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

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Message-ID: <aIzOEJgbmjae7AOS@lappy>
Date: Fri, 1 Aug 2025 10:24:16 -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 Fri, Aug 01, 2025 at 04:13:32PM +0200, David Hildenbrand wrote:
>On 01.08.25 16:06, David Hildenbrand wrote:
>>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?
>
>loongarch doesn't use HIGHMEM, so it probably doesn't matter. Could be 
>considered a cleanup, though.

Yup, it's just a cleanup for loongarch.

It was the only other place besides mm/userfaultfd.c that had that
inversion, so keeping the tree warning clear will make it easier to spot
newly introduced issues in the future.

-- 
Thanks,
Sasha