Re: [PATCH] mm/huge_memory: fix dereferencing invalid pmd migration entry

[Zi Yan]

On 14 Apr 2025, at 3:27, Gavin Guo wrote:

> When migrating a THP, concurrent access to the PMD migration entry
> during a deferred split scan can lead to a page fault, as illustrated

It is an access violation, right? Because pmd_folio(*pmd_migration_entry)
does not return a folio address. Page fault made this sounded like not
a big issue.

> below. To prevent this page fault, it is necessary to check the PMD
> migration entry and return early. In this context, there is no need to
> use pmd_to_swp_entry and pfn_swap_entry_to_page to verify the equality
> of the target folio. Since the PMD migration entry is locked, it cannot
> be served as the target.

You mean split_huge_pmd_address() locks the PMD page table, so that
page migration cannot proceed, or the THP is locked by migration,
so that it cannot be split? The sentence is a little confusing to me.

>
> BUG: unable to handle page fault for address: ffffea60001db008
> CPU: 0 UID: 0 PID: 2199114 Comm: tee Not tainted 6.14.0+ #4 NONE
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> RIP: 0010:split_huge_pmd_locked+0x3b5/0x2b60
> Call Trace:
> <TASK>
> try_to_migrate_one+0x28c/0x3730
> rmap_walk_anon+0x4f6/0x770
> unmap_folio+0x196/0x1f0
> split_huge_page_to_list_to_order+0x9f6/0x1560
> deferred_split_scan+0xac5/0x12a0
> shrinker_debugfs_scan_write+0x376/0x470
> full_proxy_write+0x15c/0x220
> vfs_write+0x2fc/0xcb0
> ksys_write+0x146/0x250
> do_syscall_64+0x6a/0x120
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> The bug is found by syzkaller on an internal kernel, then confirmed on
> upstream.
>
> Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Gavin Guo <gavinguo@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 2a47682d1ab7..0cb9547dcff2 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3075,6 +3075,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>  void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
>  			   pmd_t *pmd, bool freeze, struct folio *folio)
>  {
> +	bool pmd_migration = is_pmd_migration_entry(*pmd);
> +
>  	VM_WARN_ON_ONCE(folio && !folio_test_pmd_mappable(folio));
>  	VM_WARN_ON_ONCE(!IS_ALIGNED(address, HPAGE_PMD_SIZE));
>  	VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));
> @@ -3085,10 +3087,18 @@ void split_huge_pmd_locked(struct vm_area_struct *vma, unsigned long address,
>  	 * require a folio to check the PMD against. Otherwise, there
>  	 * is a risk of replacing the wrong folio.
>  	 */
> -	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
> -	    is_pmd_migration_entry(*pmd)) {
> -		if (folio && folio != pmd_folio(*pmd))
> -			return;
> +	if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) || pmd_migration) {
> +		if (folio) {
> +			/*
> +			 * Do not apply pmd_folio() to a migration entry; and
> +			 * folio lock guarantees that it must be of the wrong
> +			 * folio anyway.

Why does the folio lock imply it is a wrong folio?

> +			 */
> +			if (pmd_migration)
> +				return;
> +			if (folio != pmd_folio(*pmd))
> +				return;
> +		}

Why not just

if (folio && pmd_migration)
	return;

if (pmd_trans_huge() …) {
	…
}
?

Thanks.

Best Regards,
Yan, Zi