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=-10.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 2A184C47425 for ; Wed, 30 Sep 2020 23:44:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DA71121481 for ; Wed, 30 Sep 2020 23:44:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1601509497; bh=SSqSVJUZq5m2CFytxoaIhalJNHYAEM0gt/cv9d1YWNU=; h=References:In-Reply-To:From:Date:Subject:To:Cc:List-ID:From; b=2EBAJbUIVb83Tij9X0ycq6ShD9+LYSS1gODi3WYD9zAk/oDz1XpOr1snyiSWPdCU9 6/XkFVt/LHTeGEqpXpRSBG2ctwmd6uTji/vdCKQwz0vEgJxVm1VhIz9ePXl/+SZx5t EOUpCI7+7J/QMf1zckKOyZGyoUGSdGBjMQTD/62I= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731571AbgI3Xoy (ORCPT ); Wed, 30 Sep 2020 19:44:54 -0400 Received: from mail.kernel.org ([198.145.29.99]:41518 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731871AbgI3Xoy (ORCPT ); Wed, 30 Sep 2020 19:44:54 -0400 Received: from mail-wm1-f52.google.com (mail-wm1-f52.google.com [209.85.128.52]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 302042184D for ; Wed, 30 Sep 2020 23:44:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1601509493; bh=SSqSVJUZq5m2CFytxoaIhalJNHYAEM0gt/cv9d1YWNU=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=zKh5u/vQ4DHmmkQhSWp+wdV0skJheTi2Ff/1SoaNF1ZoLo3R9XRZgCJDc9uIIZ9qm 1PpdnxTGA1gFcjoSNicwPMLd24HvAwjLbqL2J1EeJuCR3zFck9U5kGKKHardh6gyRs 4QiSjs2FaHFOuqYC9WQ/xKIKuse5MaZog1dPpeOw= Received: by mail-wm1-f52.google.com with SMTP id v12so1153410wmh.3 for ; Wed, 30 Sep 2020 16:44:53 -0700 (PDT) X-Gm-Message-State: AOAM531cXOFyuEH2xpW5wwzlW8ASzO3p65jYystzOrm0vDjjfzFSnAbz 60s4vIR4/PXITGJCzujEoJ5bo3Uf3szHEa6/OK4Faw== X-Google-Smtp-Source: ABdhPJyMRLhY+lYah0ZMRcHOsRvGLLMnGNegO49w3/xDIxX2DcDbv8Z7wMY4A1dUcTaUYFV25yfdu9/ydBCNezB8fvU= X-Received: by 2002:a1c:f70b:: with SMTP id v11mr4785295wmh.21.1601509491674; Wed, 30 Sep 2020 16:44:51 -0700 (PDT) MIME-Version: 1.0 References: <99B32E59-CFF2-4756-89BD-AEA0021F355F@amacapital.net> In-Reply-To: From: Andy Lutomirski Date: Wed, 30 Sep 2020 16:44:40 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation To: "Yu, Yu-cheng" Cc: Andy Lutomirski , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , LKML , "open list:DOCUMENTATION" , Linux-MM , linux-arch , Linux API , Arnd Bergmann , Balbir Singh , Borislav Petkov , Cyrill Gorcunov , Dave Hansen , Eugene Syromiatnikov , Florian Weimer , "H.J. Lu" , Jann Horn , Jonathan Corbet , Kees Cook , Mike Kravetz , Nadav Amit , Oleg Nesterov , Pavel Machek , Peter Zijlstra , Randy Dunlap , "Ravi V. Shankar" , Vedvyas Shanbhogue , Dave Martin , Weijiang Yang , Pengfei Xu Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Wed, Sep 30, 2020 at 3:33 PM Yu, Yu-cheng wrote: > > On 9/29/2020 1:00 PM, Andy Lutomirski wrote: > > On Tue, Sep 29, 2020 at 12:57 PM Andy Lutomirski wrote: > >> > >> On Tue, Sep 29, 2020 at 11:37 AM Yu, Yu-cheng wrote: > >>> > >>> On 9/28/2020 10:37 AM, Andy Lutomirski wrote: > >>>> On Mon, Sep 28, 2020 at 9:59 AM Yu-cheng Yu wrote: > >>>>> > >>>>> On Fri, 2020-09-25 at 09:51 -0700, Andy Lutomirski wrote: > >>>>>>> On Sep 25, 2020, at 9:48 AM, Yu, Yu-cheng wrote: > >>>>> + > >>>>> + cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER); > >>>>> + if (!cet) { > >>>>> + /* > >>>>> + * This is an unlikely case where the task is > >>>>> + * CET-enabled, but CET xstate is in INIT. > >>>>> + */ > >>>>> + WARN_ONCE(1, "CET is enabled, but no xstates"); > >>>> > >>>> "unlikely" doesn't really cover this. > >>>> > >>>>> + fpregs_unlock(); > >>>>> + goto sigsegv; > >>>>> + } > >>>>> + > >>>>> + if (cet->user_ssp && ((cet->user_ssp + 8) < TASK_SIZE_MAX)) > >>>>> + cet->user_ssp += 8; > >>>> > >>>> This looks buggy. The condition should be "if SHSTK is on, then add 8 > >>>> to user_ssp". If the result is noncanonical, then some appropriate > >>>> exception should be generated, probably by the FPU restore code -- see > >>>> below. You should be checking the SHSTK_EN bit, not SSP. > >>> > >>> Updated. Is this OK? I will resend the whole series later. > >>> > >>> Thanks, > >>> Yu-cheng > >>> > >>> ====== > >>> > >>> From 09803e66dca38d7784e32687d0693550948199ed Mon Sep 17 00:00:00 2001 > >>> From: