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

[Nadav Amit]

> On Jan 5, 2021, at 7:08 AM, Peter Xu <peterx@xxxxxxxxxx> wrote:
> 
> 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.

Sorry for missing your feedback before. What you suggest makes perfect
sense.