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=UW85lMy3eLQqGbmm6QqWXIyMK9LIJXV5KbNrmX3uSc8=; b=I6UYIJbbz0u4RDVGjwQ5xW1LFC1bzvH3mLF56bItwQOge6o/VR0FgsldFsppv3g90u brz4edwoFBS2jkHUKDwWwmBTKUjRfhIDAjyU8RoTq/MOUi6OXpXjOxSba842/1aUlB5c VTv1B7HTrIcRZvF5yK3UfkNY8DTC/SvTEBKGo= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1724418633; x=1725023433; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=UW85lMy3eLQqGbmm6QqWXIyMK9LIJXV5KbNrmX3uSc8=; b=uEGmM/ASE/CS1gJMqw+mPFwFQIIuIZovgYywGg1LuJiNZEDrfF96FWKHlIZNgsGHej eafDhLG4hylnK3W0CR0xv+XHuYqBjNSIqtubFX4jA3I9ndXg3za1wzF07Zs39z6VQCd4 H05GwJDkgYE74eC1kaaN2QxYsB+6CYKQjzfivVmrtFWJVnnW5o2/JUUgZj6EpF3vPbTZ w6ee3TTsGU5cv2sgED1U+YEHCKgLJwtbdPyyY6giENnS18fZfSqcqBfbz3ddm+s/mxHu O/qNGbUYK91lxuCpxHiYVyHN0v2cMgzJNnZZvLRCV89cuB3pGLjIvLMeH39epiAuHuSY qWgA== X-Gm-Message-State: AOJu0YzPvTzDJWDn9g4fNLJK7fPxKTrZwjVPPdOXGvgGDBYSp/dVA6PE w/Af2PkYEz38GciK5XykmAykY1zNH4uaIdb6Vl18GgyXJ6QxQt9UIVFVoiHVSiNh8ZjR2u1ztsR u X-Google-Smtp-Source: AGHT+IF5Jv2uhbCVqySaIYOjBbU2IqNlpZqEyA63pgmRS5QFhGyO9e7Xi1DkxU8XFbqZ3yqs15660Q== X-Received: by 2002:a17:907:d59a:b0:a86:91c3:9517 with SMTP id a640c23a62f3a-a86a52eb66bmr168805666b.35.1724418632091; Fri, 23 Aug 2024 06:10:32 -0700 (PDT) From: Andrew Cooper To: Xen-devel Cc: Frediano Ziglio , Andrew Cooper , Jan Beulich , =?utf-8?q?Roger_Pau_Monn=C3=A9?= Subject: [PATCH v3] x86/boot: Preserve the value clobbered by the load-base calculation Date: Fri, 23 Aug 2024 14:10:29 +0100 Message-Id: <20240823131029.1025984-1-andrew.cooper3@citrix.com> X-Mailer: git-send-email 2.39.2 MIME-Version: 1.0 From: Frediano Ziglio Right now, Xen clobbers the value at 0xffc when performing it's load-base calculation. We've got plenty of free registers at this point, so the value can be preserved easily. This fixes a real bug booting under Coreboot+SeaBIOS, where 0xffc happens to be the cbmem pointer (e.g. Coreboot's dmesg ring, among other things). However, there's also a better choice of memory location to use than 0xffc, as all our supported boot protocols have a pointer to an info structure in %ebx. Update the documentation to match. Fixes: 1695e53851e5 ("x86/boot: Fix the boot time relocation calculations") Fixes: d96bb172e8c9 ("x86/entry: Early PVH boot code") Signed-off-by: Frediano Ziglio Signed-off-by: Andrew Cooper Reviewed-by: Jason Andryuk Reviewed-by: Jan Beulich --- CC: Jan Beulich CC: Roger Pau Monné CC: Frediano Ziglio v3: * Use plain MOV for preserve/restore. It's easier to follow than pop/pop. * Update how-xen-boots.rst too. Superceeds https://lore.kernel.org/xen-devel/20240822140044.441126-1-frediano.ziglio@cloud.com/T/#u --- docs/hypervisor-guide/x86/how-xen-boots.rst | 12 ++++++-- xen/arch/x86/boot/head.S | 33 ++++++++++++++------- 2 files changed, 31 insertions(+), 14 deletions(-) base-commit: 8ffcf184affbc2ff1860dabe1757388509d5ed67 diff --git a/docs/hypervisor-guide/x86/how-xen-boots.rst b/docs/hypervisor-guide/x86/how-xen-boots.rst index ca77d7c8a333..f1878ad7897f 100644 --- a/docs/hypervisor-guide/x86/how-xen-boots.rst +++ b/docs/hypervisor-guide/x86/how-xen-boots.rst @@ -96,6 +96,12 @@ Xen, once loaded into memory, identifies its position in order to relocate system structures. For 32bit entrypoints, this necessarily requires a call instruction, and therefore a stack, but none of the ABIs provide one. -Overall, given that on a BIOS-based system, the IVT and BDA occupy the first -5/16ths of the first page of RAM, with the rest free to use, Xen assumes the -top of the page is safe to use. +In each supported 32bit entry protocol, ``%ebx`` is a pointer to an info +structure, and it is highly likely that this structure does not overlap with +Xen. Therefore we use this as as a temporary stack, preserving the prior +value, in order to calculate Xen's position in memory. + +If this heuristic happens to be wrong (most likely because we were booted by +some other protocol), the calculation stills works as long as ``%ebx`` points +at RAM and does not alias the currently-executing instructions. This is +reasonably likely, and the best we can manage given no other information. diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S index d8ac0f0494db..994819826b01 100644 --- a/xen/arch/x86/boot/head.S +++ b/xen/arch/x86/boot/head.S @@ -414,17 +414,23 @@ __pvh_start: cli /* - * We need one push/pop to determine load address. Use the same - * absolute stack address as the native path, for lack of a better - * alternative. + * We need one call (i.e. push) to determine the load address. See + * __start for a discussion on how to do this safely using the PVH + * info structure. */ - mov $0x1000, %esp + + /* Preserve the field we're about to clobber. */ + mov (%ebx), %edx + lea 4(%ebx), %esp /* Calculate the load base address. */ call 1f 1: pop %esi sub $sym_offs(1b), %esi + /* Restore the clobbered field. */ + mov %edx, (%ebx) + /* Set up stack. */ lea STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp @@ -460,21 +466,26 @@ __start: /* * Multiboot (both 1 and 2) specify the stack pointer as undefined * when entering in BIOS circumstances. This is unhelpful for - * relocatable images, where one push/pop is required to calculate - * images load address. + * relocatable images, where one call (i.e. push) is required to + * calculate images load address. * - * On a BIOS-based system, the IVT and BDA occupy the first 5/16ths of - * the first page of RAM, with the rest free for use. Use the top of - * this page for a temporary stack, being one of the safest locations - * to clobber. + * This early in boot, there is one area of memory we know about with + * reasonable confidence that it isn't overlapped by Xen, and that's + * the Multiboot info structure in %ebx. Use it as a temporary stack. */ - mov $0x1000, %esp + + /* Preserve the field we're about to clobber. */ + mov (%ebx), %edx + lea 4(%ebx), %esp /* Calculate the load base address. */ call 1f 1: pop %esi sub $sym_offs(1b), %esi + /* Restore the clobbered field. */ + mov %edx, (%ebx) + /* Set up stack. */ lea STACK_SIZE - CPUINFO_sizeof + sym_esi(cpu0_stack), %esp