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

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Message-ID: <a4d8b292-154a-4d14-90e4-6c822acf1cfb@redhat.com>
Date: Tue, 8 Jul 2025 17:10:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>, Sasha Levin <sashal@...nel.org>
Cc: 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 01.07.25 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?

-- 
Cheers,

David / dhildenb