Re: [PATCH] mm/huge_memory: Fix to make vma_adjust_trans_huge() use find_vma() correctly

[David Hildenbrand <david@redhat.com>]

On 21.11.24 14:44, David Hildenbrand wrote:
> On 21.11.24 13:41, Jeongjun Park wrote:
>> vma_adjust_trans_huge() uses find_vma() to get the VMA, but find_vma() uses
>> the returned pointer without any verification, even though it may return NULL.
>> In this case, NULL pointer dereference may occur, so to prevent this,
>> vma_adjust_trans_huge() should be fix to verify the return value of find_vma().
>>
>> Cc: <stable@vger.kernel.org>
>> Fixes: 685405020b9f ("mm/khugepaged: stop using vma linked list")
> 
> If that's an issue, wouldn't it have predated that commit?
> 
> struct vm_area_struct *next = vma->vm_next;
> unsigned long nstart = next->vm_start;
> 
> Would have also assumed that there is a next VMA that can be
> dereferenced, no?
> 

And looking into the details, we only assume that there is a next VMA if 
we are explicitly told to by the caller of vma_adjust_trans_huge() using 
"adjust_next".

There is only one such caller, 
vma_merge_existing_range()->commit_merge() where we set adj_start -> 
"adjust_next" where we seem to have a guarantee that there is a next VMA.

So I don't think there is an issue here (although the code does look 
confusing ...).

Not sure, though, if a

if (WARN_ON_ONCE(!next))
	return;

would be reasonable.

-- 
Cheers,

David / dhildenb