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

Andy Lutomirski
2018-11-08T20:46:29Z

On Thu, Oct 11, 2018 at 8:20 AM Yu-cheng Yu <yu-cheng.yu@intel.com> 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@intel.com>
> ---
> 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.