mm: always lock new vma before inserting into vma tree

While it's not strictly necessary to lock a newly created vma before
adding it into the vma tree (as long as no further changes are performed
to it), it seems like a good policy to lock it and prevent accidental
changes after it becomes visible to the page faults. Lock the vma before
adding it into the vma tree.

[akpm@linux-foundation.org: fix reject fixing in vma_link(), per Jann]
Link: https://lkml.kernel.org/r/20230804152724.3090321-6-surenb@google.comSuggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -401,6 +401,8 @@ static int vma_link(struct mm_struct *mm, struct vm_area_struct *vma)
 	if (vma_iter_prealloc(&vmi, vma))
 		return -ENOMEM;
 
+	vma_start_write(vma);
+
 	vma_iter_store(&vmi, vma);
 
 	if (vma->vm_file) {
@@ -463,7 +465,8 @@ static inline void vma_prepare(struct vma_prepare *vp)
 	vma_start_write(vp->vma);
 	if (vp->adj_next)
 		vma_start_write(vp->adj_next);
-	/* vp->insert is always a newly created VMA, no need for locking */
+	if (vp->insert)
+		vma_start_write(vp->insert);
 	if (vp->remove)
 		vma_start_write(vp->remove);
 	if (vp->remove2)
@@ -3093,6 +3096,7 @@ static int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	vma->vm_pgoff = addr >> PAGE_SHIFT;
 	vm_flags_init(vma, flags);
 	vma->vm_page_prot = vm_get_page_prot(flags);
+	vma_start_write(vma);
 	if (vma_iter_store_gfp(vmi, vma, GFP_KERNEL))
 		goto mas_store_fail;
 
@@ -3341,7 +3345,6 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
 			get_file(new_vma->vm_file);
 		if (new_vma->vm_ops && new_vma->vm_ops->open)
 			new_vma->vm_ops->open(new_vma);
-		vma_start_write(new_vma);
 		if (vma_link(mm, new_vma))
 			goto out_vma_link;
 		*need_rmap_locks = false;