mm: fix oops when filemap_map_pmd() without prealloc_pte

syzbot reports oops in lockdep's __lock_acquire(), called from
__pte_offset_map_lock() called from filemap_map_pages(); or when I run the
repro, the oops comes in pmd_install(), called from filemap_map_pmd()
called from filemap_map_pages(), just before the __pte_offset_map_lock().

The problem is that filemap_map_pmd() has been assuming that when it finds
pmd_none(), a page table has already been prepared in prealloc_pte; and
indeed do_fault_around() has been careful to preallocate one there, when
it finds pmd_none(): but what if *pmd became none in between?

My 6.6 mods in mm/khugepaged.c, avoiding mmap_lock for write, have made it
easy for *pmd to be cleared while servicing a page fault; but even before
those, a huge *pmd might be zapped while a fault is serviced.

The difference in symptomatic stack traces comes from the "memory model"
in use: pmd_install() uses pmd_populate() uses page_to_pfn(): in some
models that is strict, and will oops on the NULL prealloc_pte; in other
models, it will construct a bogus value to be populated into *pmd, then
__pte_offset_map_lock() oops when trying to access split ptlock pointer
(or some other symptom in normal case of ptlock embedded not pointer).

Link: https://lore.kernel.org/linux-mm/[email protected]/
Link: https://lkml.kernel.org/r/[email protected]
Fixes: f9ce0be71d1f ("mm: Cleanup faultaround and finish_fault() codepaths")
Signed-off-by: Hugh Dickins <[email protected]>
Reported-and-tested-by: [email protected]
Closes: https://lore.kernel.org/linux-mm/[email protected]/
Reviewed-by: David Hildenbrand <[email protected]>
Cc: Jann Horn <[email protected]>,
Cc: José Pekkarinen <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: <[email protected]>    [5.12+]
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 1 insertions, 1 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 32eedf3afd45..f1c8c278310f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3371,7 +3371,7 @@ static bool filemap_map_pmd(struct vm_fault *vmf, struct folio *folio,
 		}
 	}
 
-	if (pmd_none(*vmf->pmd))
+	if (pmd_none(*vmf->pmd) && vmf->prealloc_pte)
 		pmd_install(mm, vmf->pmd, &vmf->prealloc_pte);
 
 	return false;