From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id 8EA796B0005 for ; Tue, 12 Jun 2018 16:18:48 -0400 (EDT) Received: by mail-pf0-f198.google.com with SMTP id y8-v6so102392pfl.17 for ; Tue, 12 Jun 2018 13:18:48 -0700 (PDT) Received: from mga02.intel.com (mga02.intel.com. [134.134.136.20]) by mx.google.com with ESMTPS id t66-v6si789154pgc.6.2018.06.12.13.18.47 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 12 Jun 2018 13:18:47 -0700 (PDT) Message-ID: <1528834538.9849.13.camel@2b52.sc.intel.com> Subject: Re: [PATCH 00/10] Control Flow Enforcement - Part (3) From: Yu-cheng Yu Date: Tue, 12 Jun 2018 13:15:38 -0700 In-Reply-To: <1528824280.9447.30.camel@2b52.sc.intel.com> References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <1528815820.8271.16.camel@2b52.sc.intel.com> <1528820489.9324.14.camel@2b52.sc.intel.com> <1528824280.9447.30.camel@2b52.sc.intel.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: bsingharora@gmail.com, 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 Tue, 2018-06-12 at 10:24 -0700, Yu-cheng Yu wrote: > On Tue, 2018-06-12 at 09:31 -0700, Andy Lutomirski wrote: > > On Tue, Jun 12, 2018 at 9:24 AM Yu-cheng Yu wrote: > > > > > > On Tue, 2018-06-12 at 09:00 -0700, Andy Lutomirski wrote: > > > > On Tue, Jun 12, 2018 at 8:06 AM Yu-cheng Yu wrote: > > > > > > > > > > On Tue, 2018-06-12 at 20:56 +1000, Balbir Singh wrote: > > > > > > > > > > > > On 08/06/18 00:37, Yu-cheng Yu wrote: > > > > > > > This series introduces CET - Shadow stack > > > > > > > > > > > > > > At the high level, shadow stack is: > > > > > > > > > > > > > > Allocated from a task's address space with vm_flags VM_SHSTK; > > > > > > > Its PTEs must be read-only and dirty; > > > > > > > Fixed sized, but the default size can be changed by sys admin. > > > > > > > > > > > > > > For a forked child, the shadow stack is duplicated when the next > > > > > > > shadow stack access takes place. > > > > > > > > > > > > > > For a pthread child, a new shadow stack is allocated. > > > > > > > > > > > > > > The signal handler uses the same shadow stack as the main program. > > > > > > > > > > > > > > > > > > > Even with sigaltstack()? > > > > > > > > > > > > > > > > > > Balbir Singh. > > > > > > > > > > Yes. > > > > > > > > > > > > > I think we're going to need some provision to add an alternate signal > > > > stack to handle the case where the shadow stack overflows. > > > > > > The shadow stack stores only return addresses; its consumption will not > > > exceed a percentage of (program stack size + sigaltstack size) before > > > those overflow. When that happens, there is usually very little we can > > > do. So we set a default shadow stack size that supports certain nested > > > calls and allow sys admin to adjust it. > > > > > > > Of course there's something you can do: add a sigaltstack-like stack > > switching mechanism. Have a reserve shadow stack and, when a signal > > is delivered (possibly guarded by other conditions like "did the > > shadow stack overflow"), switch to a new shadow stack and maybe write > > a special token to the new shadow stack that says "signal delivery > > jumped here and will restore to the previous shadow stack and > > such-and-such address on return". > > If (shstk size == (stack size + sigaltstack size)), then shstk will not > overflow before program stack overflows and sigaltstack also overflows. > > Let me think about this. The reserve shadow stack will help only when the shstk overflows but signal stack/sigaltstack still has room and we can deliver a signal. If the shstk is large enough to cover any nested calls that will overflow both the program stack and sigaltstack then we don't need a reserve shstk. We can estimate how big the shstk needs to be; in the worst case it should not be greater than (program stack size + sigaltstack size). The default shstk size we choose pass all signal tests in GLIBC. In case there is a need to increase it for a very large RLIMIT_STACK or very large sigaltstack, the sys admin can increase the default shstk size. Yu-cheng