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

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Message-ID: <aIt4u9gzwGOtjXPi@lappy>
Date: Thu, 31 Jul 2025 10:07:55 -0400
From: Sasha Levin <sashal@...nel.org>
To: David Hildenbrand <david@...hat.com>
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 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>

Thanks for the review!

I'll send this patch out properly.

-- 
Thanks,
Sasha