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From: Kees Cook <keescook@chromium.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: "H. J. Lu" <hjl.tools@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	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>, Ingo Molnar <mingo@redhat.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, Florian Weimer <fweimer@redhat.com>
Subject: Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack
Date: Mon, 18 Jun 2018 17:52:29 -0700	[thread overview]
Message-ID: <CAGXu5jK0gospOXRpN6zYiQPXOZeE=YpVAz2qu4Zc3-32v85+EQ@mail.gmail.com> (raw)
In-Reply-To: <CALCETrVsh5t-V1Sm88LsZE_+DS0GE_bMWbcoX3SjD6GnrB08Pw@mail.gmail.com>

On Mon, Jun 18, 2018 at 3:03 PM, Andy Lutomirski <luto@kernel.org> wrote:
> On Tue, Jun 12, 2018 at 12:34 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> On Tue, Jun 12, 2018 at 11:59 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> > On Tue, 12 Jun 2018, H.J. Lu wrote:
>> >> On Tue, Jun 12, 2018 at 9:34 AM, Andy Lutomirski <luto@kernel.org> wrote:
>> >> > On Tue, Jun 12, 2018 at 9:05 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>> >> >> On Tue, Jun 12, 2018 at 9:01 AM, Andy Lutomirski <luto@kernel.org> wrote:
>> >> >> > On Tue, Jun 12, 2018 at 4:43 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>> >> >> >> On Tue, Jun 12, 2018 at 3:03 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> >> >> >> > That works for stuff which loads all libraries at start time, but what
>> >> >> >> > happens if the program uses dlopen() later on? If CET is force locked and
>> >> >> >> > the library is not CET enabled, it will fail.
>> >> >> >>
>> >> >> >> That is to prevent disabling CET by dlopening a legacy shared library.
>> >> >> >>
>> >> >> >> > I don't see the point of trying to support CET by magic. It adds complexity
>> >> >> >> > and you'll never be able to handle all corner cases correctly. dlopen() is
>> >> >> >> > not even a corner case.
>> >> >> >>
>> >> >> >> That is a price we pay for security.  To enable CET, especially shadow
>> >> >> >> shack, the program and all of shared libraries it uses should be CET
>> >> >> >> enabled.  Most of programs can be enabled with CET by compiling them
>> >> >> >> with -fcf-protection.
>> >> >> >
>> >> >> > If you charge too high a price for security, people may turn it off.
>> >> >> > I think we're going to need a mode where a program says "I want to use
>> >> >> > the CET, but turn it off if I dlopen an unsupported library".  There
>> >> >> > are programs that load binary-only plugins.
>> >> >>
>> >> >> You can do
>> >> >>
>> >> >> # export GLIBC_TUNABLES=glibc.tune.hwcaps=-SHSTK
>> >> >>
>> >> >> which turns off shadow stack.
>> >> >>
>> >> >
>> >> > Which exactly illustrates my point.  By making your security story too
>> >> > absolute, you'll force people to turn it off when they don't need to.
>> >> > If I'm using a fully CET-ified distro and I'm using a CET-aware
>> >> > program that loads binary plugins, and I may or may not have an old
>> >> > (binary-only, perhaps) plugin that doesn't support CET, then the
>> >> > behavior I want is for CET to be on until I dlopen() a program that
>> >> > doesn't support it.  Unless there's some ABI reason why that can't be
>> >> > done, but I don't think there is.
>> >>
>> >> We can make it opt-in via GLIBC_TUNABLES.  But by default, the legacy
>> >> shared object is disallowed when CET is enabled.
>> >
>> > That's a bad idea. Stuff has launchers which people might not be able to
>> > change. So they will simply turn of CET completely or it makes them hack
>> > horrible crap into init, e.g. the above export.
>> >
>> > Give them sane kernel options:
>> >
>> >      cet = off, relaxed, forced
>> >
>> > where relaxed allows to run binary plugins. Then let dlopen() call into the
>> > kernel with the filepath of the library to check for CET and that will tell
>> > you whether its ok or or not and do the necessary magic in the kernel when
>> > CET has to be disabled due to a !CET library/application.
>> >
>> > That's also making the whole thing independent of magic glibc environment
>> > options and allows it to be used all over the place in the same way.
>>
>> This is very similar to our ARCH_CET_EXEC proposal which controls how
>> CET should be enforced.   But Andy thinks it is a bad idea.
>
> I do think it's a bad idea to have a new piece of state that survives
> across exec().  It's going to have nasty usability problems and nasty
> security problems.
>
> We may need a mode by which glibc can turn CET *back off* even after a
> program had it on if it dlopens() an old binary.  Or maybe there won't
> be demand.  I can certainly understand why the CET_LOCK feature is
> there, although I think we need a way to override it using something
> like ptrace().  I'm not convinced that CET_LOCK is really needed, but
> someone who understand the thread model should chime in.
>
> Kees, do you know anyone who has a good enough understanding of
> usermode exploits and how they'll interact with CET?

Adding Florian to CC, but if something gets CET enabled, it really
shouldn't have a way to turn it off. If there's a way to turn it off,
all the ROP research will suddenly turn to exactly one gadget before
doing the rest of the ROP: turning off CET. Right now ROP is: use
stack-pivot gadget, do everything else. Allowed CET to turn off will
just add one step: use CET-off gadget, use stack-pivot gadget, do
everything else. :P

Following Linus's request for "slow introduction" of new security
features, likely the best approach is to default to "relaxed" (with a
warning about down-grades), and allow distros/end-users to pick
"forced" if they know their libraries are all CET-enabled.

-Kees

-- 
Kees Cook
Pixel Security

  reply	other threads:[~2018-06-19  0:52 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
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 [this message]
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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