Re: [PATCH 09/14] arm64: ssbd: Introduce thread flag to control userspace mitigation

[Mark Rutland]

On Tue, May 22, 2018 at 04:06:43PM +0100, Marc Zyngier wrote:
> In order to allow userspace to be mitigated on demand, let's
> introduce a new thread flag that prevents the mitigation from
> being turned off when exiting to userspace, and doesn't turn
> it on on entry into the kernel (with the assumtion that the

Nit: s/assumtion/assumption/

> mitigation is always enabled in the kernel itself).
> 
> This will be used by a prctl interface introduced in a later
> patch.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>

On the assumption that this flag cannot be flipped while a task is in
userspace:

Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>

Mark.

> ---
>  arch/arm64/include/asm/thread_info.h | 1 +
>  arch/arm64/kernel/entry.S            | 2 ++
>  2 files changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index 740aa03c5f0d..cbcf11b5e637 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -94,6 +94,7 @@ void arch_release_task_struct(struct task_struct *tsk);
>  #define TIF_32BIT		22	/* 32bit process */
>  #define TIF_SVE			23	/* Scalable Vector Extension in use */
>  #define TIF_SVE_VL_INHERIT	24	/* Inherit sve_vl_onexec across exec */
> +#define TIF_SSBD		25	/* Wants SSB mitigation */
>  
>  #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
>  #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index e6f6e2339b22..28ad8799406f 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -147,6 +147,8 @@ alternative_cb	arm64_enable_wa2_handling
>  alternative_cb_end
>  	ldr_this_cpu	\tmp2, arm64_ssbd_callback_required, \tmp1
>  	cbz	\tmp2, \targ
> +	ldr	\tmp2, [tsk, #TSK_TI_FLAGS]
> +	tbnz	\tmp2, #TIF_SSBD, \targ
>  	mov	w0, #ARM_SMCCC_ARCH_WORKAROUND_2
>  	mov	w1, #\state
>  alternative_cb	arm64_update_smccc_conduit
> -- 
> 2.14.2
> 
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