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From: Andy Lutomirski <luto@kernel.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: Andrew Lutomirski <luto@kernel.org>,
	Yu-cheng Yu <yu-cheng.yu@intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
	linux-arch <linux-arch@vger.kernel.org>, X86 ML <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. J. Lu" <hjl.tools@gmail.com>,
	"Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Jonathan Corbet <corbet@lwn.net>, Oleg Nesterov <oleg@redhat.com>,
	Arnd Bergmann <arnd@arndb.de>,
	mike.kravetz@oracle.com
Subject: Re: [PATCH 04/10] x86/cet: Handle thread shadow stack
Date: Fri, 8 Jun 2018 08:01:43 -0700	[thread overview]
Message-ID: <CALCETrV_V68nVhCpUSGXrwUKCu4utbdp01snmG=G=+_xAo0KJA@mail.gmail.com> (raw)
In-Reply-To: <6ee29e8b-4a0a-3459-a1ee-03923ba4e15d@redhat.com>

On Fri, Jun 8, 2018 at 7:53 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> On 06/07/2018 10:53 PM, Andy Lutomirski wrote:
> > On Thu, Jun 7, 2018 at 12:47 PM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> On 06/07/2018 08:21 PM, Andy Lutomirski wrote:
> >>> On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@intel.com> 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.

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

  reply	other threads:[~2018-06-08 15:02 UTC|newest]

