linux-kernel - Re: [PATCH v2 35/46] hugetlb: add check to prevent refcount overflow via HGM

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Date:   Fri, 24 Feb 2023 10:05:11 -0800
From:   James Houghton <jthoughton@...gle.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Peter Xu <peterx@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        "Zach O'Keefe" <zokeefe@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Frank van der Linden <fvdl@...gle.com>,
        Jiaqi Yan <jiaqiyan@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 35/46] hugetlb: add check to prevent refcount overflow
 via HGM

On Fri, Feb 24, 2023 at 9:42 AM James Houghton <jthoughton@...gle.com> wrote:
>
> > @@ -5397,7 +5397,10 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
> >                 } else {
> >                         ptepage = pte_page(entry);
> >                         hpage = compound_head(ptepage);
> > -                       get_page(hpage);
> > +                       if (try_get_page(hpage)) {
> > +                               ret = -EFAULT;
> > +                               break;
>
> spin_unlock(src_ptl) and spin_unlock(dst_ptl) is required here.
>
> I'll make sure there's a selftest that actually makes sure that
> refcount overflowing is handled gracefully for v3.

And this should be !try_get_page(). This hunk was a last-minute
addition to this commit; apparently I hadn't retested fork() after I
made this change. Sorry! The hugetlb-hgm selftest immediately catches
this problem.

- James