linux-kernel - Re: [PATCH] powerpc/nohash: fix pte_access

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Date:   Tue, 21 Aug 2018 16:47:58 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        aneesh.kumar@...ux.vnet.ibm.com
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] powerpc/nohash: fix pte_access_permitted()



Le 21/08/2018 à 16:25, Aneesh Kumar K.V a écrit :
> Christophe Leroy <christophe.leroy@....fr> writes:
> 
>> Commit 5769beaf180a8 ("powerpc/mm: Add proper pte access check helper
>> for other platforms") replaced generic pte_access_permitted() by an
>> arch specific one.
>>
>> The generic one is defined as
>> (pte_present(pte) && (!(write) || pte_write(pte)))
>>
>> The arch specific one is open coded checking that _PAGE_USER and
>> _PAGE_WRITE (_PAGE_RW) flags are set, but lacking to check that
>> _PAGE_RO and _PAGE_PRIVILEGED are unset, leading to a useless test
>> on targets like the 8xx which defines _PAGE_RW and _PAGE_USER as 0.
>>
>> Commit 5fa5b16be5b31 ("powerpc/mm/hugetlb: Use pte_access_permitted
>> for hugetlb access check") replaced some tests performed with
>> pte helpers by a call to pte_access_permitted(), leading to the same
>> issue.
>>
>> This patch rewrites powerpc/nohash pte_access_permitted()
>> using pte helpers.
>>
> 
> Thanks for fixing this. I should have used the helper instead of
> opencoding it on nohash platforms. This is another reason why I was also
> suggesting we should avoid consolidating pte accessors across platforms
> and user accessors instead of opencoding.

I fully agree. I have opened a topic for that at 
https://github.com/linuxppc/linux/issues/177

Christophe

> 
> https://lore.kernel.org/lkml/87lgcusc6z.fsf@linux.vnet.ibm.com/T/#u
> 
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> 
>> Fixes: 5769beaf180a8 ("powerpc/mm: Add proper pte access check helper for other platforms")
>> Fixes: 5fa5b16be5b31 ("powerpc/mm/hugetlb: Use pte_access_permitted for hugetlb access check")
>> Cc: stable@...r.kernel.org # v4.15+
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>> ---
>>   arch/powerpc/include/asm/nohash/pgtable.h | 9 +++------
>>   1 file changed, 3 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/nohash/pgtable.h b/arch/powerpc/include/asm/nohash/pgtable.h
>> index 2160be2e4339..b321c82b3624 100644
>> --- a/arch/powerpc/include/asm/nohash/pgtable.h
>> +++ b/arch/powerpc/include/asm/nohash/pgtable.h
>> @@ -51,17 +51,14 @@ static inline int pte_present(pte_t pte)
>>   #define pte_access_permitted pte_access_permitted
>>   static inline bool pte_access_permitted(pte_t pte, bool write)
>>   {
>> -	unsigned long pteval = pte_val(pte);
>>   	/*
>>   	 * A read-only access is controlled by _PAGE_USER bit.
>>   	 * We have _PAGE_READ set for WRITE and EXECUTE
>>   	 */
>> -	unsigned long need_pte_bits = _PAGE_PRESENT | _PAGE_USER;
>> -
>> -	if (write)
>> -		need_pte_bits |= _PAGE_WRITE;
>> +	if (!pte_present(pte) || !pte_user(pte) || !pte_read(pte))
>> +		return false;
>>   
>> -	if ((pteval & need_pte_bits) != need_pte_bits)
>> +	if (write && !pte_write(pte))
>>   		return false;
>>   
>>   	return true;
>> -- 
>> 2.13.3