Thread overview: 98+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-07 14:37 [PATCH 00/10] Control Flow Enforcement - Part (3) Yu-cheng Yu
2018-06-07 14:37 ` [PATCH 01/10] x86/cet: User-mode shadow stack support Yu-cheng Yu
2018-06-07 16:37   ` Andy Lutomirski
2018-06-07 17:46     ` Yu-cheng Yu
2018-06-07 17:55       ` Dave Hansen
2018-06-07 18:23       ` Andy Lutomirski
2018-06-12 11:56   ` Balbir Singh
2018-06-12 15:03     ` Yu-cheng Yu
2018-06-07 14:37 ` [PATCH 02/10] x86/cet: Introduce WRUSS instruction Yu-cheng Yu
2018-06-07 16:40   ` Andy Lutomirski
2018-06-07 16:51     ` Yu-cheng Yu
2018-06-07 18:41     ` Peter Zijlstra
2018-06-07 20:31       ` Yu-cheng Yu
2018-06-11  8:17     ` Peter Zijlstra
2018-06-11 15:02       ` Yu-cheng Yu
2018-06-14  1:30   ` Balbir Singh
2018-06-14 14:43     ` Yu-cheng Yu
2018-06-07 14:38 ` [PATCH 03/10] x86/cet: Signal handling for shadow stack Yu-cheng Yu
2018-06-07 18:30   ` Andy Lutomirski
2018-06-07 18:58     ` Florian Weimer
2018-06-07 19:51       ` Yu-cheng Yu
2018-06-07 20:07     ` Cyrill Gorcunov
2018-06-07 20:57       ` Andy Lutomirski
2018-06-08 12:07         ` Cyrill Gorcunov
2018-06-07 20:12     ` Yu-cheng Yu
2018-06-07 20:17       ` Dave Hansen
2018-06-07 14:38 ` [PATCH 04/10] x86/cet: Handle thread " Yu-cheng Yu
2018-06-07 18:21   ` Andy Lutomirski
2018-06-07 19:47     ` Florian Weimer
2018-06-07 20:53       ` Andy Lutomirski
2018-06-08 14:53         ` Florian Weimer
2018-06-08 15:01           ` Andy Lutomirski [this message]
2018-06-08 15:50             ` Yu-cheng Yu
2018-06-07 14:38 ` [PATCH 05/10] x86/cet: ELF header parsing of Control Flow Enforcement Yu-cheng Yu
2018-06-07 18:38   ` Andy Lutomirski
2018-06-07 20:40     ` Yu-cheng Yu
2018-06-07 14:38 ` [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack Yu-cheng Yu
2018-06-07 18:48   ` Andy Lutomirski
2018-06-07 20:30     ` Yu-cheng Yu
2018-06-07 21:01       ` Andy Lutomirski
2018-06-07 22:02         ` H.J. Lu
2018-06-07 23:01           ` Andy Lutomirski
2018-06-08  4:09             ` H.J. Lu
2018-06-08  4:38               ` Andy Lutomirski
2018-06-08 12:24                 ` H.J. Lu
2018-06-08 14:57                   ` Andy Lutomirski
2018-06-08 15:52                     ` Cyrill Gorcunov
2018-06-08  4:22           ` H.J. Lu
2018-06-08  4:35             ` Andy Lutomirski
2018-06-08 12:17               ` H.J. Lu
2018-06-12 10:03           ` Thomas Gleixner
2018-06-12 11:43             ` H.J. Lu
2018-06-12 16:01               ` Andy Lutomirski
2018-06-12 16:05                 ` H.J. Lu
2018-06-12 16:34                   ` Andy Lutomirski
2018-06-12 16:51                     ` H.J. Lu
2018-06-12 18:59                       ` Thomas Gleixner
2018-06-12 19:34                         ` H.J. Lu
2018-06-18 22:03                           ` Andy Lutomirski
2018-06-19  0:52                             ` Kees Cook
2018-06-19  6:40                               ` Florian Weimer
2018-06-19 14:50                               ` Andy Lutomirski
2018-06-19 16:44                                 ` Kees Cook
2018-06-19 16:59                                   ` Yu-cheng Yu
2018-06-19 17:07                                     ` Kees Cook
2018-06-19 17:20                                       ` Andy Lutomirski
2018-06-19 20:12                                         ` Kees Cook
2018-06-19 20:47                                           ` Andy Lutomirski
2018-06-19 22:38                                             ` Yu-cheng Yu
2018-06-20  0:50                                               ` Andy Lutomirski
2018-06-21 23:07                                                 ` Yu-cheng Yu
2018-06-07 14:38 ` [PATCH 07/10] mm: Prevent mprotect from changing " Yu-cheng Yu
2018-06-07 14:38 ` [PATCH 08/10] mm: Prevent mremap of " Yu-cheng Yu
2018-06-07 18:48   ` Andy Lutomirski
2018-06-07 20:18     ` Yu-cheng Yu
2018-06-07 14:38 ` [PATCH 09/10] mm: Prevent madvise from changing " Yu-cheng Yu
2018-06-07 20:54   ` Andy Lutomirski
2018-06-07 21:09   ` Nadav Amit
2018-06-07 21:18     ` Yu-cheng Yu
2018-06-07 14:38 ` [PATCH 10/10] mm: Prevent munmap and remap_file_pages of " Yu-cheng Yu
2018-06-07 18:50   ` Andy Lutomirski
2018-06-07 20:15     ` Yu-cheng Yu
2018-06-12 10:56 ` [PATCH 00/10] Control Flow Enforcement - Part (3) Balbir Singh
2018-06-12 15:03   ` Yu-cheng Yu
2018-06-12 16:00     ` Andy Lutomirski
2018-06-12 16:21       ` Yu-cheng Yu
2018-06-12 16:31         ` Andy Lutomirski
2018-06-12 17:24           ` Yu-cheng Yu
2018-06-12 20:15             ` Yu-cheng Yu
2018-06-14  1:07     ` Balbir Singh
2018-06-14 14:56       ` Yu-cheng Yu
2018-06-17  3:16         ` Balbir Singh
2018-06-18 21:44           ` Andy Lutomirski
2018-06-19  8:52             ` Balbir Singh
2018-06-26  2:46 ` Jann Horn
2018-06-26 14:56   ` Yu-cheng Yu
2018-06-26  5:26 ` Andy Lutomirski
2018-06-26 14:56   ` Yu-cheng Yu

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