From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl0-f72.google.com (mail-pl0-f72.google.com [209.85.160.72]) by kanga.kvack.org (Postfix) with ESMTP id A32E46B000C for ; Fri, 8 Jun 2018 11:01:59 -0400 (EDT) Received: by mail-pl0-f72.google.com with SMTP id x2-v6so7498188plv.0 for ; Fri, 08 Jun 2018 08:01:59 -0700 (PDT) Received: from mail.kernel.org (mail.kernel.org. [198.145.29.99]) by mx.google.com with ESMTPS id c21-v6si27956900pgw.50.2018.06.08.08.01.58 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 08 Jun 2018 08:01:58 -0700 (PDT) Received: from mail-wm0-f54.google.com (mail-wm0-f54.google.com [74.125.82.54]) (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 762E3208AD for ; Fri, 8 Jun 2018 15:01:57 +0000 (UTC) Received: by mail-wm0-f54.google.com with SMTP id x6-v6so3872562wmc.3 for ; Fri, 08 Jun 2018 08:01:57 -0700 (PDT) MIME-Version: 1.0 References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-5-yu-cheng.yu@intel.com> <3c1bdf85-0c52-39ed-a799-e26ac0e52391@redhat.com> <6ee29e8b-4a0a-3459-a1ee-03923ba4e15d@redhat.com> In-Reply-To: <6ee29e8b-4a0a-3459-a1ee-03923ba4e15d@redhat.com> From: Andy Lutomirski Date: Fri, 8 Jun 2018 08:01:43 -0700 Message-ID: Subject: Re: [PATCH 04/10] x86/cet: Handle thread shadow stack Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Florian Weimer Cc: Andrew Lutomirski , 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 On Fri, Jun 8, 2018 at 7:53 AM Florian Weimer wrote: > > On 06/07/2018 10:53 PM, Andy Lutomirski wrote: > > On Thu, Jun 7, 2018 at 12:47 PM Florian Weimer wro= te: > >> > >> On 06/07/2018 08:21 PM, Andy Lutomirski wrote: > >>> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu wr= ote: > >>>> > >>>> 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 =E2=80=A6 > >> > >> 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. Doing it this way would have been my first incliniation. It would avoid all the oddities of the kernel magically creating a VMA when clone() is called, guessing the shadow stack size, etc. But I'm okay with having the kernel do it automatically, too. I think it would be very nice to have a way for user code to find out the size of the shadow stack and change it, though. (And relocate it, but maybe that's impossible. The CET documentation doesn't have a clear description of the shadow stack layout.) > > > 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. True. I proposed in a different email that ptrace() have full control of the shadow stack (read, write, lock, unlock, etc). > > Does CRIU implement MPX support? Dunno. But given that MPX seems to be dying, I'm not sure it matters. --Andy