Re: [RFC PATCH v2 1/2] mm/userfaultfd: fix memory corruption due to writeprotect

[Peter Xu <peterx@redhat.com>]

On Fri, Dec 25, 2020 at 01:25:28AM -0800, Nadav Amit wrote:
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index ab709023e9aa..c08c4055b051 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -75,7 +75,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>  		oldpte = *pte;
>  		if (pte_present(oldpte)) {
>  			pte_t ptent;
> -			bool preserve_write = prot_numa && pte_write(oldpte);
> +			bool preserve_write = (prot_numa || uffd_wp_resolve) &&
> +					      pte_write(oldpte);

Irrelevant of the other tlb issue, this is a standalone one and I commented in
v1 about simply ignore the change if necessary; unluckily that seems to be
ignored..  so I'll try again - would below be slightly better?

    if (uffd_wp_resolve && !pte_uffd_wp(oldpte))
        continue;

Firstly, current patch is confusing at least to me, because "uffd_wp_resolve"
means "unprotect the pte", whose write bit should mostly be cleared already
when uffd_wp_resolve is applicable.  Then "preserve_write" for that pte looks
odd already.

Meanwhile, if that really happens (when pte write bit set, but during a
uffd_wp_resolve request) imho there is really nothing we can do, so we should
simply avoid touching that at all, and also avoid ptep_modify_prot_start,
pte_modify, ptep_modify_prot_commit, calls etc., which takes extra cost.

Thanks,

-- 
Peter Xu