[PATCH 1/2] mm/huge_memory: Fix use of NULL folio in move_pages_huge_pmd()

[Chris Down <chris@chrisdown.name>]

move_pages_huge_pmd() handles UFFDIO_MOVE for both normal THPs and huge
zero pages. For the huge zero page path, src_folio is explicitly set to
NULL (used as a sentinel to skip folio operations like lock and rmap).

In the huge zero page branch, src_folio is NULL, so folio_mk_pmd(NULL,
pgprot) passes NULL through folio_pfn() and page_to_pfn(). With
SPARSEMEM_VMEMMAP this silently produces a bogus PFN, installing a PMD
pointing to non-existent physical memory. On other memory models it
is a NULL dereference.

Use page_folio(src_page) to obtain the valid huge zero folio from the
page, which was obtained from pmd_page() and remains valid throughout.

Fixes: e3981db444a0 ("mm: add folio_mk_pmd()")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Down <chris@chrisdown.name>
---
 mm/huge_memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 44ff8a648afd..fed57951a7cd 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2794,7 +2794,7 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
 		_dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
 	} else {
 		src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
-		_dst_pmd = folio_mk_pmd(src_folio, dst_vma->vm_page_prot);
+		_dst_pmd = folio_mk_pmd(page_folio(src_page), dst_vma->vm_page_prot);
 	}
 	set_pmd_at(mm, dst_addr, dst_pmd, _dst_pmd);
 
-- 
2.51.2