page_folio() calls page_fixed_fake_head() which will misidentify this page as being a fake head and load off the end of 'precise'. We may have a pointer to a fake head, but that's OK because it contains the right information for dump_page().
gcc-15 is smart enough to catch this with -Warray-bounds:
In function 'page_fixed_fake_head', inlined from '_compound_head' at ../include/linux/page-flags.h:251:24, inlined from '__dump_page' at ../mm/debug.c:123:11: ../include/asm-generic/rwonce.h:44:26: warning: array subscript 9 is outside +array bounds of 'struct page[1]' [-Warray-bounds=]
Reported-by: Kees Cook kees@kernel.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Fixes: fae7d834c43c (mm: add __dump_folio()) Cc: stable@vger.kernel.org --- mm/debug.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/mm/debug.c b/mm/debug.c index aa57d3ffd4ed..95b6ab809c0e 100644 --- a/mm/debug.c +++ b/mm/debug.c @@ -124,19 +124,22 @@ static void __dump_page(const struct page *page) { struct folio *foliop, folio; struct page precise; + unsigned long head; unsigned long pfn = page_to_pfn(page); unsigned long idx, nr_pages = 1; int loops = 5;
again: memcpy(&precise, page, sizeof(*page)); - foliop = page_folio(&precise); - if (foliop == (struct folio *)&precise) { + head = precise.compound_head; + if ((head & 1) == 0) { + foliop = (struct folio *)&precise; idx = 0; if (!folio_test_large(foliop)) goto dump; foliop = (struct folio *)page; } else { + foliop = (struct folio *)(head - 1); idx = folio_page_idx(foliop, page); }