On Thu, 16 Jan 2025 10:50:05 +0800 Liu Ye <liuye@kylinos.cn> wrote:

    Add VM_WARN_ON to prevent 'adjust' from accessing NULL pointers
    when 'adjust' is NULL and 'expanded' is false or 'adj_start' is
    not zero.

...

--- a/mm/vma.c
+++ b/mm/vma.c
@@ -641,6 +641,7 @@ static int commit_merge(struct vma_merge_struct *vmg,
 
 	init_multi_vma_prep(&vp, vmg->vma, adjust, remove, remove2);
 
+	VM_WARN_ON(!adjust && (!expanded || adj_start));
 	VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
 		   vp.anon_vma != adjust->anon_vma);
 

This won't prevent a null deref.  It will emit a warning which
duplicates all the information which we're about to emit from the oops
handler.

Are there any reports of an oops from a NULL deref of `adjust'?

This issue is not from any report yet, but by cppcheck tool only.

mm/vma.c:652:29: warning: Possible null pointer dereference: adjust [nullPointer]
  vma_iter_config(vmg->vmi, adjust->vm_start + adj_start,
                                                            ^
mm/vma.c:1072:24: note: Calling function 'commit_merge', 2nd argument 'NULL' value is 0
 if (commit_merge(vmg, NULL, remove_next ? next : NULL, NULL, 0, true))
                                                   ^
mm/vma.c:652:29: note: Null pointer dereference
  vma_iter_config(vmg->vmi, adjust->vm_start + adj_start,
                                                            ^
mm/vma.c:653:5: warning: Possible null pointer dereference: adjust [nullPointer]
    adjust->vm_end);
    ^
mm/vma.c:1072:24: note: Calling function 'commit_merge', 2nd argument 'NULL' value is 0
 if (commit_merge(vmg, NULL, remove_next ? next : NULL, NULL, 0, true))
                                                  ^
mm/vma.c:653:5: note: Null pointer dereference
    adjust->vm_end);
    ^

Before calling commit_merge, the correct relationship between adjust,
adj_start, and expanded must be ensured, such as the functions
vma_merge_existing_range and vma_expand. Therefore, VM_WARN_ON is added
inside the function to detect incorrect relationships. Of course,
commit_merge is not used anywhere else at present, so adding VM_WARN_ON
is just a suggestion.