From mboxrd@z Thu Jan 1 00:00:00 1970 From: Florian Weimer Subject: Re: [PATCH 04/10] x86/cet: Handle thread shadow stack Date: Fri, 8 Jun 2018 16:53:03 +0200 Message-ID: <6ee29e8b-4a0a-3459-a1ee-03923ba4e15d@redhat.com> References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-5-yu-cheng.yu@intel.com> <3c1bdf85-0c52-39ed-a799-e26ac0e52391@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Andy Lutomirski Cc: Yu-cheng Yu , LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , "H. J. Lu" , "Shanbhogue, Vedvyas" , "Ravi V. Shankar" , Dave Hansen , Jonathan Corbet , Oleg Nesterov , Arnd Bergmann , mike.kravetz@oracle.com List-Id: linux-arch.vger.kernel.org On 06/07/2018 10:53 PM, Andy Lutomirski wrote: > On Thu, Jun 7, 2018 at 12:47 PM Florian Weimer wrote: >> >> On 06/07/2018 08:21 PM, Andy Lutomirski wrote: >>> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu wrote: >>>> >>>> When fork() specifies CLONE_VM but not CLONE_VFORK, the child >>>> needs a separate program stack and a separate shadow stack. >>>> This patch handles allocation and freeing of the thread shadow >>>> stack. >>> >>> Aha -- you're trying to make this automatic. I'm not convinced this >>> is a good idea. The Linux kernel has a long and storied history of >>> enabling new hardware features in ways that are almost entirely >>> useless for userspace. >>> >>> Florian, do you have any thoughts on how the user/kernel interaction >>> for the shadow stack should work? >> >> I have not looked at this in detail, have not played with the emulator, >> and have not been privy to any discussions before these patches have >> been posted, however … >> >> I believe that we want as little code in userspace for shadow stack >> management as possible. One concern I have is that even with the code >> we arguably need for various kinds of stack unwinding, we might have >> unwittingly built a generic trampoline that leads to full CET bypass. > > I was imagining an API like "allocate a shadow stack for the current > thread, fail if the current thread already has one, and turn on the > shadow stack". glibc would call clone and then call this ABI pretty > much immediately (i.e. before making any calls from which it expects > to return). Ahh. So you propose not to enable shadow stack enforcement on the new thread even if it is enabled for the current thread? For the cases where CLONE_VM is involved? It will still need a new assembler wrapper which sets up the shadow stack, and it's probably required to disable signals. I think it should be reasonable safe and actually implementable. But the benefits are not immediately obvious to me. > We definitely want strong enough user control that tools like CRIU can > continue to work. I haven't looked at the SDM recently enough to > remember for sure, but I'm reasonably confident that user code can > learn the address of its own shadow stack. If nothing else, CRIU > needs to be able to restore from a context where there's a signal on > the stack and the signal frame contains a shadow stack pointer. CRIU also needs the shadow stack *contents*, which shouldn't be directly available to the process. So it needs very special interfaces anyway. Does CRIU implement MPX support? Thanks, Florian From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx3-rdu2.redhat.com ([66.187.233.73]:42054 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751152AbeFHOxL (ORCPT ); Fri, 8 Jun 2018 10:53:11 -0400 Subject: Re: [PATCH 04/10] x86/cet: Handle thread shadow stack References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-5-yu-cheng.yu@intel.com> <3c1bdf85-0c52-39ed-a799-e26ac0e52391@redhat.com> From: Florian Weimer Message-ID: <6ee29e8b-4a0a-3459-a1ee-03923ba4e15d@redhat.com> Date: Fri, 8 Jun 2018 16:53:03 +0200 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Andy Lutomirski Cc: Yu-cheng Yu , LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , X86 ML , "H. Peter Anvin" , Thomas Gleixner , Ingo Molnar , "H. J. Lu" , "Shanbhogue, Vedvyas" , "Ravi V. Shankar" , Dave Hansen , Jonathan Corbet , Oleg Nesterov , Arnd Bergmann , mike.kravetz@oracle.com Message-ID: <20180608145303.tLOSFZwztZ3RNxiLsGeGzRunP5CkxCehAW_t0v8He14@z> On 06/07/2018 10:53 PM, Andy Lutomirski wrote: > On Thu, Jun 7, 2018 at 12:47 PM Florian Weimer wrote: >> >> On 06/07/2018 08:21 PM, Andy Lutomirski wrote: >>> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu wrote: >>>> >>>> When fork() specifies CLONE_VM but not CLONE_VFORK, the child >>>> needs a separate program stack and a separate shadow stack. >>>> This patch handles allocation and freeing of the thread shadow >>>> stack. >>> >>> Aha -- you're trying to make this automatic. I'm not convinced this >>> is a good idea. The Linux kernel has a long and storied history of >>> enabling new hardware features in ways that are almost entirely >>> useless for userspace. >>> >>> Florian, do you have any thoughts on how the user/kernel interaction >>> for the shadow stack should work? >> >> I have not looked at this in detail, have not played with the emulator, >> and have not been privy to any discussions before these patches have >> been posted, however … >> >> I believe that we want as little code in userspace for shadow stack >> management as possible. One concern I have is that even with the code >> we arguably need for various kinds of stack unwinding, we might have >> unwittingly built a generic trampoline that leads to full CET bypass. > > I was imagining an API like "allocate a shadow stack for the current > thread, fail if the current thread already has one, and turn on the > shadow stack". glibc would call clone and then call this ABI pretty > much immediately (i.e. before making any calls from which it expects > to return). Ahh. So you propose not to enable shadow stack enforcement on the new thread even if it is enabled for the current thread? For the cases where CLONE_VM is involved? It will still need a new assembler wrapper which sets up the shadow stack, and it's probably required to disable signals. I think it should be reasonable safe and actually implementable. But the benefits are not immediately obvious to me. > We definitely want strong enough user control that tools like CRIU can > continue to work. I haven't looked at the SDM recently enough to > remember for sure, but I'm reasonably confident that user code can > learn the address of its own shadow stack. If nothing else, CRIU > needs to be able to restore from a context where there's a signal on > the stack and the signal frame contains a shadow stack pointer. CRIU also needs the shadow stack *contents*, which shouldn't be directly available to the process. So it needs very special interfaces anyway. Does CRIU implement MPX support? Thanks, Florian