[PATCH] mm: Avoid calling folio_page() with an out-of-bounds index

["Li Zhe" <lizhe.67@bytedance.com>]

In folio_zero_user(), the page pointer is calculated via folio_page()
before checking if the number of pages to be cleared is greater than zero.
Furthermore, folio_page() does not verify that the page number lies
within folio.

When 'addr_hint' is near the end of a large folio, the range 'r[0]'
represents an empty interval. In this scenario, 'nr_pages' will be
calculated as 0 and 'r[0].start' can be an index that is out-of-bounds
for folio_page(). The code unconditionally calls folio_page() on a wrong
index, even though the subsequent clearing logic is correctly skipped.

While this does not cause a functional bug today, calculating a page
pointer for an out-of-bounds index is logically unsound and fragile. It
could pose a risk for future refactoring or trigger warnings from static
analysis tools.

To fix this, move the call to folio_page() inside the 'if (nr_pages > 0)'
block. This ensures that the page pointer is only calculated when it is
actually needed for a valid, non-empty range of pages, thus making the code
more robust and logically correct.

Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
---
 mm/memory.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 07778814b4a8..6f8c55d604b5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -7343,12 +7343,14 @@ void folio_zero_user(struct folio *folio, unsigned long addr_hint)
 	r[0] = DEFINE_RANGE(r[2].end + 1, pg.end);
 
 	for (i = 0; i < ARRAY_SIZE(r); i++) {
-		const unsigned long addr = base_addr + r[i].start * PAGE_SIZE;
 		const long nr_pages = (long)range_len(&r[i]);
-		struct page *page = folio_page(folio, r[i].start);
 
-		if (nr_pages > 0)
+		if (nr_pages > 0) {
+			const unsigned long addr = base_addr + r[i].start * PAGE_SIZE;
+			struct page *page = folio_page(folio, r[i].start);
+
 			clear_contig_highpages(page, addr, nr_pages);
+		}
 	}
 }
 
-- 
2.20.1