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

Yu, Yu-cheng
2020-11-28T22:24:42Z

On 11/27/2020 9:10 AM, Borislav Petkov wrote:
> On Tue, Nov 10, 2020 at 08:21:50AM -0800, Yu-cheng Yu wrote:
>> +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.

>
>> + 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.

>> +
>> + 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.
>