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

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Date:   Fri, 7 Dec 2018 08:38:11 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        peterz@...radead.org, will.deacon@....com, mingo@...hat.com,
        james.morse@....com, hpa@...or.com
Subject: Re: [PATCH v2 2/2] arm64: uaccess: Implement unsafe accessors



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)
>>   	__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;
> 
> Nitpick: could write this in 1-2 lines.
> 

True, I can do that in 1 line.

>> +	} 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.

Thanks,

Julien