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

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Message-ID: <7fc4f5d1-a587-1a24-8fae-fc8e26195804@arm.com>
Date:   Fri, 21 Dec 2018 14:57:10 +0000
From:   James Morse <james.morse@....com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Julien Thierry <julien.thierry@....com>
Cc:     peterz@...radead.org, will.deacon@....com,
        linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
        linux-arm-kernel@...ts.infradead.org,
        Suzuki K Poulose <Suzuki.Poulose@....com>
Subject: Re: [PATCH v2 2/2] arm64: uaccess: Implement unsafe accessors

Hi guys,

On 10/12/2018 14:59, Catalin Marinas wrote:
> On Fri, Dec 07, 2018 at 08:38:11AM +0000, Julien Thierry wrote:
>> On 12/06/2018 06:25 PM, Catalin Marinas wrote:
>>> On Mon, Dec 03, 2018 at 01:55:18PM +0000, Julien Thierry wrote:
>>>> 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)
>>>> +#define unsafe_user_region_active	uaccess_region_active
>>>> +static inline bool uaccess_region_active(void)
>>>> +{
>>>> +	if (system_uses_ttbr0_pan()) {

>>>> +	} 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);
>>>> +	}
>>>
>>> ARM64_ALT_PAN_NOT_UAO implies ARM64_HAS_PAN which implies SCTLR_EL1.SPAN
>>> is 0 at run-time. Is this to cope with the case of being called prior to
>>> cpu_enable_pan()?
>>>
>>
>> Yes, the issue I can into is that for cpufeatures, .cpu_enable() callbacks
>> are called inside stop_machine() which obviously might_sleep and so attempts
>> to check whether user_access is on. But for features that get enabled before
>> PAN, the PAN bit will be set.
> 
> OK, so the PSTATE.PAN bit only makes sense when SCTLR_EL1.SPAN is 0, IOW
> the PAN hardware feature has been enabled. Maybe you could write it
> (together with some comment):
> 
> 	} else if (cpus_have_const_cap(ARM64_ALT_PAN_NOT_UAO) &&
> 		 !(read_sysreg(sctlr_el1) & SCTLR_EL1_SPAN)) {
> 		 /* only if PAN is present and enabled */
> 		return !read_sysreg_s(SYS_PSTATE_PAN)
> 	}
> 
> On the cpufeature.c side of things, it seems that we enable the
> static_branch before calling the cpu_enable. I wonder whether changing
> the order here would help with avoid the SCTLR_EL1 read (not sure what
> else it would break; cc'ing Suzuki).

Avoiding the system-register read would be good. Can we check
alternatives_applied? It gets set later, and is obviously connected to the PAN
alternatives being patched in to the uaccess routines.


Thanks,

James