Thanks!
On 4/29/24 4:40 PM, 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>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
Tested-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> ---
> 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);
> } else if (is_swap_pte(ptent)) {
> ptent = pte_swp_mkuffd_wp(ptent);
> --
> 2.25.1
>
--
BR,
Muhammad Usama Anjum
