[PATCH 6.18.y] mm: khugepaged: fix swap entry value to folio_pfn()

[Sasha Levin <sashal@kernel.org>]

From: Vernon Yang <yanglincheng@kylinos.cn>

[ Upstream commit 942c8ec27aa4ffc587511650718349964e1696e6 ]

Patch series "mm: khugepaged: fix tracepoint UAF", v5.

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 patch (of 3):

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.

Link: https://lore.kernel.org/20260909025804.3233645-1-vernon2gm@gmail.com
Link: https://lore.kernel.org/20260909025804.3233645-2-vernon2gm@gmail.com
Fixes: d41fd2016ed0 ("mm/khugepaged: add tracepoint to hpage_collapse_scan_file()")
Signed-off-by: Vernon Yang <yanglincheng@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Cc: <stable@vger.kernel.org>
[ sashal: Retained int result instead of enum scan_result in hpage_collapse_scan_file(). ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/trace/events/huge_memory.h | 6 +++---
 mm/khugepaged.c                    | 7 ++++++-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
index 2276614691dfd..1464d45c90abf 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -167,10 +167,10 @@ 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)
@@ -183,7 +183,7 @@ 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 e2ac26c05064f..cc3a09a0c2596 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -2308,6 +2308,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 	int present, swap;
 	int node = NUMA_NO_NODE;
 	int result = SCAN_SUCCEED;
+	unsigned long failed_pfn = -1;
 
 	present = 0;
 	swap = 0;
@@ -2344,6 +2345,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 		    folio->index == start) {
 			/* Maybe PMD-mapped */
 			result = SCAN_PTE_MAPPED_HUGEPAGE;
+			failed_pfn = folio_pfn(folio);
 			/*
 			 * For SCAN_PTE_MAPPED_HUGEPAGE, further processing
 			 * by the caller won't touch the page cache, and so
@@ -2357,6 +2359,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 		node = folio_nid(folio);
 		if (hpage_collapse_scan_abort(node, cc)) {
 			result = SCAN_SCAN_ABORT;
+			failed_pfn = folio_pfn(folio);
 			folio_put(folio);
 			break;
 		}
@@ -2364,12 +2367,14 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 
 		if (!folio_test_lru(folio)) {
 			result = SCAN_PAGE_LRU;
+			failed_pfn = folio_pfn(folio);
 			folio_put(folio);
 			break;
 		}
 
 		if (folio_expected_ref_count(folio) + 1 != folio_ref_count(folio)) {
 			result = SCAN_PAGE_COUNT;
+			failed_pfn = folio_pfn(folio);
 			folio_put(folio);
 			break;
 		}
@@ -2401,7 +2406,7 @@ static int hpage_collapse_scan_file(struct mm_struct *mm, unsigned long addr,
 		}
 	}
 
-	trace_mm_khugepaged_scan_file(mm, folio, file, present, swap, result);
+	trace_mm_khugepaged_scan_file(mm, failed_pfn, file, present, swap, result);
 	return result;
 }
 
-- 
2.53.0