Re: [PATCH v2 2/3] mm: khugepaged: fix folio is used after pte_unmap_unlock()

[Lorenzo Stoakes <ljs@kernel.org>]

On Mon, Aug 17, 2026 at 05:12:01PM +0100, Lorenzo Stoakes (ARM) wrote:
> On Sat, Aug 15, 2026 at 01:19:23PM +0800, Vernon Yang wrote:
> > From: Vernon Yang <yanglincheng@kylinos.cn>
> >
> > After the page table lock has dropped, the folio can be freed
> > concurrently. The trace_mm_khugepaged_scan_pmd() is left with
> > a dangling folio pointer.
> >
> > So using the folio_pfn() before dropping the page table lock,
> > closing use-after-free window.
> >
> > Fixes: 7d2eba0557c1 ("mm: add tracepoint for scanning pages")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> > ---
> >  include/trace/events/huge_memory.h | 6 +++---
> >  mm/khugepaged.c                    | 4 +++-
> >  2 files changed, 6 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> > index d3572d4ef453..5dc71d292f47 100644
> > --- a/include/trace/events/huge_memory.h
> > +++ b/include/trace/events/huge_memory.h
> > @@ -55,10 +55,10 @@ SCAN_STATUS
> >
> >  TRACE_EVENT(mm_khugepaged_scan_pmd,
> >
> > -	TP_PROTO(struct mm_struct *mm, struct folio *folio,
> > +	TP_PROTO(struct mm_struct *mm, unsigned long pfn,
> >  		 int referenced, int none_or_zero, int status, int unmapped),
> >
> > -	TP_ARGS(mm, folio, referenced, none_or_zero, status, unmapped),
> > +	TP_ARGS(mm, pfn, referenced, none_or_zero, status, unmapped),
> >
> >  	TP_STRUCT__entry(
> >  		__field(struct mm_struct *, mm)
> > @@ -71,7 +71,7 @@ TRACE_EVENT(mm_khugepaged_scan_pmd,
> >
> >  	TP_fast_assign(
> >  		__entry->mm = mm;
> > -		__entry->pfn = folio ? folio_pfn(folio) : -1;
> > +		__entry->pfn = pfn;
> >  		__entry->referenced = referenced;
> >  		__entry->none_or_zero = none_or_zero;
> >  		__entry->status = status;
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index e7830761d3a2..7c8c48577408 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -1603,6 +1603,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> >  	enum scan_result result = SCAN_FAIL;
> >  	struct page *page = NULL;
> >  	struct folio *folio = NULL;
> > +	unsigned long pfn = -1;
> >  	unsigned long addr;
> >  	unsigned long enabled_orders;
> >  	spinlock_t *ptl;
> > @@ -1778,6 +1779,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> >  		result = SCAN_SUCCEED;
> >  	}
> >  out_unmap:
> > +	pfn = folio ? folio_pfn(folio) : -1;

You already defaulted the value to -1, better as:

	if (folio)
		pfn = folio_pfn(folio);

> >  	pte_unmap_unlock(pte, ptl);
> >  	if (result == SCAN_SUCCEED) {
> >  		/* collapse_huge_page expects the lock to be dropped before calling */
> > @@ -1788,7 +1790,7 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
> >  		*lock_dropped = true;
> >  	}
> >  out:
> > -	trace_mm_khugepaged_scan_pmd(mm, folio, referenced,
> > +	trace_mm_khugepaged_scan_pmd(mm, pfn, referenced,
>
> Same comment as 1/3 I don't see why we should be storing a pfn value used
> nowhere else just for tracing.

Disregard, I didn't account for lifetime.

>
> >  				     none_or_zero, result, unmapped);
> >  	return result;
> >  }
> > --
> > 2.53.0
> >
>
> --
> Cheers, Lorenzo

--
Cheers, Lorenzo