[PATCH v3 0/3] mm: khugepaged: fix tracepoint UAF

[PATCH v3 0/3] mm: khugepaged: fix tracepoint UAF

[Vernon Yang]

From: Vernon Yang <yanglincheng@kylinos.cn>

The khugepaged tracepoints take a folio pointer and call folio_pfn(),
but by then the folio may no longer be valid: freed after folio_put(),
folio_unlock() or pte_unmap_unlock(), or not a folio at all but an
xarray-encoded swap entry. On classic SPARSEMEM, dereferencing it oopses
khugepaged as soon as the trace event is enabled; on other memory models
it merely prints a bogus pfn.

Pass the pfn to the tracepoints directly, captured while the folio is
still pinned, closing the use-after-free windows in
mm_khugepaged_scan_file(), mm_khugepaged_scan_pmd() and
mm_khugepaged_collapse_file().

This series is based on mm-new.

V2 -> V3:
- Place folio_pfn() inside the xas_for_each() loop in PATCH#1.
- Already defaulted the pfn value to -1, to simple it in PATCH#2.

V1 -> V2:
- Instead of passing the folio, just pass the pfn directly.
- Using the folio_pfn() before dropping the reference or the page table
  lock.

V2 : https://lore.kernel.org/linux-mm/20260815051924.194810-1-vernon2gm@gmail.com/
V1 : https://lore.kernel.org/linux-mm/20260811133655.267739-1-vernon2gm@gmail.com/

Vernon Yang (3):
  mm: khugepaged: fix swap entry value to folio_pfn()
  mm: khugepaged: fix folio is used after pte_unmap_unlock()
  mm: khugepaged: fix folio is used after folio_put/unlock()

 include/trace/events/huge_memory.h | 18 +++++++++---------
 mm/khugepaged.c                    | 15 ++++++++++++---
 2 files changed, 21 insertions(+), 12 deletions(-)


base-commit: a032d41a86cb82a747bc14d9c82b3e153a9a9ab7
--
2.53.0

[PATCH v3 1/3] mm: khugepaged: fix swap entry value to folio_pfn()

[Vernon Yang]

From: Vernon Yang <yanglincheng@kylinos.cn>

When the swap entries found exceed max_ptes_swap, the loop is left via
break with folio still holding the xarray value that encodes the swap
entry, not valid folio pointer.

That value is passed to trace_mm_khugepaged_scan_file(), which feeds it
to folio_pfn(). On FLATMEM and SPARSEMEM_VMEMMAP, the page_to_pfn() is
plain pointer arithmetic, so the trace event merely prints bogus
scan_pfn. On classic SPARSEMEM, the page_to_pfn() reads page->flags,
dereferencing the tiny encoded integer and oopsing khugepaged whenever
the trace event is enabled.

So when folio is the swap entry value, simply set pfn to -1, just like
exhausted scan naturally.

And the folio_put() has maybe dropped the last reference of folio. The
trace_mm_khugepaged_scan_file() is left with a dangling folio pointer.
so using the folio_pfn() before dropping the reference, closing
use-after-free window.

Fixes: d41fd2016ed0 ("mm/khugepaged: add tracepoint to hpage_collapse_scan_file()")
Cc: stable@vger.kernel.org
Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
---
 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 5a48c5406cce..7b526528f85b 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -178,10 +178,10 @@ TRACE_EVENT(mm_collapse_huge_page_swapin,
 
 TRACE_EVENT(mm_khugepaged_scan_file,
 
-	TP_PROTO(struct mm_struct *mm, struct folio *folio, struct file *file,
+	TP_PROTO(struct mm_struct *mm, unsigned long pfn, struct file *file,
 		 int present, int swap, int result),
 
-	TP_ARGS(mm, folio, file, present, swap, result),
+	TP_ARGS(mm, pfn, file, present, swap, result),
 
 	TP_STRUCT__entry(
 		__field(struct mm_struct *, mm)
@@ -194,7 +194,7 @@ TRACE_EVENT(mm_khugepaged_scan_file,
 
 	TP_fast_assign(
 		__entry->mm = mm;
-		__entry->pfn = folio ? folio_pfn(folio) : -1;
+		__entry->pfn = pfn;
 		__assign_str(filename);
 		__entry->present = present;
 		__entry->swap = swap;
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 79effd3f3da4..00337405c0e0 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2689,6 +2689,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 	int present, swap;
 	int node = NUMA_NO_NODE;
 	enum scan_result result = SCAN_SUCCEED;
+	unsigned long pfn;
 
 	present = 0;
 	swap = 0;
@@ -2719,6 +2720,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 			continue;
 		}
 
+		pfn = folio_pfn(folio);
 		if (is_pmd_order(folio_order(folio))) {
 			result = SCAN_PTE_MAPPED_HUGEPAGE;
 			/*
@@ -2779,7 +2781,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 		}
 	}
 
-	trace_mm_khugepaged_scan_file(mm, folio, file, present, swap, result);
+	trace_mm_khugepaged_scan_file(mm, (!folio || xa_is_value(folio)) ? -1 : pfn,
+				      file, present, swap, result);
 	return result;
 }
 
