[PATCH v3 2/2] arm64: kasan: mte: move GCR_EL1 switch to task switch when KASAN disabled

[Catalin Marinas]

On Wed, Sep 15, 2021 at 12:03:36PM -0700, Peter Collingbourne wrote:
> -static void mte_update_sctlr_user(struct task_struct *task)
> +static void mte_update_sctlr_user_and_gcr_excl(struct task_struct *task)
>  {
>  	/*
>  	 * This must be called with preemption disabled and can only be called
> @@ -177,6 +177,24 @@ static void mte_update_sctlr_user(struct task_struct *task)
>  	else if (resolved_mte_tcf & MTE_CTRL_TCF_SYNC)
>  		sctlr |= SCTLR_EL1_TCF0_SYNC;
>  	task->thread.sctlr_user = sctlr;
> +
> +	/*
> +	 * SYS_GCR_EL1 will be set to current->thread.mte_ctrl value by
> +	 * mte_set_user_gcr() in kernel_exit, but only if KASAN is enabled.
> +	 */
> +	if (!kasan_hw_tags_enabled())
> +		write_sysreg_s(((mte_ctrl >> MTE_CTRL_GCR_USER_EXCL_SHIFT) &
> +				SYS_GCR_EL1_EXCL_MASK) | SYS_GCR_EL1_RRND,
> +			       SYS_GCR_EL1);
> +}

The only nitpick I have is that I'd like to keep the GCR_EL1 updating in
a separate function, mte_update_gcr_user() or something (it can be
static inline) that you'd explicitly call from mte_thread_switch() and
set_mte_ctrl(). The original mte_update_sctlr_user() was not writing any
system registers.

Maybe a better alternative would be to move the update_sctlr_el1() call
in mte_update_sctlr_user() as well (and rename it more generically to
mte_update_user_ctrl()) but with a new bool update_sctlr argument that's
false on the thread switch path.

Either way:

Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>