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

[Dev Jain <dev.jain@arm.com>]

On 30/06/25 8:49 am, Sasha Levin 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.
>
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Cc: stable@vger.kernel.org
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>   mm/userfaultfd.c | 9 +++++----
>   1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 8253978ee0fb1..7c298e9cbc18f 100644
> --- 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;

Won't the out label take care of the unmapping? I think CONFIG_HIGHPTE
is involved in the explanation.

>   		}
>