linux-kernel - Re: [PATCH] mm/userfaultfd: fix missing PTE unmap for non-migration entries

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Message-ID: <214e78a0-7774-4b1e-8d85-9a66d2384744@redhat.com>
Date: Thu, 31 Jul 2025 14:56:25 +0200
From: David Hildenbrand <david@...hat.com>
To: Sasha Levin <sashal@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, peterx@...hat.com,
 aarcange@...hat.com, surenb@...gle.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] mm/userfaultfd: fix missing PTE unmap for non-migration
 entries

On 31.07.25 14:37, Sasha Levin wrote:
> On Tue, Jul 08, 2025 at 05:42:16PM +0200, David Hildenbrand wrote:
>> On 08.07.25 17:33, Sasha Levin wrote:
>>> On Tue, Jul 08, 2025 at 05:10:44PM +0200, David Hildenbrand wrote:
>>>> On 01.07.25 02:57, Andrew Morton wrote:
>>>>> On Sun, 29 Jun 2025 23:19:58 -0400 Sasha Levin <sashal@...nel.org> wrote:
>>>>>
>>>>>> When handling non-swap entries in move_pages_pte(), the error handling
>>>>>> for entries that are NOT migration entries fails to unmap the page table
>>>>>> entries before jumping to the error handling label.
>>>>>>
>>>>>> This results in a kmap/kunmap imbalance which on CONFIG_HIGHPTE systems
>>>>>> triggers a WARNING in kunmap_local_indexed() because the kmap stack is
>>>>>> corrupted.
>>>>>>
>>>>>> Example call trace on ARM32 (CONFIG_HIGHPTE enabled):
>>>>>>    WARNING: CPU: 1 PID: 633 at mm/highmem.c:622 kunmap_local_indexed+0x178/0x17c
>>>>>>    Call trace:
>>>>>>      kunmap_local_indexed from move_pages+0x964/0x19f4
>>>>>>      move_pages from userfaultfd_ioctl+0x129c/0x2144
>>>>>>      userfaultfd_ioctl from sys_ioctl+0x558/0xd24
>>>>>>
>>>>>> The issue was introduced with the UFFDIO_MOVE feature but became more
>>>>>> frequent with the addition of guard pages (commit 7c53dfbdb024 ("mm: add
>>>>>> PTE_MARKER_GUARD PTE marker")) which made the non-migration entry code
>>>>>> path more commonly executed during userfaultfd operations.
>>>>>>
>>>>>> Fix this by ensuring PTEs are properly unmapped in all non-swap entry
>>>>>> paths before jumping to the error handling label, not just for migration
>>>>>> entries.
>>>>>
>>>>> I don't get it.
>>>>>
>>>>>> --- a/mm/userfaultfd.c
>>>>>> +++ b/mm/userfaultfd.c
>>>>>> @@ -1384,14 +1384,15 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
>>>>>>   		entry = pte_to_swp_entry(orig_src_pte);
>>>>>>   		if (non_swap_entry(entry)) {
>>>>>> +			pte_unmap(src_pte);
>>>>>> +			pte_unmap(dst_pte);
>>>>>> +			src_pte = dst_pte = NULL;
>>>>>>   			if (is_migration_entry(entry)) {
>>>>>> -				pte_unmap(src_pte);
>>>>>> -				pte_unmap(dst_pte);
>>>>>> -				src_pte = dst_pte = NULL;
>>>>>>   				migration_entry_wait(mm, src_pmd, src_addr);
>>>>>>   				err = -EAGAIN;
>>>>>> -			} else
>>>>>> +			} else {
>>>>>>   				err = -EFAULT;
>>>>>> +			}
>>>>>>   			goto out;
>>>>>
>>>>> where we have
>>>>>
>>>>> out:
>>>>> 	...
>>>>> 	if (dst_pte)
>>>>> 		pte_unmap(dst_pte);
>>>>> 	if (src_pte)
>>>>> 		pte_unmap(src_pte);
>>>>
>>>> AI slop?
>>>
>>> Nah, this one is sadly all me :(
>>
>> Haha, sorry :P
> 
> So as I was getting nowhere with this, I asked AI to help me :)
> 
> If you're not interested in reading LLM generated code, feel free to
> stop reading now...
> 
> After it went over the logs, and a few prompts to point it the right
> way, it ended up generating a patch (below) that made sense, and fixed
> the warning that LKFT was being able to trigger.
> 
> If anyone who's more familiar with the code than me (and the AI) agrees
> with the patch and ways to throw their Reviewed-by, I'll send out the
> patch.

Seems to check out for me. In particular, out pte_unmap() everywhere 
else in that function (and mremap.c:move_ptes) are ordered properly.

Even if it would not fix the issue, it would be a cleanup :)

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb