Re: [PATCH] powerpc/nohash: fix pte_access_permitted()

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Christophe Leroy <christophe.leroy@c-s.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. 

https://lore.kernel.org/lkml/87lgcusc6z.fsf@linux.vnet.ibm.com/T/#u

Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.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@vger.kernel.org # v4.15+
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.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