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Tue, 05 Jan 2021 07:08:15 -0800 (PST) Date: Tue, 5 Jan 2021 10:08:13 -0500 From: Peter Xu To: Nadav Amit Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Nadav Amit , Andrea Arcangeli , Yu Zhao , Andy Lutomirski , Pavel Emelyanov , Mike Kravetz , Mike Rapoport , Minchan Kim , Will Deacon , Peter Zijlstra Subject: Re: [RFC PATCH v2 1/2] mm/userfaultfd: fix memory corruption due to writeprotect Message-ID: <20210105150813.GB149908@xz-x1> References: <20201225092529.3228466-1-namit@vmware.com> <20201225092529.3228466-2-namit@vmware.com> MIME-Version: 1.0 In-Reply-To: <20201225092529.3228466-2-namit@vmware.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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