From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH 01/10] x86/cet: User-mode shadow stack support Date: Thu, 7 Jun 2018 11:23:18 -0700 Message-ID: References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-2-yu-cheng.yu@intel.com> <1528393611.4636.70.camel@2b52.sc.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: In-Reply-To: <1528393611.4636.70.camel@2b52.sc.intel.com> Sender: linux-kernel-owner@vger.kernel.org To: Yu-cheng Yu , Florian Weimer Cc: Andrew Lutomirski , 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 Thu, Jun 7, 2018 at 10:50 AM Yu-cheng Yu wrote: > > On Thu, 2018-06-07 at 09:37 -0700, Andy Lutomirski wrote: > > On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu wrote: > > > > > > This patch adds basic shadow stack enabling/disabling routines. > > > A task's shadow stack is allocated from memory with VM_SHSTK > > > flag set and read-only protection. The shadow stack is > > > allocated to a fixed size and that can be changed by the system > > > admin. > > > > How do threads work? Can a user program mremap() its shadow stack to > > make it bigger? > > A pthread's shadow stack is allocated/freed by the kernel. This patch > has the supporting routines that handle both non-pthread and pthread. > > In [PATCH 04/10] "Handle thread shadow stack", we allocate pthread > shadow stack in copy_thread_tls(), and free it in deactivate_mm(). > > If clone of a pthread fails, shadow stack is freed in > cet_disable_free_shstk() below (I will add more comments): > > If (Current thread existing) > Disable and free shadow stack > > If (Clone of a pthread fails) > Free the pthread shadow stack > > We block mremap, mprotect, madvise, and munmap on a vma that has > VM_SHSTK (in separate patches). Why? mremap() seems like a sensible way to enlarge a shadow stack. munmap() seems like a good way to get rid of one, and mmap() seems like a nice way to create a new shadow stack if one were needed (for green threads or similar). From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:60816 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936236AbeFGSXb (ORCPT ); Thu, 7 Jun 2018 14:23:31 -0400 Received: from mail-wm0-f44.google.com (mail-wm0-f44.google.com [74.125.82.44]) (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 20781208B0 for ; Thu, 7 Jun 2018 18:23:31 +0000 (UTC) Received: by mail-wm0-f44.google.com with SMTP id 69-v6so7970795wmf.3 for ; Thu, 07 Jun 2018 11:23:31 -0700 (PDT) MIME-Version: 1.0 References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-2-yu-cheng.yu@intel.com> <1528393611.4636.70.camel@2b52.sc.intel.com> In-Reply-To: <1528393611.4636.70.camel@2b52.sc.intel.com> From: Andy Lutomirski Date: Thu, 7 Jun 2018 11:23:18 -0700 Message-ID: Subject: Re: [PATCH 01/10] x86/cet: User-mode shadow stack support Content-Type: text/plain; charset="UTF-8" Sender: linux-arch-owner@vger.kernel.org List-ID: To: Yu-cheng Yu , Florian Weimer Cc: Andrew Lutomirski , 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: <20180607182318.T1RcpHDQD8sUkPbXQ-9EDEUxu6j-EajAsSJQ-zTkwwo@z> On Thu, Jun 7, 2018 at 10:50 AM Yu-cheng Yu wrote: > > On Thu, 2018-06-07 at 09:37 -0700, Andy Lutomirski wrote: > > On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu wrote: > > > > > > This patch adds basic shadow stack enabling/disabling routines. > > > A task's shadow stack is allocated from memory with VM_SHSTK > > > flag set and read-only protection. The shadow stack is > > > allocated to a fixed size and that can be changed by the system > > > admin. > > > > How do threads work? Can a user program mremap() its shadow stack to > > make it bigger? > > A pthread's shadow stack is allocated/freed by the kernel. This patch > has the supporting routines that handle both non-pthread and pthread. > > In [PATCH 04/10] "Handle thread shadow stack", we allocate pthread > shadow stack in copy_thread_tls(), and free it in deactivate_mm(). > > If clone of a pthread fails, shadow stack is freed in > cet_disable_free_shstk() below (I will add more comments): > > If (Current thread existing) > Disable and free shadow stack > > If (Clone of a pthread fails) > Free the pthread shadow stack > > We block mremap, mprotect, madvise, and munmap on a vma that has > VM_SHSTK (in separate patches). Why? mremap() seems like a sensible way to enlarge a shadow stack. munmap() seems like a good way to get rid of one, and mmap() seems like a nice way to create a new shadow stack if one were needed (for green threads or similar).