Re: [PATCH v15 05/26] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack

On 11/27/2020 9:10 AM, Borislav Petkov wrote:

On Tue, Nov 10, 2020 at 08:21:50AM -0800, Yu-cheng Yu wrote:

quoted

+config X86_CET
+	def_bool n
+
+config ARCH_HAS_SHADOW_STACK
+	def_bool n
+
+config X86_SHADOW_STACK_USER

Is X86_SHADOW_STACK_KERNEL coming too?

Regardless, you can add it when it comes and you can use only X86_CET
for now and drop this one and simplify this pile of Kconfig symbols.

We have X86_BRANCH_TRACKING_USER too.  My thought was, X86_CET means any 
of kernel/user shadow stack/ibt.

quoted

+	prompt "Intel Shadow Stacks for user-mode"
+	def_bool n
+	depends on CPU_SUP_INTEL && X86_64
+	depends on AS_HAS_SHADOW_STACK
+	select ARCH_USES_HIGH_VMA_FLAGS
+	select X86_CET
+	select ARCH_HAS_SHADOW_STACK
+	help
+	  Shadow Stacks provides protection against program stack
+	  corruption.  It's a hardware feature.  This only matters
+	  if you have the right hardware.  It's a security hardening
+	  feature and apps must be enabled to use it.  You get no
+	  protection "for free" on old userspace.  The hardware can
+	  support user and kernel, but this option is for user space
+	  only.
+	  Support for this feature is only known to be present on
+	  processors released in 2020 or later.  CET features are also
+	  known to increase kernel text size by 3.7 KB.

This help text needs some rewriting. You can find an inspiration about
more adequate style in that same Kconfig file.

I will work on it.

quoted

+
+	  If unsure, say N.
+
  config EFI
  	bool "EFI runtime service support"
  	depends on ACPI

diff --git a/scripts/as-x86_64-has-shadow-stack.sh b/scripts/as-x86_64-has-shadow-stack.sh
new file mode 100755
index 000000000000..fac1d363a1b8
--- /dev/null
+++ b/scripts/as-x86_64-has-shadow-stack.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+
+echo "wrussq %rax, (%rbx)" | $* -x assembler -c -

						      2> /dev/null

otherwise you get

{standard input}: Assembler messages:
{standard input}:1: Error: no such instruction: `wrussq %rax,(%rbx)

on non-enlightened toolchains during build.

Yes, I will fix this in the next revision.

Yu-cheng

Thx.