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

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Message-ID: <593b222e-1a62-475c-9502-76e128d3625d@redhat.com>
Date: Fri, 1 Aug 2025 16:06:14 +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 01.08.25 15:26, Sasha Levin wrote:
> On Thu, Jul 31, 2025 at 02:56:25PM +0200, David Hildenbrand wrote:
>> 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>
> 
> David, I ended up LLM generating a .cocci script to detect this type of
> issues, and it ended up detecting a similar issue in
> arch/loongarch/mm/init.c.

Does loongarch have these kmap_local restrictions?

> 
> Would you be open to reviewing both the .cocci script as well as the
> loongarch fix?

Sure, if it's prechecked by you no problem.

-- 
Cheers,

David / dhildenb