[PATCH v2 1/3] mm: khugepaged: fix swap entry value to folio_pfn()

From: Vernon Yang <[email protected]>

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: [email protected]
Signed-off-by: Vernon Yang <[email protected]>
---
 include/trace/events/huge_memory.h |  6 +++---
 mm/khugepaged.c                    | 14 +++++++++-----
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index 291fae364c62..d3572d4ef453 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 617bca76db49..e7830761d3a2 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2683,6 +2683,7 @@ 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;
@@ -2720,27 +2721,23 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 			 * PMD-sized THP implies that we can only try
 			 * retracting the PTE table.
 			 */
-			folio_put(folio);
 			break;
 		}
 
 		node = folio_nid(folio);
 		if (collapse_scan_abort(node, cc)) {
 			result = SCAN_SCAN_ABORT;
-			folio_put(folio);
 			break;
 		}
 		cc->node_load[node]++;
 
 		if (!folio_test_lru(folio)) {
 			result = SCAN_PAGE_LRU;
-			folio_put(folio);
 			break;
 		}
 
 		if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) {
 			result = SCAN_PAGE_COUNT;
-			folio_put(folio);
 			break;
 		}
 
@@ -2759,7 +2756,14 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
 			cond_resched_rcu();
 		}
 	}
+	if (!folio || xa_is_value(folio)) {
+		pfn = -1;
+	} else {
+		pfn = folio_pfn(folio);
+		folio_put(folio);
+	}
 	rcu_read_unlock();
+
 	if (result == SCAN_PTE_MAPPED_HUGEPAGE)
 		cc->progress++;
 	else
@@ -2774,7 +2778,7 @@ 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, pfn, file, present, swap, result);
 	return result;
 }
 
-- 
2.53.0