Re: [PATCH 2/7] mm/hugetlb: Fix hugetlb_follow_page_mask() on permission checks

[Peter Xu]

On Wed, Jun 14, 2023 at 05:31:36PM +0200, David Hildenbrand wrote:
> On 13.06.23 23:53, Peter Xu wrote:
> > It seems hugetlb_follow_page_mask() was missing permission checks.  For
> > example, one follow_page() can get the hugetlb page with FOLL_WRITE even if
> > the page is read-only.
> 
> I'm curious if there even is a follow_page() user that operates on hugetlb
> ...
> 
> s390x secure storage does not apply to hugetlb IIRC.

You're the expert, so I'll rely on you. :)

> 
> ksm.c? no.
> 
> huge_memory.c ? no
> 
> So what remains is most probably mm/migrate.c, which never sets FOLL_WRITE.
> 
> Or am I missing something a user?

Yes, non of the rest are with WRITE.

Then I assume no fixes /backport needed at all (which is what this patch
already does).  It's purely to be prepared only.  I'll mention that in the
new version.

Thanks,

> 
> >  > And it wasn't there even in the old follow_page_mask(), where we can
> > reference from before commit 57a196a58421 ("hugetlb: simplify hugetlb
> > handling in follow_page_mask").
> > 
> > Let's add them, namely, either the need to CoW due to missing write bit, or
> > proper CoR on !AnonExclusive pages over R/O pins to reject the follow page.
> > That brings this function closer to follow_hugetlb_page().
> > 
> > I just doubt how many of us care for that, for FOLL_PIN follow_page doesn't
> > really happen at all.  But we'll care, and care more if we switch over
> > slow-gup to use hugetlb_follow_page_mask().  We'll also care when to return
> > -EMLINK then, as that's the gup internal api to mean "we should do CoR".
> > 
> > When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
> > clear that it just should never fail.
> > 
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> >   mm/hugetlb.c | 22 ++++++++++++++++------
> >   1 file changed, 16 insertions(+), 6 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 82dfdd96db4c..9c261921b2cf 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -6481,8 +6481,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> >   	ptl = huge_pte_lock(h, mm, pte);
> >   	entry = huge_ptep_get(pte);
> >   	if (pte_present(entry)) {
> > -		page = pte_page(entry) +
> > -				((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> > +		page = pte_page(entry);
> > +
> > +		if (gup_must_unshare(vma, flags, page)) {
> > +			/* Tell the caller to do Copy-On-Read */
> > +			page = ERR_PTR(-EMLINK);
> > +			goto out;
> > +		}
> > +
> > +		if ((flags & FOLL_WRITE) && !pte_write(entry)) {
> > +			page = NULL;
> > +			goto out;
> > +		}
> > +
> > +		page += ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> > +
> >   		/*
> >   		 * Note that page may be a sub-page, and with vmemmap
> >   		 * optimizations the page struct may be read only.
> > @@ -6492,10 +6505,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> >   		 * try_grab_page() should always be able to get the page here,
> >   		 * because we hold the ptl lock and have verified pte_present().
> >   		 */
> > -		if (try_grab_page(page, flags)) {
> > -			page = NULL;
> > -			goto out;
> > -		}
> > +		WARN_ON_ONCE(try_grab_page(page, flags));
> >   	}
> >   out:
> >   	spin_unlock(ptl);
> 
> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Peter Xu