mm/khugepaged: invoke MMU notifiers in shmem/file collapse paths

Any codepath that zaps page table entries must invoke MMU notifiers to
ensure that secondary MMUs (like KVM) don't keep accessing pages which
aren't mapped anymore. Secondary MMUs don't hold their own references to
pages that are mirrored over, so failing to notify them can lead to page
use-after-free.

I'm marking this as addressing an issue introduced in commit f3f0e1d2150b
("khugepaged: add support of collapse for tmpfs/shmem pages"), but most of
the security impact of this only came in commit 27e1f8273113 ("khugepaged:
enable collapse pmd for pte-mapped THP"), which actually omitted flushes
for the removal of present PTEs, not just for the removal of empty page
tables.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: Jann Horn <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Reviewed-by: Yang Shi <[e-mail]>
Cc: John Hubbard <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/khugepaged.c

···
 				  unsigned long addr, pmd_t *pmdp)
 {
 	pmd_t pmd;
+	struct mmu_notifier_range range;
 
 	mmap_assert_write_locked(mm);
 	if (vma->vm_file)
···
 	if (vma->anon_vma)
 		lockdep_assert_held_write(&vma->anon_vma->root->rwsem);
 
+	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, NULL, mm, addr,
+				addr + HPAGE_PMD_SIZE);
+	mmu_notifier_invalidate_range_start(&range);
 	pmd = pmdp_collapse_flush(vma, addr, pmdp);
 	tlb_remove_table_sync_one();
+	mmu_notifier_invalidate_range_end(&range);
 	mm_dec_nr_ptes(mm);
 	page_table_check_pte_clear_range(mm, addr, pmd);
 	pte_free(mm, pmd_pgtable(pmd));