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=0.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,HK_RANDOM_FROM, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 ACF18C5ACD6 for ; Wed, 18 Mar 2020 13:02:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7867820774 for ; Wed, 18 Mar 2020 13:02:06 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="BT3oKviF" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726783AbgCRNCF (ORCPT ); Wed, 18 Mar 2020 09:02:05 -0400 Received: from us-smtp-delivery-74.mimecast.com ([216.205.24.74]:31281 "EHLO us-smtp-delivery-74.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726801AbgCRNCF (ORCPT ); Wed, 18 Mar 2020 09:02:05 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1584536523; 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: in-reply-to:in-reply-to:references:references; bh=umU8Z9JoL7Qhg365QcEJh2Mxe4az67bMGgKE4dWEnUw=; b=BT3oKviFAm3rBgfuaPdmNwRweOOEUEMqPm9oRfFALe+msvhVpdKNaOfBHkfrCVJ/TuyVfE P8Ecq1CIh77ay9Sox89tFMtl7y9vZZr708Cs2ZrD24M+MMuRds3kU/vVKCcRnSAAuNRY9r +kedfmtS65biGsxI8hYMCX/4OmIpZtQ= Received: from mail-il1-f199.google.com (mail-il1-f199.google.com [209.85.166.199]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-477-D8TIbLb2NW-1SM2bcu7hPA-1; Wed, 18 Mar 2020 09:02:02 -0400 X-MC-Unique: D8TIbLb2NW-1SM2bcu7hPA-1 Received: by mail-il1-f199.google.com with SMTP id c18so1877067ils.2 for ; Wed, 18 Mar 2020 06:02:02 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=umU8Z9JoL7Qhg365QcEJh2Mxe4az67bMGgKE4dWEnUw=; b=Hqj1Y+hxOhUn2KkrTmUajMxmYdCr3OT+qACuvlFVgzd7AiH3Ib6A9tE+kOuxyM+UAA d3J69LnZcop1wjpgDiXffEHnMZKi0l9Il6n9SGsx7iO/dC/SmAgpooqZ7ENQ3RyabBm8 iPnE3wXbH0FdPV+sgxgXu/o5mO8R/o59MxY3/0rSFxFQG9mPI4CimnmoeOei1IO0Q8RY JzBZULdZzc0V7KbB8o17kp4rj7f5zgzaI23wuLD2UqFVfgx1D5cA9pQDlBd1EripahCM g8f4qqpy+bxk+B3Q62uxQOU7zyvs3WMU4/dKvcsnpDKUKxaALS/Gvb3nH84B8pqybZqi AfZA== X-Gm-Message-State: ANhLgQ1NnDff6FMNHn7G3WKYKOt8Ro/1E05ZuDDxVzB8NvabCz26U/2q iCaRj2vajQy18MYUneeqaWCEDFCHSEcXDk1bR4NGgFKtVCmqSZ8ijPCIz+FxCbJcWtIYm0cI9h+ SIenqiK4qqsjOocsjenrCBcU4KGfr9oDRm2Lh X-Received: by 2002:a02:2302:: with SMTP id u2mr2616688jau.67.1584536520842; Wed, 18 Mar 2020 06:02:00 -0700 (PDT) X-Google-Smtp-Source: ADFU+vue0J337X7n3fMgGPXHgf7ihPUf0i9k0D3EkKwRboLLQkEbfoL5NML/4YXkkbv93kMa/Ws2N+WhXw/EJ/fj3co= X-Received: by 2002:a02:2302:: with SMTP id u2mr2616605jau.67.1584536520187; Wed, 18 Mar 2020 06:02:00 -0700 (PDT) MIME-Version: 1.0 References: <20200303233609.713348-1-jarkko.sakkinen@linux.intel.com> <20200303233609.713348-22-jarkko.sakkinen@linux.intel.com> <20200315012523.GC208715@linux.intel.com> <94ce05323c4de721c4a6347223885f2ad9f541af.camel@linux.intel.com> <5dc2ec4bc9433f9beae824759f411c32b45d4b74.camel@linux.intel.com> <20200316225322.GJ24267@linux.intel.com> <20200316235934.GM24267@linux.intel.com> In-Reply-To: From: Nathaniel McCallum Date: Wed, 18 Mar 2020 09:01:49 -0400 Message-ID: Subject: Re: [PATCH v28 21/22] x86/vdso: Implement a vDSO for Intel SGX enclave call To: "Xing, Cedric" Cc: Sean Christopherson , Jarkko Sakkinen , linux-kernel@vger.kernel.org, x86@kernel.org, linux-sgx@vger.kernel.org, akpm@linux-foundation.org, dave.hansen@intel.com, Neil Horman , "Huang, Haitao" , andriy.shevchenko@linux.intel.com, tglx@linutronix.de, "Svahn, Kai" , bp@alien8.de, Josh Triplett , luto@kernel.org, kai.huang@intel.com, David Rientjes , Patrick Uiterwijk , Andy Lutomirski , Jethro Beekman , Connor Kuehl , Harald Hoyer , Lily Sturmann X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Sender: linux-sgx-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org Archived-At: List-Archive: List-Post: On Tue, Mar 17, 2020 at 6:23 PM Xing, Cedric wrote: > > On 3/17/2020 9:50 AM, Nathaniel McCallum wrote: > > On Mon, Mar 16, 2020 at 8:18 PM Xing, Cedric wrote: > >> > >> On 3/16/2020 4:59 PM, Sean Christopherson wrote: > >>> On Mon, Mar 16, 2020 at 04:50:26PM -0700, Xing, Cedric wrote: > >>>> On 3/16/2020 3:53 PM, Sean Christopherson wrote: > >>>>> On Mon, Mar 16, 2020 at 11:38:24PM +0200, Jarkko Sakkinen wrote: > >>>>>>> My suggestions explicitly