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From: Florian Weimer <fweimer@redhat.com>
To: Andy Lutomirski <luto@kernel.org>, Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: 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: Thu, 7 Jun 2018 21:47:22 +0200	[thread overview]
Message-ID: <3c1bdf85-0c52-39ed-a799-e26ac0e52391@redhat.com> (raw)
In-Reply-To: <CALCETrUqXvh2FDXe6bveP10TFpzptEyQe2=mdfZFGKU0T+NXsA@mail.gmail.com>

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 also expect that we'd only have donor mappings in userspace anyway, 
and that the memory is not actually accessible from userspace if it is 
used for a shadow stack.

> My intuition would be that all
> shadow stack management should be entirely controlled by userspace --
> newly cloned threads (with CLONE_VM) should have no shadow stack
> initially, and newly started processes should have no shadow stack
> until they ask for one.

If the new thread doesn't have a shadow stack, we need to disable 
signals around clone, and we are very likely forced to rewrite the early 
thread setup in assembler, to avoid spurious calls (including calls to 
thunks to get EIP on i386).  I wouldn't want to do this If we can avoid 
it.  Just using C and hoping to get away with it doesn't sound greater, 
either.  And obviously there is the matter that the initial thread setup 
code ends up being that universal trampoline.

Thanks,
Florian

  reply	other threads:[~2018-06-07 19:47 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 [this message]
2018-06-07 20:53       ` Andy Lutomirski
2018-06-08 14:53         ` Florian Weimer
2018-06-08 15:01           ` Andy Lutomirski
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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