linux-kernel - Re: [PATCH v2 2/2] arm64: uaccess: Implement unsafe accessors

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Message-ID: <f933b06a-0f4a-3bdb-86ef-644ac0a16163@arm.com>
Date:   Fri, 21 Dec 2018 14:57:15 +0000
From:   James Morse <james.morse@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        mingo@...hat.com, peterz@...radead.org, catalin.marinas@....com,
        will.deacon@....com, hpa@...or.com
Subject: Re: [PATCH v2 2/2] arm64: uaccess: Implement unsafe accessors

Hi Julien,

On 03/12/2018 13:55, Julien Thierry wrote:
> Current implementation of get/put_user_unsafe default to get/put_user
> which toggle PAN before each access, despite having been told by the caller
> that multiple accesses to user memory were about to happen.
> 
> Provide implementations for user_access_begin/end to turn PAN off/on and
> implement unsafe accessors that assume PAN was already turned off.

> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 842fb95..4e6477b 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -108,6 +108,8 @@
>  #define SYS_DC_CSW			sys_insn(1, 0, 7, 10, 2)
>  #define SYS_DC_CISW			sys_insn(1, 0, 7, 14, 2)
>  
> +#define SYS_PSTATE_PAN			sys_reg(3, 0, 4, 2, 3)

Nit: Could we keep this list in encoding order please.
(it makes conflicts easier to resolve in the future)

>  #define SYS_OSDTRRX_EL1			sys_reg(2, 0, 0, 0, 2)
>  #define SYS_MDCCINT_EL1			sys_reg(2, 0, 0, 2, 0)
>  #define SYS_MDSCR_EL1			sys_reg(2, 0, 0, 2, 2)
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 07c3408..cabfcae 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -233,6 +233,23 @@ static inline void uaccess_enable_not_uao(void)
>  	__uaccess_enable(ARM64_ALT_PAN_NOT_UAO);
>  }
>  
> +#define unsafe_user_region_active	uaccess_region_active
> +static inline bool uaccess_region_active(void)
> +{
> +	if (system_uses_ttbr0_pan()) {
> +		u64 ttbr;
> +
> +		ttbr = read_sysreg(ttbr1_el1);
> +		return ttbr & TTBR_ASID_MASK;
> +	} else if (cpus_have_const_cap(ARM64_ALT_PAN_NOT_UAO)) {
> +		return (read_sysreg(sctlr_el1) & SCTLR_EL1_SPAN) ?
> +				false :
> +				!read_sysreg_s(SYS_PSTATE_PAN);
> +	}
> +
> +	return false;
> +}

(Reading the SCTLR bit is a bit of a heavy-hammer, as suggested elsewhere on the
thread, can we use alternatives_applied here?)

It may be worth splitting this into three patches, so the 'unsafe' bits can be
merged without the debug option.


Either way, for the unsafe parts:
Reviewed-by: James Morse <james.morse@....com>


Thanks!

James