Re: [PATCH v3 2/3] mm: khugepaged: fix folio is used after pte_unmap_unlock()

On 8/24/26 11:29, Vernon Yang wrote:
> From: Vernon Yang <[email protected]>
> 
> 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: [email protected]
> Signed-off-by: Vernon Yang <[email protected]>
> ---
>  include/trace/events/huge_memory.h | 6 +++---
>  mm/khugepaged.c                    | 5 ++++-
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 7b526528f85b..fa828967e1fb 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 00337405c0e0..4e0fca5942dd 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1618,6 +1618,7 @@ 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;
> @@ -1780,6 +1781,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  		result = SCAN_SUCCEED;
>  	}
>  out_unmap:
> +	if (folio)
> +		pfn = folio_pfn(folio);

Should we reset the folio to NULL at the beginning of the loop? Then we really
only trace the PFN if it really was problematic.

-- 
Cheers,

David