maintained robustness, and in fact increased > >>>>>>> it. If you think we've lost capability, please speak with specificity > >>>>>>> rather than in vague generalities. Under my suggestions we can: > >>>>>>> 1. call the vDSO from C > >>>>>>> 2. pass context to the handler > >>>>>>> 3. have additional stack manipulation options in the handler > >>>>>>> > >>>>>>> The cost for this is a net 2 additional instructions. No existing > >>>>>>> capability is lost. > >>>>>> > >>>>>> My vague generality in this case is just that the whole design > >>>>>> approach so far has been to minimize the amount of wrapping to > >>>>>> EENTER. > >>>>> > >>>>> Yes and no. If we wanted to minimize the amount of wrapping around the > >>>>> vDSO's ENCLU then we wouldn't have the exit handler shenanigans in the > >>>>> first place. The whole process has been about balancing the wants of each > >>>>> use case against the overall quality of the API and code. > >>>>> > >>>> The design of this vDSO API was NOT to minimize wrapping, but to allow > >>>> maximal flexibility. More specifically, we strove not to restrict how info > >>>> was exchanged between the enclave and its host process. After all, calling > >>>> convention is compiler specific - i.e. the enclave could be built by a > >>>> different compiler (e.g. MSVC) that doesn't share the same list of CSRs as > >>>> the host process. Therefore, the API has been implemented to pass through > >>>> virtually all registers except those used by EENTER itself. Similarly, all > >>>> registers are passed back from enclave to the caller (or the exit handler) > >>>> except those used by EEXIT. %rbp is an exception because the vDSO API has to > >>>> anchor the stack, using either %rsp or %rbp. We picked %rbp to allow the > >>>> enclave to allocate space on the stack. > >>> > >>> And unless I'm missing something, using %rcx to pass @leaf would still > >>> satisfy the above, correct? Ditto for saving/restoring %rbx. > >>> > >>> I.e. a runtime that's designed to work with enclave's using a different > >>> calling convention wouldn't be able to take advantage of being able to call > >>> the vDSO from C, but neither would it take on any meaningful burden. > >>> > >> Not exactly. > >> > >> If called directly from C code, the caller would expect CSRs to be > >> preserved. > > > > Correct. This requires collaboration between the caller of the vDSO > > and the enclave. > > > >> Then who should preserve CSRs? > > > > The enclave. > > > >> It can't be the enclave > >> because it may not follow the same calling convention. > > > > This is incorrect. You are presuming there is not tight integration > > between the caller of the vDSO and the enclave. In my case, the > > integration is total and complete. We have working code today that > > does this. > > > >> Moreover, the > >> enclave may run into an exception, in which case it doesn't have the > >> ability to restore CSRs. > > > > There are two solutions to this: > > 1. Write the handler in assembly and don't return to C on AEX. > > 2. The caller can simply preserve the registers. Nothing stops that. > > > > We have implemented #1. > > > What if the enclave cannot proceed due to an unhandled exception so the > execution has to get back to the C caller of the vDSO API? mov $60, %rax mov $1, %rdi syscall We exit in all such cases. > It seems to me the caller has to preserve CSRs by itself, otherwise it > cannot continue execution after any enclave exception. Passing @leaf in > %ecx will allow saving/restoring CSRs in C by setjmp()/longjmp(), with > the help of an exit handler. But if the C caller has already preserved > CSRs, why preserve CSRs again inside the enclave? It looks to me things > can be simplified only if the host process handles no enclave exceptions > (or exceptions inside the enclave will crash the calling thread). Thus > the only case of enclave EEXIT'ing back to its caller is considered > valid, hence the enclave will always be able to restore CSRs, so that > neither vDSO nor its caller has to preserve CSRs. > > Is my understanding correct? >