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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 DDB62C0044C for ; Wed, 7 Nov 2018 15:34:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A487C2085B for ; Wed, 7 Nov 2018 15:34:55 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A487C2085B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731203AbeKHBFp (ORCPT ); Wed, 7 Nov 2018 20:05:45 -0500 Received: from mga06.intel.com ([134.134.136.31]:20949 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727387AbeKHBFo (ORCPT ); Wed, 7 Nov 2018 20:05:44 -0500 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False Received: from orsmga002.jf.intel.com ([10.7.209.21]) by orsmga104.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 07 Nov 2018 07:34:53 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.54,475,1534834800"; d="scan'208";a="106678567" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.154]) by orsmga002.jf.intel.com with ESMTP; 07 Nov 2018 07:34:53 -0800 Date: Wed, 7 Nov 2018 07:34:52 -0800 From: Sean Christopherson To: Andy Lutomirski Cc: Dave Hansen , Jann Horn , Linus Torvalds , Rich Felker , Dave Hansen , Jethro Beekman , Jarkko Sakkinen , Florian Weimer , Linux API , X86 ML , linux-arch , LKML , Peter Zijlstra , nhorman@redhat.com, npmccallum@redhat.com, "Ayoun, Serge" , shay.katz-zamir@intel.com, linux-sgx@vger.kernel.org, Andy Shevchenko , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Carlos O'Donell , adhemerval.zanella@linaro.org Subject: Re: RFC: userspace exception fixups Message-ID: <20181107153452.GB22972@linux.intel.com> References: <1C426267-492F-4AE7-8BE8-C7FE278531F9@amacapital.net> <209cf4a5-eda9-2495-539f-fed22252cf02@intel.com> <9B76E95B-5745-412E-8007-7FAA7F83D6FB@amacapital.net> <1541541565.8854.13.camel@intel.com> <7FF4802E-FBC5-4E6D-A8F6-8A65114F18C7@amacapital.net> <20181106233515.GB11101@linux.intel.com> <20181107000235.GC11101@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 06, 2018 at 05:17:14PM -0800, Andy Lutomirski wrote: > On Tue, Nov 6, 2018 at 4:02 PM Sean Christopherson > wrote: > > > > On Tue, Nov 06, 2018 at 03:39:48PM -0800, Andy Lutomirski wrote: > > > On Tue, Nov 6, 2018 at 3:35 PM Sean Christopherson > > > wrote: > > > > > > > > On Tue, Nov 06, 2018 at 03:00:56PM -0800, Andy Lutomirski wrote: > > > > > > > > > > > > > > > >> On Nov 6, 2018, at 1:59 PM, Sean Christopherson wrote: > > > > > >> > > > > > >>> On Tue, 2018-11-06 at 13:41 -0800, Andy Lutomirski wrote: > > > > > >> Sean, how does the current SDK AEX handler decide whether to do > > > > > >> EENTER, ERESUME, or just bail and consider the enclave dead? It seems > > > > > >> like the *CPU* could give a big hint, but I don't see where there is > > > > > >> any architectural indication of why the AEX code got called or any > > > > > >> obvious way for the user code to know whether the exit was fixed up by > > > > > >> the kernel? > > > > > > > > > > > > The SDK "unconditionally" does ERESUME at the AEP location, but that's > > > > > > bit misleading because its signal handler may muck with the context's > > > > > > RIP, e.g. to abort the enclave on a fatal fault. > > > > > > > > > > > > On an event/exception from within an enclave, the event is immediately > > > > > > delivered after loading synthetic state and changing RIP to the AEP. > > > > > > In other words, jamming CPU state is essentially a bunch of vectoring > > > > > > ucode preamble, but from software's perspective it's a normal event > > > > > > that happens to point at the AEP instead of somewhere in the enclave. > > > > > > And because the signals the SDK cares about are all synchronous, the > > > > > > SDK can simply hardcode ERESUME at the AEP since all of the fault logic > > > > > > resides in its signal handler. IRQs and whatnot simply trampoline back > > > > > > into the enclave. > > > > > > > > > > > > Userspace can do something funky instead of ERESUME, but only *after* > > > > > > IRET/RSM/VMRESUME has returned to the AEP location, and in Linux's > > > > > > case, after the trap handler has run. > > > > > > > > > > > > Jumping back a bit, how much do we care about preventing userspace > > > > > > from doing stupid things? > > > > > > > > > > My general feeling is that userspace should be allowed to do apparently > > > > > stupid things. For example, as far as the kernel is concerned, Wine and > > > > > DOSEMU are just user programs that do stupid things. Linux generally tries > > > > > to provide a reasonably complete view of architectural behavior. This is > > > > > in contrast to, say, Windows, where IIUC doing an unapproved WRFSBASE May > > > > > cause very odd behavior indeed. So magic fixups that do non-architectural > > > > > things are not so great. > > > > > > > > Sorry if I'm beating a dead horse, but what if we only did fixup on ENCLU > > > > with a specific (ignored) prefix pattern? I.e. effectively make the magic > > > > fixup opt-in, falling back to signals. Jamming RIP to skip ENCLU isn't > > > > that far off the architecture, e.g. EENTER stuffs RCX with the next RIP so > > > > that the enclave can EEXIT to immediately after the EENTER location. > > > > > > > > > > How does that even work, though? On an AEX, RIP points to the ERESUME > > > instruction, not the EENTER instruction, so if we skip it we just end > > > up in lala land. > > > > Userspace would obviously need to be aware of the fixup behavior, but > > it actually works out fairly nicely to have a separate path for ERESUME > > fixup since a fault on EENTER is generally fatal, whereas as a fault on > > ERESUME might be recoverable. > > > > Hmm. > > > > > do_eenter: > > mov tcs, %rbx > > lea async_exit, %rcx > > mov $EENTER, %rax > > ENCLU > > Or SOME_SILLY_PREFIX ENCLU? Yeah, forgot to include that. > > > > /* > > * EEXIT or EENTER faulted. In the latter case, %RAX already holds some > > * fault indicator, e.g. -EFAULT. > > */ > > eexit_or_eenter_fault: > > ret > > But userspace wants to know whether it was a fault or not. So I think > we either need two landing pads or we need to hijack a flag bit (are > there any known-zeroed flag bits after EEXIT?) to say whether it was a > fault. And, if it was a fault, we should give the vector, the > sanitized error code, and possibly CR2. As Jethro mentioned, RAX will always be 4 on a successful EEXIT, so we can use RAX to indicate a fault. That's what I was trying to imply with EFAULT. Here's the reg stuffing I use for the POC: regs->ax = EFAULT; regs->di = trapnr; regs->si = error_code; regs->dx = address; Well-known RAX values also means the kernel fault handlers only need to look for SOME_SILLY_PREFIX ENCLU if RAX==2 || RAX==3, i.e. the fault occurred on EENTER or in an enclave (RAX is set to ERESUME's leaf as part of the asynchronous enlcave exit flow). > > > > async_exit: > > ENCLU > > Same prefix here, right? > > > > > fixup_handler: > > > > This whole thing is a bit odd, but not necessarily a terrible idea.