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

[David Hildenbrand <david@redhat.com>]

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@kernel.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

-- 
Cheers,

David / dhildenb