linux-kernel - Re: [PATCH] arm64: mm: disable PAN during caches_clean_inval_user

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Date: Mon, 8 Jan 2024 15:54:58 +0000
From: Mark Rutland <mark.rutland@....com>
To: "Brandt, Oliver - Lenze" <oliver.brandt@...ze.com>
Cc: "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"will@...nel.org" <will@...nel.org>,
	"catalin.marinas@....com" <catalin.marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: mm: disable PAN during caches_clean_inval_user_pou

Hi Oliver,

On Mon, Jan 08, 2024 at 01:00:39PM +0000, Brandt, Oliver - Lenze wrote:
> Using the cacheflush() syscall from an 32-bit user-space fails when
> ARM64_PAN is used. We 'll get an endless loop:
> 
> 	1. executing "dc cvau, x2" results in raising an abort
> 	2. abort handler does not fix the reason for the abort and
> 	   returns to 1.
> 
> Disabling PAN for the time of the cache maintenance fixes this.

Hmm... the ARM ARM says PSTATE.PAN is not supposed to affect DC CVAU.

Looking at the latest ARM ARM (ARM DDI 0487J.a), R_PMTWB states:

| The PSTATE.PAN bit has no effect on all of the following:
|
| o Instruction fetches.
| o Data cache instructions, except DC ZVA.
| o If FEAT_PAN2 is not implemented, then address translation instructions.
| o If FEAT_PAN2 is implemented, then the address translation instructions
|   other than AT S1E1RP and AT S1E1WP.

So IIUC, DC CVAU shouldn't be affected by PAN.

This could be a CPU bug; which CPU are you seeing this with?

Mark.

> Fixes: 338d4f49d6f7 ("arm64: kernel: Add support for Privileged Access Never")
> Cc: stable@...r.kernel.org
> Signed-off-by: Oliver Brandt <oliver.brandt@...ze.com>
> ---
>  arch/arm64/mm/cache.S | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm64/mm/cache.S b/arch/arm64/mm/cache.S
> index 503567c864fde..333c4c2baa568 100644
> --- a/arch/arm64/mm/cache.S
> +++ b/arch/arm64/mm/cache.S
> @@ -70,10 +70,12 @@ SYM_FUNC_ALIAS(__pi_caches_clean_inval_pou, caches_clean_inval_pou)
>   */
>  SYM_FUNC_START(caches_clean_inval_user_pou)
>  	uaccess_ttbr0_enable x2, x3, x4
> +	ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN, CONFIG_ARM64_PAN)
>  
>  	caches_clean_inval_pou_macro 2f
>  	mov	x0, xzr
>  1:
> +	ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN, CONFIG_ARM64_PAN)
>  	uaccess_ttbr0_disable x1, x2
>  	ret
>  2:
> -- 
> 2.43.0