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Subject: Re: [PATCH v5 19/26] powerpc/book3s64/kuap: Move KUAP related
 function outside radix
To: Michael Ellerman <mpe@ellerman.id.au>, linuxppc-dev@lists.ozlabs.org
References: <20200619135850.47155-1-aneesh.kumar@linux.ibm.com>
 <20200619135850.47155-20-aneesh.kumar@linux.ibm.com>
 <87eepo6cjm.fsf@mpe.ellerman.id.au>
From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Message-ID: <d09d0150-860a-1e6a-1d4a-01ae8d7e97b9@linux.ibm.com>
Date: Mon, 6 Jul 2020 20:11:45 +0530
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On 7/6/20 6:11 PM, Michael Ellerman wrote:
> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:
>> The next set of patches adds support for kuap with hash translation.
> 
> That's no longer true of this series.
> 
>> diff --git a/arch/powerpc/mm/book3s64/pkeys.c b/arch/powerpc/mm/book3s64/pkeys.c
>> index 0d72c0246052..e93b65a0e6e7 100644
>> --- a/arch/powerpc/mm/book3s64/pkeys.c
>> +++ b/arch/powerpc/mm/book3s64/pkeys.c
>> @@ -199,6 +200,24 @@ void __init pkey_early_init_devtree(void)
>>   	return;
>>   }
>>   
>> +#ifdef CONFIG_PPC_KUAP
>> +void __init setup_kuap(bool disabled)
>> +{
>> +	if (disabled || !early_radix_enabled())
>> +		return;
>> +
>> +	if (smp_processor_id() == boot_cpuid) {
>> +		pr_info("Activating Kernel Userspace Access Prevention\n");
>> +		cur_cpu_spec->mmu_features |= MMU_FTR_RADIX_KUAP;
>> +	}
>> +
>> +	/* Make sure userspace can't change the AMR */
>> +	mtspr(SPRN_UAMOR, 0);
>> +	mtspr(SPRN_AMR, AMR_KUAP_BLOCKED);
>> +	isync();
>> +}
>> +#endif
> 
> This makes this code depend on CONFIG_PPC_MEM_KEYS=y, which it didn't
> used to.
> 
> That risks breaking people's existing .configs, if they have
> PPC_MEM_KEYS=n they will now lose KUAP.
> 
> And I'm not convinced the two features should be tied together, at least
> at the user-visible Kconfig level.
> 

That simplifies the addition of hash kuap a lot. Especially in the 
exception entry and return paths.  I did try to consider them as 
independent options. But then the feature fixup in asm code gets 
unnecessarily complicated. Also the UAMOR handling also get complicated.

-aneesh