-- 
2.53.0

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

[Vernon Yang]

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                    | 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);
 	pte_unmap_unlock(pte, ptl);
 	if (result == SCAN_SUCCEED) {
 		/* collapse_huge_page() expects the lock to be dropped before calling */
@@ -1790,7 +1793,7 @@ 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,
 				     none_or_zero, result, unmapped);
 	return result;
 }
-- 
2.53.0

[PATCH v3 3/3] mm: khugepaged: fix folio is used after folio_put/unlock()

[Vernon Yang]

From: Vernon Yang <yanglincheng@kylinos.cn>

On the rollback path, folio_put() has already dropped the last reference
of new_folio. On the success path, new_folio is already unlocked and can
be freed concurrently. The trace_mm_khugepaged_collapse_file() is left
with a dangling folio pointer.

So using the folio_pfn() before dropping the reference, closing
use-after-free window.

Fixes: 4c9473e87e75 ("mm/khugepaged: add tracepoint to collapse_file()")
Cc: stable@vger.kernel.org
Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
---
 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 fa828967e1fb..5fb4d92cfd84 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -211,10 +211,10 @@ TRACE_EVENT(mm_khugepaged_scan_file,
 );
 
 TRACE_EVENT(mm_khugepaged_collapse_file,
-	TP_PROTO(struct mm_struct *mm, struct folio *new_folio, pgoff_t index,
+	TP_PROTO(struct mm_struct *mm, unsigned long new_pfn, pgoff_t index,
 			unsigned long addr, bool is_shmem, struct file *file,
 			int nr, int result),
-	TP_ARGS(mm, new_folio, index, addr, is_shmem, file, nr, result),
+	TP_ARGS(mm, new_pfn, index, addr, is_shmem, file, nr, result),
 	TP_STRUCT__entry(
 		__field(struct mm_struct *, mm)
 		__field(unsigned long, hpfn)
@@ -228,7 +228,7 @@ TRACE_EVENT(mm_khugepaged_collapse_file,
 
 	TP_fast_assign(
 		__entry->mm = mm;
-		__entry->hpfn = new_folio ? folio_pfn(new_folio) : -1;
+		__entry->hpfn = new_pfn;
 		__entry->index = index;
 		__entry->addr = addr;
 		__entry->is_shmem = is_shmem;
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 4e0fca5942dd..24347f1a94ae 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2254,6 +2254,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 	struct address_space *mapping = file->f_mapping;
 	struct page *dst;
 	struct folio *folio, *tmp, *new_folio;
+	unsigned long new_pfn = -1;
 	pgoff_t index = 0, end = start + HPAGE_PMD_NR;
 	LIST_HEAD(pagelist);
 	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
@@ -2633,6 +2634,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 	retract_page_tables(mapping, start);
 	if (cc && !cc->is_khugepaged)
 		result = SCAN_PTE_MAPPED_HUGEPAGE;
+	new_pfn = folio_pfn(new_folio);
 	folio_unlock(new_folio);
 
 	/*
@@ -2671,12 +2673,13 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
 	}
 
 	new_folio->mapping = NULL;
+	new_pfn = folio_pfn(new_folio);
 
 	folio_unlock(new_folio);
 	folio_put(new_folio);
 out:
 	VM_BUG_ON(!list_empty(&pagelist));
-	trace_mm_khugepaged_collapse_file(mm, new_folio, index, addr, is_shmem, file, HPAGE_PMD_NR, result);
+	trace_mm_khugepaged_collapse_file(mm, new_pfn, index, addr, is_shmem, file, HPAGE_PMD_NR, result);
 	return result;
 }
 
-- 
2.53.0

Re: [PATCH v3 1/3] mm: khugepaged: fix swap entry value to folio_pfn()

[David Hildenbrand (Arm)]

