Re: [PATCH v5 05/27] Documentation/x86: Add CET description

Borislav Petkov
2018-11-13T18:43:37Z

On Thu, Oct 11, 2018 at 08:15:01AM -0700, Yu-cheng Yu wrote:
> Explain how CET works and the no_cet_shstk/no_cet_ibt kernel
> parameters.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> .../admin-guide/kernel-parameters.txt | 6 +
> Documentation/index.rst | 1 +
> Documentation/x86/index.rst | 11 +
> Documentation/x86/intel_cet.rst | 266 ++++++++++++++++++
> 4 files changed, 284 insertions(+)
> create mode 100644 Documentation/x86/index.rst
> create mode 100644 Documentation/x86/intel_cet.rst

So this patch should probably come first in the series so that a reader
can know what to expect...

> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 92eb1f42240d..3854423f7c86 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2764,6 +2764,12 @@
> noexec=on: enable non-executable mappings (default)
> noexec=off: disable non-executable mappings
>
> + no_cet_ibt [X86-64] Disable indirect branch tracking for user-mode
> + applications
> +
> + no_cet_shstk [X86-64] Disable shadow stack support for user-mode
> + applications
> +
> nosmap [X86]
> Disable SMAP (Supervisor Mode Access Prevention)
> even if it is supported by processor.
> diff --git a/Documentation/index.rst b/Documentation/index.rst
> index 5db7e87c7cb1..1cdc139adb40 100644
> --- a/Documentation/index.rst
> +++ b/Documentation/index.rst

Please integrate scripts/checkpatch.pl into your patch creation
workflow. Some of the warnings/errors *actually* make sense:

WARNING: Missing or malformed SPDX-License-Identifier tag in line 1
#76: FILE: Documentation/x86/index.rst:1:
+=======================

WARNING: Missing or malformed SPDX-License-Identifier tag in line 1
#93: FILE: Documentation/x86/intel_cet.rst:1:
+=========================================

> @@ -104,6 +104,7 @@ implementation.
> :maxdepth: 2
>
> sh/index
> + x86/index
>
> Filesystem Documentation
> ------------------------
> diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
> new file mode 100644
> index 000000000000..9c34d8cbc8f0
> --- /dev/null
> +++ b/Documentation/x86/index.rst
> @@ -0,0 +1,11 @@
> +=======================
> +X86 Documentation
> +=======================
> +
> +Control Flow Enforcement
> +========================
> +
> +.. toctree::
> + :maxdepth: 1
> +
> + intel_cet
> diff --git a/Documentation/x86/intel_cet.rst b/Documentation/x86/intel_cet.rst
> new file mode 100644
> index 000000000000..946f4802a51f
> --- /dev/null
> +++ b/Documentation/x86/intel_cet.rst
> @@ -0,0 +1,266 @@
> +=========================================
> +Control Flow Enforcement Technology (CET)
> +=========================================
> +
> +[1] Overview
> +============
> +
> +Control Flow Enforcement Technology (CET) provides protection against
> +return/jump-oriented programming (ROP) attacks. It can be implemented
> +to protect both the kernel and applications. In the first phase,
> +only the user-mode protection is implemented on the 64-bit kernel.

s/the// is implemented in 64-bit mode.

> +However, 32-bit applications are supported under the compatibility
> +mode.

Drop "However":

"32-bit applications are, of course, supported in compatibility mode."

> +
> +CET includes shadow stack (SHSTK) and indirect branch tracking (IBT).

"CET introduces two a shadow stack and an indirect branch tracking mechanism."

> +The SHSTK is a secondary stack allocated from memory. The processor

s/The//

> +automatically pushes/pops a secure copy to the SHSTK every return
> +address and,

that reads funny - pls reorganize. Also, what is a "secure copy"?

You mean a copy of every return address which software cannot access?

> by comparing the secure copy to the program stack copy,
> +verifies function returns are as intended.

... have not been corrupted/modified."

> The IBT verifies all
> +indirect CALL/JMP targets are intended and marked by the compiler with
> +'ENDBR' op codes.

"opcode" - one word. And before you use "ENDBR" you need to explain it
above what it is.

/me reads further... encounters ENDBR's definition...

ah, ok, so you should say something like

"... and marked by the compiler with the ENDBR opcode (see below)."

> +
> +There are two kernel configuration options:
> +
> + INTEL_X86_SHADOW_STACK_USER, and
> + INTEL_X86_BRANCH_TRACKING_USER.
> +
> +To build a CET-enabled kernel, Binutils v2.31 and GCC v8.1 or later
> +are required. To build a CET-enabled application, GLIBC v2.28 or
> +later is also required.
> +
> +There are two command-line options for disabling CET features:
> +
> + no_cet_shstk - disables SHSTK, and
> + no_cet_ibt - disables IBT.
> +
> +At run time, /proc/cpuinfo shows the availability of SHSTK and IBT.

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.