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Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory

From Kees Cook <keescook@chromium.org>
Newsgroups linux.kernel
Subject Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory
Date 2015-11-25 18:30 +0100
Message-ID <qyM3h-RQ-9@gated-at.bofh.it> (permalink)
References <qyttF-59Z-27@gated-at.bofh.it> <qyEp4-4c9-15@gated-at.bofh.it>
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On Wed, Nov 25, 2015 at 1:13 AM, Mathias Krause <minipli@googlemail.com> wrote:
> On 24 November 2015 at 22:38, Kees Cook <keescook@chromium.org> wrote:
>> Many things are written to only during __init, and never changed
>> again. These cannot be made "const" since the compiler will do the wrong
>> thing (we do actually need to write to them). Instead, move these items
>> into a memory region that will be made read-only during mark_rodata_ro()
>> which happens after all kernel __init code has finished.
>>
>> This introduces __read_only as a way to mark such memory, and uses it on
>> the x86 vDSO to kill an extant kernel exploitation method.
>
> ...just some random notes on the experience with kernels implementing
> such a feature for quite a lot of locations, not just the vDSO.
>
> While having that annotation makes perfect sense, not only from a
> security perspective but also from a micro-optimization point of view
> (much like the already existing __read_mostly annotation), it has its
> drawbacks. Violating the "r/o after init" rule by writing to such
> annotated variables from non-init code goes unnoticed as far as it
> concerns the toolchain. Neither the compiler nor the linker will flag
> that incorrect use. It'll just trap at runtime and that's bad.

Well, or, it's good: that's the point of it. Either from the
perspective of robustness or from that of security.

> I myself had some educating experience seeing my machine triple fault
> when resuming from a S3 sleep. The root cause was a variable that was
> annotated __read_only but that was (unnecessarily) modified during CPU
> bring-up phase. Debugging that kind of problems is sort of a PITA, you
> could imagine.

As PaX Team mentions, it should be easy to catch these traps and
report them. That could certainly be a nice addition.

> So, prior extending the usage of the __read_only annotation some
> toolchain support is needed. Maybe a gcc plugin that'll warn/error on
> code that writes to such a variable but is not __init itself. The
> initify and checker plugins from the PaX patch might be worth to look
> at for that purpose, as they're doing similar things already. Adding
> such a check to sparse might be worth it, too.
> A modpost check probably won't work as it's unable to tell if it's a
> legitimate access (r/o) or a violation (/w access). So the gcc plugin
> is the way to go, IMHO.

There are many more pieces to add to the annotation use, but I don't
want to risk getting us into a Catch-22. Emese's work on the plugin
side will see this usage and utility grow, and I think getting these
basic building blocks in place is the right place to start.

-Kees


-- 
Kees Cook
Chrome OS & Brillo Security
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Thread

[PATCH 0/2] introduce post-init read-only memory Kees Cook <keescook@chromium.org> - 2015-11-24 22:40 +0100
  [PATCH 1/2] x86: introduce post-init read-only memory Kees Cook <keescook@chromium.org> - 2015-11-24 22:40 +0100
    Re: [PATCH 1/2] x86: introduce post-init read-only memory Andy Lutomirski <luto@amacapital.net> - 2015-11-25 01:40 +0100
      Re: [PATCH 1/2] x86: introduce post-init read-only memory Kees Cook <keescook@chromium.org> - 2015-11-25 01:50 +0100
        Re: [kernel-hardening] Re: [PATCH 1/2] x86: introduce post-init  read-only memory Michael Ellerman <mpe@ellerman.id.au> - 2015-11-25 02:00 +0100
          Re: [kernel-hardening] Re: [PATCH 1/2] x86: introduce post-init  read-only memory Kees Cook <keescook@chromium.org> - 2015-11-25 16:10 +0100
            Re: [kernel-hardening] Re: [PATCH 1/2] x86: introduce post-init  read-only memory Michael Ellerman <mpe@ellerman.id.au> - 2015-11-26 00:10 +0100
              Re: [kernel-hardening] Re: [PATCH 1/2] x86: introduce post-init  read-only memory Kees Cook <keescook@chromium.org> - 2015-11-26 00:40 +0100
  [PATCH 2/2] x86, vdso: mark vDSO read-only after init Kees Cook <keescook@chromium.org> - 2015-11-24 22:50 +0100
  Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Mathias Krause <minipli@googlemail.com> - 2015-11-25 10:20 +0100
    Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory Clemens Ladisch <clemens@ladisch.de> - 2015-11-25 11:10 +0100
      Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory "PaX Team" <pageexec@freemail.hu> - 2015-11-25 12:20 +0100
    Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory "PaX Team" <pageexec@freemail.hu> - 2015-11-25 12:10 +0100
      Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory Ingo Molnar <mingo@kernel.org> - 2015-11-26 10:00 +0100
        Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory "PaX Team" <pageexec@freemail.hu> - 2015-11-26 11:00 +0100
          Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory Ingo Molnar <mingo@kernel.org> - 2015-11-26 11:50 +0100
            Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory "PaX Team" <pageexec@freemail.hu> - 2015-11-26 13:20 +0100
              Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory Ingo Molnar <mingo@kernel.org> - 2015-11-27 09:10 +0100
                Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory "PaX Team" <pageexec@freemail.hu> - 2015-11-27 16:40 +0100
                Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Andy Lutomirski <luto@amacapital.net> - 2015-11-27 17:40 +0100
                Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory Ingo Molnar <mingo@kernel.org> - 2015-11-29 09:10 +0100
                Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory "PaX Team" <pageexec@freemail.hu> - 2015-11-29 12:20 +0100
                Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory Ingo Molnar <mingo@kernel.org> - 2015-11-29 16:40 +0100
                Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Mathias Krause <minipli@googlemail.com> - 2015-11-29 19:10 +0100
                Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory Ingo Molnar <mingo@kernel.org> - 2015-11-30 09:10 +0100
        Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Andy Lutomirski <luto@amacapital.net> - 2015-11-26 17:20 +0100
          Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory Ingo Molnar <mingo@kernel.org> - 2015-11-27 09:10 +0100
            Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-27 19:10 +0100
            Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Linus Torvalds <torvalds@linux-foundation.org> - 2015-11-27 19:10 +0100
              Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Kees Cook <keescook@chromium.org> - 2015-11-27 21:10 +0100
                Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Andy Lutomirski <luto@amacapital.net> - 2015-11-27 21:10 +0100
                Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory Ingo Molnar <mingo@kernel.org> - 2015-11-29 09:10 +0100
    Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Kees Cook <keescook@chromium.org> - 2015-11-25 18:30 +0100
    Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory "H. Peter Anvin" <hpa@zytor.com> - 2015-11-25 18:40 +0100
      Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory Kees Cook <keescook@chromium.org> - 2015-11-25 20:00 +0100
        Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only  memory "H. Peter Anvin" <hpa@zytor.com> - 2015-11-25 20:10 +0100

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