Re: [PATCH v1] fs/proc/task_mmu: Fix loss of young/dirty bits during pagemap scan

[David Hildenbrand]

On 29.04.24 13:40, Ryan Roberts wrote:
> make_uffd_wp_pte() was previously doing:
>
>    pte = ptep_get(ptep);
>    ptep_modify_prot_start(ptep);
>    pte = pte_mkuffd_wp(pte);
>    ptep_modify_prot_commit(ptep, pte);
>
> But if another thread accessed or dirtied the pte between the first 2
> calls, this could lead to loss of that information. Since
> ptep_modify_prot_start() gets and clears atomically, the following is
> the correct pattern and prevents any possible race. Any access after the
> first call would see an invalid pte and cause a fault:
>
>    pte = ptep_modify_prot_start(ptep);
>    pte = pte_mkuffd_wp(pte);
>    ptep_modify_prot_commit(ptep, pte);
>
> Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
>   fs/proc/task_mmu.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 23fbab954c20..af4bc1da0c01 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1825,7 +1825,7 @@ static void make_uffd_wp_pte(struct vm_area_struct *vma,
>   		pte_t old_pte;
>
>   		old_pte = ptep_modify_prot_start(vma, addr, pte);
> -		ptent = pte_mkuffd_wp(ptent);
> +		ptent = pte_mkuffd_wp(old_pte);
>   		ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb