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

[Chris Down]

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, and is 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.

After commit d82d09e48219 ("mm/huge_memory: mark PMD mappings of the
huge zero folio special"), moved huge zero PMDs must remain special so
vm_normal_page_pmd() continues to treat them as special mappings.

move_pages_huge_pmd() currently reconstructs the destination PMD in the
huge zero page branch, which drops PMD state such as pmd_special() on
architectures with CONFIG_ARCH_HAS_PTE_SPECIAL. As a result,
vm_normal_page_pmd() can treat the moved huge zero PMD as a normal page
and corrupt its refcount.

Instead of reconstructing the PMD from the folio, derive the destination
entry from src_pmdval after pmdp_huge_clear_flush(), then handle the PMD
metadata the same way move_huge_pmd() does for moved entries by marking
it soft-dirty and clearing uffd-wp.

Fixes: e3981db444a0 ("mm: add folio_mk_pmd()")
Cc: stable@xxxxxxxxxxxxxxx
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Tested-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
---
 mm/huge_memory.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8e2746ea74ad..c5d3e7bf6414 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2797,7 +2797,8 @@ 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 = move_soft_dirty_pmd(src_pmdval);
+		_dst_pmd = clear_uffd_wp_pmd(_dst_pmd);
 	}
 	set_pmd_at(mm, dst_addr, dst_pmd, _dst_pmd);

--
2.53.0