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    SubjectRe: [PATCH v19 02/25] x86/cet/shstk: Add Kconfig option for user-mode control-flow protection
    On Wed, Feb 03, 2021 at 02:55:24PM -0800, Yu-cheng Yu wrote:
    > Shadow Stack provides protection against function return address
    > corruption. It is active when the processor supports it, the kernel has
    > CONFIG_X86_CET enabled, and the application is built for the feature.
    > This is only implemented for the 64-bit kernel. When it is enabled, legacy
    > non-Shadow Stack applications continue to work, but without protection.
    >
    > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    > ---
    > arch/x86/Kconfig | 22 ++++++++++++++++++++++
    > arch/x86/Kconfig.assembler | 5 +++++
    > 2 files changed, 27 insertions(+)
    >
    > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
    > index 21f851179ff0..074b3c0e6bf6 100644
    > --- a/arch/x86/Kconfig
    > +++ b/arch/x86/Kconfig
    > @@ -1951,6 +1951,28 @@ config X86_SGX
    >
    > If unsure, say N.
    >
    > +config ARCH_HAS_SHADOW_STACK
    > + def_bool n
    > +
    > +config X86_CET
    > + prompt "Intel Control-flow protection for user-mode"
    > + def_bool n
    > + depends on X86_64
    > + depends on AS_WRUSS
    > + select ARCH_USES_HIGH_VMA_FLAGS
    > + select ARCH_HAS_SHADOW_STACK

    This seems backwards to me? Shouldn't 'config X86_64' do the 'select
    ARCH_HAS_SHADOW_STACK' and 'config X86_CET' do a 'depends on
    ARCH_HAS_SHADOW_STACK' instead?

    > + help
    > + Control-flow protection is a set of hardware features which place
    > + additional restrictions on indirect branches. These help
    > + mitigate ROP attacks. Applications must be enabled to use it,
    > + and old userspace does not get protection "for free".
    > + Support for this feature is present on Tiger Lake family of
    > + processors released in 2020 or later. Enabling this feature
    > + increases kernel text size by 3.7 KB.
    > + See Documentation/x86/intel_cet.rst for more information.
    > +
    > + If unsure, say N.
    > +
    > config EFI
    > bool "EFI runtime service support"
    > depends on ACPI
    > diff --git a/arch/x86/Kconfig.assembler b/arch/x86/Kconfig.assembler
    > index 26b8c08e2fc4..00c79dd93651 100644
    > --- a/arch/x86/Kconfig.assembler
    > +++ b/arch/x86/Kconfig.assembler
    > @@ -19,3 +19,8 @@ config AS_TPAUSE
    > def_bool $(as-instr,tpause %ecx)
    > help
    > Supported by binutils >= 2.31.1 and LLVM integrated assembler >= V7
    > +
    > +config AS_WRUSS
    > + def_bool $(as-instr,wrussq %rax$(comma)(%rbx))
    > + help
    > + Supported by binutils >= 2.31 and LLVM integrated assembler
    > --
    > 2.21.0
    >

    --
    Kees Cook

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