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 X-Spam-Level: X-Spam-Status: No, score=-8.6 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 31025C433DF for ; Fri, 16 Oct 2020 11:04:10 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id A25B12084C for ; Fri, 16 Oct 2020 11:04:09 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=suse.com header.i=@suse.com header.b="iXx6mzof" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A25B12084C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.7903.20870 (Exim 4.92) (envelope-from ) id 1kTNWb-0007Em-LX; Fri, 16 Oct 2020 11:03:53 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 7903.20870; Fri, 16 Oct 2020 11:03:53 +0000 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" Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kTNWb-0007Ef-Ig; Fri, 16 Oct 2020 11:03:53 +0000 Received: by outflank-mailman (input) for mailman id 7903; Fri, 16 Oct 2020 11:03:52 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kTNWa-0007Ea-B7 for xen-devel@lists.xenproject.org; Fri, 16 Oct 2020 11:03:52 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 66ed3bd4-882d-419e-a494-d653ec6b3ae3; Fri, 16 Oct 2020 11:03:51 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 5041EAF57; Fri, 16 Oct 2020 11:03:50 +0000 (UTC) Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kTNWa-0007Ea-B7 for xen-devel@lists.xenproject.org; Fri, 16 Oct 2020 11:03:52 +0000 X-Inumbo-ID: 66ed3bd4-882d-419e-a494-d653ec6b3ae3 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 66ed3bd4-882d-419e-a494-d653ec6b3ae3; Fri, 16 Oct 2020 11:03:51 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1602846230; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Va+0gUhz7qtRrDzZw2xhJD5kqAtBVfdpqPmhdFPf9j4=; b=iXx6mzof2NKNCtme8qB3jiGKUtjzUAWvvTxud64h6UqtzsJPUcLG5tPu+pOXGBTHa9VS1s caae5yFCquRQ/HliibJqUVoGwxK93wbAWj/m0U+hIdI45prx4gMA9n69zNl99Yat16bMDi Mvd5pZxT0D39dOzoXnzzITbxnm28Dcs= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 5041EAF57; Fri, 16 Oct 2020 11:03:50 +0000 (UTC) Subject: Re: [PATCH] x86/traps: 'Fix' safety of read_registers() in #DF path To: Andrew Cooper Cc: Xen-devel , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Wei Liu , Julien Grall References: <20201012134908.27497-1-andrew.cooper3@citrix.com> <307753b0-fef8-658d-f897-8c0eb99ce3e5@citrix.com> <948f0753-561b-15e8-bf8c-52ff507133d2@suse.com> From: Jan Beulich Message-ID: <00ba5885-5ee6-c772-a72e-15431cd3b1f4@suse.com> Date: Fri, 16 Oct 2020 13:03:51 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit On 16.10.2020 12:58, Andrew Cooper wrote: > On 15/10/2020 08:27, Jan Beulich wrote: >> On 14.10.2020 20:00, Andrew Cooper wrote: >>> On 13/10/2020 16:51, Jan Beulich wrote: >>>> On 12.10.2020 15:49, Andrew Cooper wrote: >>>>> All interrupts and exceptions pass a struct cpu_user_regs up into C. This >>>>> contains the legacy vm86 fields from 32bit days, which are beyond the >>>>> hardware-pushed frame. >>>>> >>>>> Accessing these fields is generally illegal, as they are logically out of >>>>> bounds for anything other than an interrupt/exception hitting ring1/3 code. >>>>> >>>>> Unfortunately, the #DF handler uses these fields as part of preparing the >>>>> state dump, and being IST, accesses the adjacent stack frame. >>>>> >>>>> This has been broken forever, but c/s 6001660473 "x86/shstk: Rework the stack >>>>> layout to support shadow stacks" repositioned the #DF stack to be adjacent to >>>>> the guard page, which turns this OoB write into a fatal pagefault: >>>>> >>>>> (XEN) *** DOUBLE FAULT *** >>>>> (XEN) ----[ Xen-4.15-unstable x86_64 debug=y Tainted: C ]---- >>>>> (XEN) ----[ Xen-4.15-unstable x86_64 debug=y Tainted: C ]---- >>>>> (XEN) CPU: 4 >>>>> (XEN) RIP: e008:[] traps.c#read_registers+0x29/0xc1 >>>>> (XEN) RFLAGS: 0000000000050086 CONTEXT: hypervisor (d1v0) >>>>> ... >>>>> (XEN) Xen call trace: >>>>> (XEN) [] R traps.c#read_registers+0x29/0xc1 >>>>> (XEN) [] F do_double_fault+0x3d/0x7e >>>>> (XEN) [] F double_fault+0x107/0x110 >>>>> (XEN) >>>>> (XEN) Pagetable walk from ffff830236f6d008: >>>>> (XEN) L4[0x106] = 80000000bfa9b063 ffffffffffffffff >>>>> (XEN) L3[0x008] = 0000000236ffd063 ffffffffffffffff >>>>> (XEN) L2[0x1b7] = 0000000236ffc063 ffffffffffffffff >>>>> (XEN) L1[0x16d] = 8000000236f6d161 ffffffffffffffff >>>>> (XEN) >>>>> (XEN) **************************************** >>>>> (XEN) Panic on CPU 4: >>>>> (XEN) FATAL PAGE FAULT >>>>> (XEN) [error_code=0003] >>>>> (XEN) Faulting linear address: ffff830236f6d008 >>>>> (XEN) **************************************** >>>>> (XEN) >>>>> >>>>> and rendering the main #DF analysis broken. >>>>> >>>>> The proper fix is to delete cpu_user_regs.es and later, so no >>>>> interrupt/exception path can access OoB, but this needs disentangling from the >>>>> PV ABI first. >>>>> >>>>> Not-really-fixes: 6001660473 ("x86/shstk: Rework the stack layout to support shadow stacks") >>>>> Signed-off-by: Andrew Cooper >>>> Reviewed-by: Jan Beulich >>>> >>>> Is it perhaps worth also saying explicitly that the other IST >>>> stacks don't suffer the same problem because show_registers() >>>> makes an local copy of the passed in struct? (Doing so also >>>> for #DF would apparently be an alternative solution.) >>> They're not safe.  They merely don't explode. >>> >>> https://lore.kernel.org/xen-devel/1532546157-5974-1-git-send-email-andrew.cooper3@citrix.com/ >>> was "fixed" by CET-SS turning the guard page from not present to >>> read-only, but the same CET-SS series swapped #DB for #DF when it comes >>> to the single OoB write problem case. >> I see. While indeed I didn't pay attention to the OoB read aspect, >> me saying "the other IST stacks don't suffer the same problem" was >> still correct, wasn't it? Anyway - not a big deal. > > I've tweaked the commit message to make this more clear. > > --8<--- > Accessing these fields is generally illegal, as they are logically out of > bounds for anything other than an interrupt/exception hitting ring1/3 code. > > show_registers() unconditionally reads these fields, but the content is > discarded before use.  This is benign right now, as all parts of the > stack are > readable, including the guard pages. > > However, read_registers() in the #DF handler writes to these fields as > part of > preparing the state dump, and being IST, hits the adjacent stack frame. > --8<-- Thanks, lgtm. Jan