From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f71.google.com (mail-pg0-f71.google.com [74.125.83.71]) by kanga.kvack.org (Postfix) with ESMTP id 8E9EA6B0003 for ; Thu, 21 Jun 2018 19:10:57 -0400 (EDT) Received: by mail-pg0-f71.google.com with SMTP id 69-v6so1814241pgg.0 for ; Thu, 21 Jun 2018 16:10:57 -0700 (PDT) Received: from mga05.intel.com (mga05.intel.com. [192.55.52.43]) by mx.google.com with ESMTPS id j12-v6si4741378pgq.312.2018.06.21.16.10.51 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 21 Jun 2018 16:10:51 -0700 (PDT) Message-ID: <1529622448.30393.22.camel@intel.com> Subject: Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack From: Yu-cheng Yu Date: Thu, 21 Jun 2018 16:07:28 -0700 In-Reply-To: <13E3C29A-3295-4A7F-90EC-A84CF34F3E1A@amacapital.net> References: <20180607143807.3611-1-yu-cheng.yu@intel.com> <20180607143807.3611-7-yu-cheng.yu@intel.com> <1528403417.5265.35.camel@2b52.sc.intel.com> <569B4719-6283-4575-A16E-D0A78D280F4E@amacapital.net> <1529427588.23068.7.camel@intel.com> <0AF8B71E-B6CC-42DE-B95C-93896196C3D7@amacapital.net> <446EB18D-EF06-4A04-AF62-E72C68D96A84@amacapital.net> <1529447937.27370.33.camel@intel.com> <13E3C29A-3295-4A7F-90EC-A84CF34F3E1A@amacapital.net> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: Kees Cook , Andy Lutomirski , "H. J. Lu" , Thomas Gleixner , LKML , linux-doc@vger.kernel.org, Linux-MM , linux-arch , X86 ML , "H. Peter Anvin" , Ingo Molnar , "Shanbhogue, Vedvyas" , "Ravi V. Shankar" , Dave Hansen , Jonathan Corbet , Oleg Nesterov , Arnd Bergmann , mike.kravetz@oracle.com, Florian Weimer On Tue, 2018-06-19 at 17:50 -0700, Andy Lutomirski wrote: > > > > > On Jun 19, 2018, at 3:38 PM, Yu-cheng Yu > > wrote: > > > > On Tue, 2018-06-19 at 13:47 -0700, Andy Lutomirski wrote: > > > > > > > > > > > > > > > On Jun 19, 2018, at 1:12 PM, Kees Cook > > > > wrote: > > > > > > > > > > > > > > > > > > > On Tue, Jun 19, 2018 at 10:20 AM, Andy Lutomirski > > > > pita > > > > > l.net> wrote: > > > > > > > > > > > > > > > > > > > > > > > On Jun 19, 2018, at 10:07 AM, Kees Cook > > > > > org> > > > > > > wrote: > > > > > > > > > > > > Does it provide anything beyond what PR_DUMPABLE does? > > > > > What do you mean? > > > > I was just going by the name of it. I wasn't sure what "ptrace > > > > CET > > > > lock" meant, so I was trying to understand if it was another > > > > "you > > > > can't ptrace me" toggle, and if so, wouldn't it be redundant > > > > with > > > > PR_SET_DUMPABLE = 0, etc. > > > > > > > No, other way around. The valid CET states are on/unlocked, > > > off/unlocked, on/locked, off/locked. arch_prctl can freely the > > > state > > > unless locked. ptrace can change it no matter what.A A The lock is > > > to > > > prevent the existence of a gadget to disable CET (unless the > > > gadget > > > involves ptrace, but I dona??t think thata??s a real concern). > > We have the arch_prctl now and only need to add ptrace lock/unlock. > > > > Back to the dlopen() "relaxed" mode. Would the following work? > > > > If the lib being loaded does not use setjmp/getcontext families > > (the > > loader knows?), then the loader leaves shstk on.A A > Will that actually work?A A Are there libs that do something like > longjmp without actually using the glibc longjmp routine?A A What about > compilers that statically match a throw to a catch and try to return > through several frames at once? > The compiler throw/catch is already handled similarly to how longjmp is handled. To summarize the dlopen() situation, ---- (1) We don't want to fall back like the following. A One reason is turning off SHSTK for threads is tricky. if ((dlopen() a legacy library) && (cet_policy==relaxed)) { /* A * We don't care if the library will actually fault; A * just turn off CET protection now. A */ Turn off CET; } (2) We cannot predict what version of a library will be dlopen'ed, and cannot turn off CET reliably from the beginning of an application. ---- Can we mandate a signal handler (to turn off CET) when ((dlopen is used ) && (cet_policy==relaxed))? > > > > Otherwise, if the > > system-wide setting is "relaxed", the loader turns off shstk and > > issues > > a warning.A A In addition, if (dlopen == relaxed), then cet is not > > locked > > in any time. > > > > The system-wide setting (somewhere in /etc?) can be: > > > > A A A dlopen=force|relaxed /* controls dlopen of non-cet libs */ > > A A A exec=force|relaxed /* controls exec of non-cet apps */ > > > > > Why do we need a whole new mechanism here?A A Cana??t all this use > regular glibc tunables? Ok, got it. Yu-cheng