Re: [PATCH v10 01/10] powerpc/mm: Implement set_memory() routines

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Michael Ellerman <mpe@ellerman.id.au>,
	Jordan Niethe <jniethe5@gmail.com>,
	linuxppc-dev@lists.ozlabs.org
Cc: ajd@linux.ibm.com, npiggin@gmail.com, cmr@codefail.de,
	naveen.n.rao@linux.ibm.com, dja@axtens.net
Subject: Re: [PATCH v10 01/10] powerpc/mm: Implement set_memory() routines
Date: Wed, 31 Mar 2021 14:03:23 +0200	[thread overview]
Message-ID: <17c3a18b-9bfb-15e6-fc09-63d6226f1879@csgroup.eu> (raw)
In-Reply-To: <87lfa37euw.fsf@mpe.ellerman.id.au>

Le 31/03/2021 à 13:16, Michael Ellerman a écrit :
> Hi Jordan,
> 
> A few nits below ...
> 
> Jordan Niethe <jniethe5@gmail.com> writes:
>> From: Russell Currey <ruscur@russell.cc>
>>
>> The set_memory_{ro/rw/nx/x}() functions are required for STRICT_MODULE_RWX,
>> and are generally useful primitives to have.  This implementation is
>> designed to be completely generic across powerpc's many MMUs.
>>
>> It's possible that this could be optimised to be faster for specific
>> MMUs, but the focus is on having a generic and safe implementation for
>> now.
>>
>> This implementation does not handle cases where the caller is attempting
>> to change the mapping of the page it is executing from, or if another
>> CPU is concurrently using the page being altered.  These cases likely
>> shouldn't happen, but a more complex implementation with MMU-specific code
>> could safely handle them, so that is left as a TODO for now.
>>
>> On hash the linear mapping is not kept in the linux pagetable, so this
>> will not change the protection if used on that range. Currently these
>> functions are not used on the linear map so just WARN for now.
>>
>> These functions do nothing if STRICT_KERNEL_RWX is not enabled.
>>
>> Reviewed-by: Daniel Axtens <dja@axtens.net>
>> Signed-off-by: Russell Currey <ruscur@russell.cc>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> [jpn: -rebase on next plus "powerpc/mm/64s: Allow STRICT_KERNEL_RWX again"
>>        - WARN on hash linear map]
>> Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
>> ---
>> v10: WARN if trying to change the hash linear map
>> ---

> 
> This ↓ should have a comment explaining what it's doing:
> 
>> +#ifdef CONFIG_PPC_BOOK3S_64
>> +	if (WARN_ON_ONCE(!radix_enabled() &&
>> +		     get_region_id(addr) == LINEAR_MAP_REGION_ID)) {
>> +		return -1;
>> +	}
>> +#endif
> 
> Maybe:
> 
> 	if (IS_ENABLED(CONFIG_PPC_BOOK3S_64) &&
> 	    WARN_ON_ONCE(!radix_enabled() && get_region_id(addr) == LINEAR_MAP_REGION_ID)) {
> 		return -1;
> 	}

get_region_id() only exists for book3s/64 at the time being, and LINEAR_MAP_REGION_ID as well.

> 
> But then Aneesh pointed out that we should also block VMEMMAP_REGION_ID.
> 
> It might be better to just check for the permitted regions.
> 
> 	if (IS_ENABLED(CONFIG_PPC_BOOK3S_64) && !radix_enabled()) {
>          	int region = get_region_id(addr);
> 
> 	    	if (WARN_ON_ONCE(region != VMALLOC_REGION_ID && region != IO_REGION_ID))
>                  	return -1;
> 	}
> 
>> +
>> +	return apply_to_existing_page_range(&init_mm, start, sz,
>> +					    change_page_attr, (void *)action);
>> +}
> 
> 
> cheers
> 