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[2003:cb:c71c:3900:7211:d436:8d8b:531c]) by smtp.gmail.com with ESMTPSA id k5-20020a05600c1c8500b003d237d60318sm2096196wms.2.2022.12.16.01.04.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 Dec 2022 01:04:28 -0800 (PST) Message-ID: <618b69be-0e99-e35f-04b3-9c63d78ece50@redhat.com> Date: Fri, 16 Dec 2022 10:04:27 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.5.1 Content-Language: en-US To: Peter Xu , linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: Andrea Arcangeli , Pengfei Xu , Nadav Amit , Andrew Morton , Miaohe Lin , Huang Ying , stable@vger.kernel.org References: <20221214200453.1772655-1-peterx@redhat.com> <20221214200453.1772655-2-peterx@redhat.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH 1/2] mm/uffd: Fix pte marker when fork() without fork event In-Reply-To: <20221214200453.1772655-2-peterx@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14.12.22 21:04, Peter Xu wrote: > When fork(), dst_vma is not guaranteed to have VM_UFFD_WP even if src may > have it and has pte marker installed. The warning is improper along with > the comment. The right thing is to inherit the pte marker when needed, or > keep the dst pte empty. > > A vague guess is this happened by an accident when there's the prior patch > to introduce src/dst vma into this helper during the uffd-wp feature got > developed and I probably messed up in the rebase, since if we replace > dst_vma with src_vma the warning & comment it all makes sense too. > > Hugetlb did exactly the right here (copy_hugetlb_page_range()). Fix the > general path. > > Reproducer: > > https://github.com/xupengfe/syzkaller_logs/blob/main/221208_115556_copy_page_range/repro.c > > Cc: # 5.19+ > Fixes: c56d1b62cce8 ("mm/shmem: handle uffd-wp during fork()") > Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=216808 > Reported-by: Pengfei Xu > Signed-off-by: Peter Xu > --- > mm/memory.c | 8 ++------ > 1 file changed, 2 insertions(+), 6 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index aad226daf41b..032ef700c3e8 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -828,12 +828,8 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, > return -EBUSY; > return -ENOENT; > } else if (is_pte_marker_entry(entry)) { > - /* > - * We're copying the pgtable should only because dst_vma has > - * uffd-wp enabled, do sanity check. > - */ > - WARN_ON_ONCE(!userfaultfd_wp(dst_vma)); > - set_pte_at(dst_mm, addr, dst_pte, pte); > + if (userfaultfd_wp(dst_vma)) > + set_pte_at(dst_mm, addr, dst_pte, pte); > return 0; > } > if (!userfaultfd_wp(dst_vma)) Staring at the code first made me go "what about other PTE markers". I then looked into the discussion in patch #2. The fix as is is suboptimal, because it 1) Removes the warning which is good, but 2) Silently drops swapin errors now So it silently breaks something else temporarily ... I remember, that theoretically we could have multiple markers stored in a single PTE marker. Wouldn't it be cleaner to be able to "clean" specific markers from a PTE marker without having to special case on each and everyone? I mean, only uffd-wp is really special such that it might disappear for the target. Something like (pseudocode): if (!userfaultfd_wp(dst_vma)) pte_marker_clear_uff_wp(entry); if (!pte_marker_empty(entry)) { pte = make_pte_marker(pte_marker_get(entry)); set_pte_at(dst_mm, addr, dst_pte, pte); } Then this fix would be correct and backport-able even without #2. And it would work for new types of markers :) I'd prefer a fix that doesn't break something else temporarily, even if the stable backport might require 5 additional minutes to do. So squashing #2 into #1 would also work. -- Thanks, David / dhildenb