Re: [PATCH v1 2/4] mm/gup: Make follow_page() succeed again on PROT_NONE PTEs/PMDs

[David Hildenbrand]

On 28.07.23 04:30, John Hubbard wrote:
> On 7/27/23 14:28, David Hildenbrand wrote:
> > We accidentally enforced PROT_NONE PTE/PMD permission checks for
> > follow_page() like we do for get_user_pages() and friends. That was
> > undesired, because follow_page() is usually only used to lookup a currently
> > mapped page, not to actually access it. Further, follow_page() does not
> > actually trigger fault handling, but instead simply fails.
>
> I see that follow_page() is also completely undocumented. And that
> reduces us to deducing how it should be used...these things that
> change follow_page()'s behavior maybe should have a go at documenting
> it too, perhaps.

I can certainly be motivated to do that. :)

> > Let's restore that behavior by conditionally setting FOLL_FORCE if
> > FOLL_WRITE is not set. This way, for example KSM and migration code will
> > no longer fail on PROT_NONE mapped PTEs/PMDS.
> >
> > Handling this internally doesn't require us to add any new FOLL_FORCE
> > usage outside of GUP code.
> >
> > While at it, refuse to accept FOLL_FORCE: we don't even perform VMA
> > permission checks like in check_vma_flags(), so especially
> > FOLL_FORCE|FOLL_WRITE would be dodgy.
> >
> > This issue was identified by code inspection. We'll add some
> > documentation regarding FOLL_FORCE next.
> >
> > Reported-by: Peter Xu <peterx@xxxxxxxxxx>
> > Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > ---
> >    mm/gup.c | 10 +++++++++-
> >    1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 2493ffa10f4b..da9a5cc096ac 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -841,9 +841,17 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
> >    	if (vma_is_secretmem(vma))
> >    		return NULL;
> >    
> > -	if (WARN_ON_ONCE(foll_flags & FOLL_PIN))
> > +	if (WARN_ON_ONCE(foll_flags & (FOLL_PIN | FOLL_FORCE)))
> >    		return NULL;
>
> This is not a super happy situation: follow_page() is now prohibited
> (see above: we should document that interface) from passing in
> FOLL_FORCE...

I guess you saw my patch #4.

If you take a look at the existing callers (that are fortunately very 
limited), you'll see that nobody cares.

Most of the FOLL flags don't make any sense for follow_page(), and 
limiting further (ab)use is at least to me very appealing.

> >    
> > +	/*
> > +	 * Traditionally, follow_page() succeeded on PROT_NONE-mapped pages
> > +	 * but failed follow_page(FOLL_WRITE) on R/O-mapped pages. Let's
> > +	 * keep these semantics by setting FOLL_FORCE if FOLL_WRITE is not set.
> > +	 */
> > +	if (!(foll_flags & FOLL_WRITE))
> > +		foll_flags |= FOLL_FORCE;
> > +
>
> ...but then we set it anyway, for special cases. It's awkward because
> FOLL_FORCE is not an "internal to gup" flag (yet?).
>
> I don't yet have suggestions, other than:
>
> 1) Yes, the FOLL_NUMA made things bad.
>
> 2) And they are still very confusing, especially the new use of
>      FOLL_FORCE.
>
> ...I'll try to let this soak in and maybe recommend something
> in a more productive way. :)

What I can offer that might be very appealing is the following:

Get rid of the flags parameter for follow_page() *completely*. Yes, then 
we can even rename FOLL_ to something reasonable in the context where it 
is nowadays used ;)


Internally, we'll then set

FOLL_GET | FOLL_DUMP | FOLL_FORCE

and document exactly what this functions does. Any user that needs 
something different should just look into using get_user_pages() instead.

I can prototype that on top of this work easily.

--
Cheers,

David / dhildenb