[PATCH 6.6.y] mm: khugepaged: fix folio is used after folio_put/unlock()

[Sasha Levin <sashal@kernel.org>]

From: Vernon Yang <yanglincheng@kylinos.cn>

[ Upstream commit 9010383e71e6ce75fcd3c8f87133c8ce6c0e6ab4 ]

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.

Link: https://lore.kernel.org/20260909025804.3233645-4-vernon2gm@gmail.com
Fixes: 4c9473e87e75 ("mm/khugepaged: add tracepoint to collapse_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>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zach O'Keefe <zokeefe@google.com>
Cc: <stable@vger.kernel.org>
[ sashal: adjusted collapse_file() declaration context to preserve struct page *tmp instead of struct folio *tmp ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 37f2443b3cdb0..a1ac19bab4a2d 100644
--- a/include/trace/events/huge_memory.h
+++ b/include/trace/events/huge_memory.h
@@ -207,10 +207,10 @@ 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)
@@ -224,7 +224,7 @@ 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 42330aeb76386..1f7c0655d8db9 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1788,6 +1788,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	struct page *page;
 	struct page *tmp, *dst;
 	struct folio *folio, *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);
@@ -1800,6 +1801,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	result = alloc_charge_folio(&new_folio, mm, cc);
 	if (result != SCAN_SUCCEED)
 		goto out;
+	new_pfn = folio_pfn(new_folio);
 
 	mapping_set_update(&xas, mapping);
 
@@ -2207,7 +2209,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
 	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