On 8/24/26 11:29, Vernon Yang wrote:
> From: Vernon Yang <yanglincheng@kylinos.cn>
> 
> When the swap entries found exceed max_ptes_swap, the loop is left via
> break with folio still holding the xarray value that encodes the swap
> entry, not valid folio pointer.
> 
> That value is passed to trace_mm_khugepaged_scan_file(), which feeds it
> to folio_pfn(). On FLATMEM and SPARSEMEM_VMEMMAP, the page_to_pfn() is
> plain pointer arithmetic, so the trace event merely prints bogus
> scan_pfn. On classic SPARSEMEM, the page_to_pfn() reads page->flags,
> dereferencing the tiny encoded integer and oopsing khugepaged whenever
> the trace event is enabled.
> 
> So when folio is the swap entry value, simply set pfn to -1, just like
> exhausted scan naturally.
> 
> And the folio_put() has maybe dropped the last reference of folio. The
> trace_mm_khugepaged_scan_file() is left with a dangling folio pointer.
> so using the folio_pfn() before dropping the reference, closing
> use-after-free window.
> 
> Fixes: d41fd2016ed0 ("mm/khugepaged: add tracepoint to hpage_collapse_scan_file()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> ---
>  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 5a48c5406cce..7b526528f85b 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -178,10 +178,10 @@ TRACE_EVENT(mm_collapse_huge_page_swapin,
>  
>  TRACE_EVENT(mm_khugepaged_scan_file,
>  
> -	TP_PROTO(struct mm_struct *mm, struct folio *folio, struct file *file,
> +	TP_PROTO(struct mm_struct *mm, unsigned long pfn, struct file *file,
>  		 int present, int swap, int result),
>  
> -	TP_ARGS(mm, folio, file, present, swap, result),
> +	TP_ARGS(mm, pfn, file, present, swap, result),
>  
>  	TP_STRUCT__entry(
>  		__field(struct mm_struct *, mm)
> @@ -194,7 +194,7 @@ TRACE_EVENT(mm_khugepaged_scan_file,
>  
>  	TP_fast_assign(
>  		__entry->mm = mm;
> -		__entry->pfn = folio ? folio_pfn(folio) : -1;
> +		__entry->pfn = pfn;
>  		__assign_str(filename);
>  		__entry->present = present;
>  		__entry->swap = swap;
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 79effd3f3da4..00337405c0e0 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2689,6 +2689,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  	int present, swap;
>  	int node = NUMA_NO_NODE;
>  	enum scan_result result = SCAN_SUCCEED;
> +	unsigned long pfn;
>  
>  	present = 0;
>  	swap = 0;
> @@ -2719,6 +2720,7 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  			continue;
>  		}
>  
> +		pfn = folio_pfn(folio);
>  		if (is_pmd_order(folio_order(folio))) {
>  			result = SCAN_PTE_MAPPED_HUGEPAGE;
>  			/*
> @@ -2779,7 +2781,8 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  		}
>  	}
>  
> -	trace_mm_khugepaged_scan_file(mm, folio, file, present, swap, result);
> +	trace_mm_khugepaged_scan_file(mm, (!folio || xa_is_value(folio)) ? -1 : pfn,
> +				      file, present, swap, result);
>  	return result;
>  }
>  

Shouldn't we just reset PFN to -1 at the beginning of the loop (and set it
initially)?

-- 
Cheers,

David

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

[David Hildenbrand (Arm)]

On 8/24/26 11:29, 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                    | 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

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

[David Hildenbrand (Arm)]

On 8/24/26 11:29, Vernon Yang wrote:
> From: Vernon Yang <yanglincheng@kylinos.cn>
> 
> On the rollback path, folio_put() has already dropped the last reference
> of new_folio. On the success path, new_folio is already unlocked and can
> be freed concurrently. The trace_mm_khugepaged_collapse_file() is left
> with a dangling folio pointer.
> 
> So using the folio_pfn() before dropping the reference, closing
> use-after-free window.
> 
> Fixes: 4c9473e87e75 ("mm/khugepaged: add tracepoint to collapse_file()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
> ---
>  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 fa828967e1fb..5fb4d92cfd84 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -211,10 +211,10 @@ TRACE_EVENT(mm_khugepaged_scan_file,
>  );
>  
>  TRACE_EVENT(mm_khugepaged_collapse_file,
> -	TP_PROTO(struct mm_struct *mm, struct folio *new_folio, pgoff_t index,
> +	TP_PROTO(struct mm_struct *mm, unsigned long new_pfn, pgoff_t index,
>  			unsigned long addr, bool is_shmem, struct file *file,
>  			int nr, int result),
> -	TP_ARGS(mm, new_folio, index, addr, is_shmem, file, nr, result),
> +	TP_ARGS(mm, new_pfn, index, addr, is_shmem, file, nr, result),
>  	TP_STRUCT__entry(
>  		__field(struct mm_struct *, mm)
>  		__field(unsigned long, hpfn)
> @@ -228,7 +228,7 @@ TRACE_EVENT(mm_khugepaged_collapse_file,
>  
>  	TP_fast_assign(
>  		__entry->mm = mm;
> -		__entry->hpfn = new_folio ? folio_pfn(new_folio) : -1;
> +		__entry->hpfn = new_pfn;
>  		__entry->index = index;
>  		__entry->addr = addr;
>  		__entry->is_shmem = is_shmem;
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 4e0fca5942dd..24347f1a94ae 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2254,6 +2254,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>  	struct address_space *mapping = file->f_mapping;
>  	struct page *dst;
>  	struct folio *folio, *tmp, *new_folio;
> +	unsigned long new_pfn = -1;
>  	pgoff_t index = 0, end = start + HPAGE_PMD_NR;
>  	LIST_HEAD(pagelist);
>  	XA_STATE_ORDER(xas, &mapping->i_pages, start, HPAGE_PMD_ORDER);
> @@ -2633,6 +2634,7 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>  	retract_page_tables(mapping, start);
>  	if (cc && !cc->is_khugepaged)
>  		result = SCAN_PTE_MAPPED_HUGEPAGE;
> +	new_pfn = folio_pfn(new_folio);

Why not set new_pfn once after successful alloc_charge_folio()?




-- 
Cheers,

David