From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sean Christopherson Subject: Re: RFC: userspace exception fixups Date: Tue, 20 Nov 2018 10:09:44 -0800 Message-ID: <20181120180944.GD31109@linux.intel.com> References: <20181118071548.GA4795@linux.intel.com> <20181119160204.GD13298@linux.intel.com> <20181120101133.GA7319@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20181120101133.GA7319@linux.intel.com> Sender: linux-kernel-owner@vger.kernel.org To: Jarkko Sakkinen Cc: Andy Lutomirski , Dave Hansen , Jethro Beekman , Florian Weimer , Linux API , Jann Horn , Linus Torvalds , X86 ML , linux-arch , LKML , Peter Zijlstra , Rich Felker , 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 List-Id: linux-arch.vger.kernel.org On Tue, Nov 20, 2018 at 12:11:33PM +0200, Jarkko Sakkinen wrote: > On Mon, Nov 19, 2018 at 09:00:08AM -0800, Andy Lutomirski wrote: > > On Mon, Nov 19, 2018 at 8:02 AM Jarkko Sakkinen > > wrote: > > > > > > On Mon, Nov 19, 2018 at 07:29:36AM -0800, Andy Lutomirski wrote: > > > > 1. The kernel needs some way to know *when* to apply this fixup. > > > > Decoding the instruction stream and doing it to all exceptions that > > > > hit an ENCLU instruction seems like a poor design. > > > > > > I'm not sure why you would ever need to do any type of fixup as the idea > > > is to just return to AEP i.e. from chosen exceptions (EPCM, #UD) the AEP > > > would work the same way as for exceptions that the kernel can deal with > > > except filling the exception information to registers. > > > > Sure, but how does the kernel know when to do that and when to send a > > signal? I don't really like decoding the instruction stream to figure > > it out. > > Hmm... why you have to decode instruction stream to find that out? Would > just depend on exception type (#GP with EPCM, #UD). #PF w/ PFEC_SGX is the only exception that indicates a fault is related to SGX. Theoretically we could avoid decoding by using a magic value for the AEP itself and doing even more magic fixup, but that wouldn't help for faults that occur on EENTER, which can be generic #GPs due to loss of EPC on SGX1 systems. > Or are you saying > that kernel should need to SIGSEGV if there is in fact ENCLU so that > there is no infinite trap loop? Sorry, I'm a bit lost here that where > does this decoding requirement comes from in the first place. I > understand how it is used in Sean's proposal... > > Anyway, this option can be probably discarded without further > consideration because apparently single stepping can cause #DB SS fault > if AEP handler is anything else than a single instruction. Not that it matters, but we could satisfy the "one instruction" requirement if the fixup changed RIP to point at an ENCLU for #DBs. > For me it seems that by ruling out options, vDSO option is what is > left. I don't like it but at least it works... > > /Jarkko From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga03.intel.com ([134.134.136.65]:39197 "EHLO mga03.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726047AbeKUEkK (ORCPT ); Tue, 20 Nov 2018 23:40:10 -0500 Date: Tue, 20 Nov 2018 10:09:44 -0800 From: Sean Christopherson Subject: Re: RFC: userspace exception fixups Message-ID: <20181120180944.GD31109@linux.intel.com> References: <20181118071548.GA4795@linux.intel.com> <20181119160204.GD13298@linux.intel.com> <20181120101133.GA7319@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181120101133.GA7319@linux.intel.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Jarkko Sakkinen Cc: Andy Lutomirski , Dave Hansen , Jethro Beekman , Florian Weimer , Linux API , Jann Horn , Linus Torvalds , X86 ML , linux-arch , LKML , Peter Zijlstra , Rich Felker , 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 Message-ID: <20181120180944.XrjtUoZrykGQ5bSciXptKYGjkRGno4KSAqxUbVPhu8I@z> On Tue, Nov 20, 2018 at 12:11:33PM +0200, Jarkko Sakkinen wrote: > On Mon, Nov 19, 2018 at 09:00:08AM -0800, Andy Lutomirski wrote: > > On Mon, Nov 19, 2018 at 8:02 AM Jarkko Sakkinen > > wrote: > > > > > > On Mon, Nov 19, 2018 at 07:29:36AM -0800, Andy Lutomirski wrote: > > > > 1. The kernel needs some way to know *when* to apply this fixup. > > > > Decoding the instruction stream and doing it to all exceptions that > > > > hit an ENCLU instruction seems like a poor design. > > > > > > I'm not sure why you would ever need to do any type of fixup as the idea > > > is to just return to AEP i.e. from chosen exceptions (EPCM, #UD) the AEP > > > would work the same way as for exceptions that the kernel can deal with > > > except filling the exception information to registers. > > > > Sure, but how does the kernel know when to do that and when to send a > > signal? I don't really like decoding the instruction stream to figure > > it out. > > Hmm... why you have to decode instruction stream to find that out? Would > just depend on exception type (#GP with EPCM, #UD). #PF w/ PFEC_SGX is the only exception that indicates a fault is related to SGX. Theoretically we could avoid decoding by using a magic value for the AEP itself and doing even more magic fixup, but that wouldn't help for faults that occur on EENTER, which can be generic #GPs due to loss of EPC on SGX1 systems. > Or are you saying > that kernel should need to SIGSEGV if there is in fact ENCLU so that > there is no infinite trap loop? Sorry, I'm a bit lost here that where > does this decoding requirement comes from in the first place. I > understand how it is used in Sean's proposal... > > Anyway, this option can be probably discarded without further > consideration because apparently single stepping can cause #DB SS fault > if AEP handler is anything else than a single instruction. Not that it matters, but we could satisfy the "one instruction" requirement if the fixup changed RIP to point at an ENCLU for #DBs. > For me it seems that by ruling out options, vDSO option is what is > left. I don't like it but at least it works... > > /Jarkko