linux-kernel - Re: [PATCH] mm/huge_memory: Fix to make vma_adjust_trans_huge() use find

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Message-ID: <25ead85f-2716-4362-8fb5-3422699e308c@redhat.com>
Date: Thu, 21 Nov 2024 14:50:57 +0100
From: David Hildenbrand <david@...hat.com>
To: Jeongjun Park <aha310510@...il.com>, akpm@...ux-foundation.org
Cc: dave@...olabs.net, willy@...radead.org, Liam.Howlett@...cle.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Subject: Re: [PATCH] mm/huge_memory: Fix to make vma_adjust_trans_huge() use
 find_vma() correctly

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@...r.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