Yu-cheng Yu > >>> Date: Thu, 29 Nov 2018 14:15:38 -0800 > >>> Subject: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and > >>> Indirect Branch > >>> Tracking for vsyscall emulation > >>> > >>> Vsyscall entry points are effectively branch targets. Mark them with > >>> ENDBR64 opcodes. When emulating the RET instruction, unwind shadow stack > >>> and reset IBT state machine. > >>> > >>> Signed-off-by: Yu-cheng Yu > >>> --- > >>> v13: > >>> - Check shadow stack address is canonical. > >>> - Change from writing to MSRs to writing to CET xstate. > >>> > >>> arch/x86/entry/vsyscall/vsyscall_64.c | 34 +++++++++++++++++++++++ > >>> arch/x86/entry/vsyscall/vsyscall_emu_64.S | 9 ++++++ > >>> arch/x86/entry/vsyscall/vsyscall_trace.h | 1 + > >>> 3 files changed, 44 insertions(+) > >>> > >>> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c > >>> b/arch/x86/entry/vsyscall/vsyscall_64.c > >>> index 44c33103a955..30b166091d46 100644 > >>> --- a/arch/x86/entry/vsyscall/vsyscall_64.c > >>> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c > >>> @@ -38,6 +38,9 @@ > >>> #include > >>> #include > >>> #include > >>> +#include > >>> +#include > >>> +#include > >>> > >>> #define CREATE_TRACE_POINTS > >>> #include "vsyscall_trace.h" > >>> @@ -286,6 +289,44 @@ bool emulate_vsyscall(unsigned long error_code, > >>> /* Emulate a ret instruction. */ > >>> regs->ip = caller; > >>> regs->sp += 8; > >>> + > >>> +#ifdef CONFIG_X86_CET > >>> + if (tsk->thread.cet.shstk_size || tsk->thread.cet.ibt_enabled) { > >>> + struct cet_user_state *cet; > >>> + struct fpu *fpu; > >>> + > >>> + fpu = &tsk->thread.fpu; > >>> + fpregs_lock(); > >>> + > >>> + if (!test_thread_flag(TIF_NEED_FPU_LOAD)) { > >>> + copy_fpregs_to_fpstate(fpu); > >>> + set_thread_flag(TIF_NEED_FPU_LOAD); > >>> + } > >>> + > >>> + cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER); > >>> + if (!cet) { > >>> + /* > >>> + * This should not happen. The task is > >>> + * CET-enabled, but CET xstate is in INIT. > >>> + */ > >> > >> Can the comment explain better, please? I would say something like: > >> > >> If the kernel thinks this task has CET enabled (because > >> tsk->thread.cet has one of the features enabled), then the > >> corresponding bits must also be set in the CET XSAVES region. If the > >> CET XSAVES region is in the INIT state, then the kernel's concept of > >> the task's CET state is corrupt. > >> > >>> + WARN_ONCE(1, "CET is enabled, but no xstates"); > >>> + fpregs_unlock(); > >>> + goto sigsegv; > >>> + } > >>> + > >>> + if (cet->user_cet & CET_SHSTK_EN) { > >>> + if (cet->user_ssp && (cet->user_ssp + 8 < TASK_SIZE_MAX)) > >>> + cet->user_ssp += 8; > >>> + } > >> > >> This makes so sense to me. Also, the vsyscall emulation code is > >> intended to be as rigid as possible to minimize the chance that it > >> gets used as an exploit gadget. So we should not silently corrupt > >> anything. Moreover, this code seems quite dangerous -- you've created > >> a gadget that does RET without actually verifying the SHSTK token. If > >> SHSTK and some form of strong indirect branch/call CFI is in use, then > >> the existance of a CFI-bypassing return primitive at a fixed address > >> seems quite problematic. > >> > >> So I think you need to write a function that reasonably accurately > >> emulates a usermode RET. > >> > > > > For what it's worth, I think there is an alternative. If you all > > (userspace people, etc) can come up with a credible way for a user > > program to statically declare that it doesn't need vsyscalls, then we > > could make SHSTK depend on *that*, and we could avoid this mess. This > > breaks orthogonality, but it's probably a decent outcome. > > > > Would an arch_prctl(DISABLE_VSYSCALL) work? The kernel then sets a > thread flag, and in emulate_vsyscall(), checks the flag. > > When CET is enabled, ld-linux will do DISABLE_VSYSCALL. > > How is that? Backwards, no? Presumably vsyscall needs to be disabled before or concurrently with CET being enabled, not after. I think the solution of making vsyscall emulation work correctly with CET is going to be better and possibly more straightforward. > > Yu-cheng