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Date: Thu, 9 Sep 2021 18:39:51 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Peter Collingbourne <pcc@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>,
 Will Deacon <will@kernel.org>, Andrey Konovalov <andreyknvl@gmail.com>,
 Evgenii Stepanov <eugenis@google.com>, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: kasan: mte: move GCR_EL1 switch to task switch
 when KASAN disabled
Message-ID: <YTpG5xT+sotOsjTD@arm.com>
References: <20210827030748.587710-1-pcc@google.com>
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On Thu, Aug 26, 2021 at 08:07:48PM -0700, Peter Collingbourne wrote:
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 923ee2ac85fd..e12ed892cde8 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -178,9 +178,9 @@ alternative_else_nop_endif
>  
>  	.macro mte_set_kernel_gcr, tmp, tmp2
>  #ifdef CONFIG_KASAN_HW_TAGS
> -alternative_if_not ARM64_MTE
> +alternative_cb	kasan_hw_tags_enable
>  	b	1f
> -alternative_else_nop_endif
> +alternative_cb_end
>  	mov	\tmp, KERNEL_GCR_EL1
>  	msr_s	SYS_GCR_EL1, \tmp
>  1:
> @@ -188,10 +188,10 @@ alternative_else_nop_endif
>  	.endm
>  
>  	.macro mte_set_user_gcr, tsk, tmp, tmp2
> -#ifdef CONFIG_ARM64_MTE
> -alternative_if_not ARM64_MTE
> +#ifdef CONFIG_KASAN_HW_TAGS
> +alternative_cb	kasan_hw_tags_enable
>  	b	1f
> -alternative_else_nop_endif
> +alternative_cb_end
>  	ldr	\tmp, [\tsk, #THREAD_MTE_CTRL]
>  
>  	mte_set_gcr \tmp, \tmp2
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 9d314a3bad3b..0e8edb5d5861 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -162,7 +162,7 @@ void mte_check_tfsr_el1(void)
>  }
>  #endif
>  
> -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
> @@ -182,6 +182,24 @@ 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);
> +}
> +
> +void __init kasan_hw_tags_enable(struct alt_instr *alt, __le32 *origptr,
> +				 __le32 *updptr, int nr_inst)
> +{
> +	BUG_ON(nr_inst != 1); /* Branch -> NOP */
> +
> +	if (kasan_hw_tags_enabled())
> +		*updptr = cpu_to_le32(aarch64_insn_gen_nop());
>  }

A bit confusing: kasan_hw_tags_enabled() checks the kasan_flag_enabled
static label. This is initialised via kasan_init_hw_tags() which is
called from smp_prepare_boot_cpu() _after_ apply_boot_alternatives().
Does this work when kasan is indeed enabled? Maybe I miss something.

-- 
Catalin

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