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

[Andrew Morton <akpm@linux-foundation.org>]

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);