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

[Christophe LEROY]

Le 21/08/2018 Ã 16:25, Aneesh Kumar K.V a ÃcritÂ:
> Christophe Leroy <christophe.leroy@xxxxxx> 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@xxxxxxxxxxxxxxxxxx/T/#u
>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
>
> > 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@xxxxxxxxxxxxxxx # v4.15+
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > ---
> >   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