mm: lock newly mapped VMA which can be modified after it becomes visible

mmap_region adds a newly created VMA into VMA tree and might modify it
afterwards before dropping the mmap_lock.  This poses a problem for page
faults handled under per-VMA locks because they don't take the mmap_lock
and can stumble on this VMA while it's still being modified.  Currently
this does not pose a problem since post-addition modifications are done
only for file-backed VMAs, which are not handled under per-VMA lock.
However, once support for handling file-backed page faults with per-VMA
locks is added, this will become a race.

Fix this by write-locking the VMA before inserting it into the VMA tree.
Other places where a new VMA is added into VMA tree do not modify it
after the insertion, so do not need the same locking.

Cc: stable@vger.kernel.org
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -2812,6 +2812,8 @@ cannot_expand:
 	if (vma->vm_file)
 		i_mmap_lock_write(vma->vm_file->f_mapping);
 
+	/* Lock the VMA since it is modified after insertion into VMA tree */
+	vma_start_write(vma);
 	vma_iter_store(&vmi, vma);
 	mm->map_count++;
 	if (vma->vm_file) {