Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack

[Andy Lutomirski]

On Thu, Oct 11, 2018 at 8:20 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> Intel Control-flow Enforcement Technology (CET) introduces the
> following MSRs into the XSAVES system states.
>
>     IA32_U_CET (user-mode CET settings),
>     IA32_PL3_SSP (user-mode shadow stack),
>     IA32_PL0_SSP (kernel-mode shadow stack),
>     IA32_PL1_SSP (ring-1 shadow stack),
>     IA32_PL2_SSP (ring-2 shadow stack).
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/include/asm/fpu/types.h            | 22 +++++++++++++++++++++
>  arch/x86/include/asm/fpu/xstate.h           |  4 +++-
>  arch/x86/include/uapi/asm/processor-flags.h |  2 ++
>  arch/x86/kernel/fpu/xstate.c                | 10 ++++++++++
>  4 files changed, 37 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
> index 202c53918ecf..e55d51d172f1 100644
> --- a/arch/x86/include/asm/fpu/types.h
> +++ b/arch/x86/include/asm/fpu/types.h
> @@ -114,6 +114,9 @@ enum xfeature {
>         XFEATURE_Hi16_ZMM,
>         XFEATURE_PT_UNIMPLEMENTED_SO_FAR,
>         XFEATURE_PKRU,
> +       XFEATURE_RESERVED,
> +       XFEATURE_SHSTK_USER,
> +       XFEATURE_SHSTK_KERNEL,
>
>         XFEATURE_MAX,
>  };
> @@ -128,6 +131,8 @@ enum xfeature {
>  #define XFEATURE_MASK_Hi16_ZMM         (1 << XFEATURE_Hi16_ZMM)
>  #define XFEATURE_MASK_PT               (1 << XFEATURE_PT_UNIMPLEMENTED_SO_FAR)
>  #define XFEATURE_MASK_PKRU             (1 << XFEATURE_PKRU)
> +#define XFEATURE_MASK_SHSTK_USER       (1 << XFEATURE_SHSTK_USER)
> +#define XFEATURE_MASK_SHSTK_KERNEL     (1 << XFEATURE_SHSTK_KERNEL)
>
>  #define XFEATURE_MASK_FPSSE            (XFEATURE_MASK_FP | XFEATURE_MASK_SSE)
>  #define XFEATURE_MASK_AVX512           (XFEATURE_MASK_OPMASK \
> @@ -229,6 +234,23 @@ struct pkru_state {
>         u32                             pad;
>  } __packed;
>
> +/*
> + * State component 11 is Control flow Enforcement user states
> + */
> +struct cet_user_state {
> +       u64 u_cet;      /* user control flow settings */
> +       u64 user_ssp;   /* user shadow stack pointer */
> +} __packed;
> +
> +/*
> + * State component 12 is Control flow Enforcement kernel states
> + */
> +struct cet_kernel_state {
> +       u64 kernel_ssp; /* kernel shadow stack */
> +       u64 pl1_ssp;    /* ring-1 shadow stack */
> +       u64 pl2_ssp;    /* ring-2 shadow stack */
> +} __packed;
> +

Why are these __packed?  It seems like it'll generate bad code for no
obvious purpose.