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[70.31.27.79]) by smtp.gmail.com with ESMTPSA id cb5-20020a05622a1f8500b0039cc0fbdb61sm4870380qtb.53.2022.11.13.16.04.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 13 Nov 2022 16:04:50 -0800 (PST) From: Peter Xu To: linux-kernel@vger.kernel.org, linux-mm@kvack.org Cc: Mike Rapoport , Nadav Amit , Andrew Morton , peterx@redhat.com, Andrea Arcangeli , Ives van Hoorne , Axel Rasmussen , Alistair Popple , stable@vger.kernel.org Subject: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte Date: Sun, 13 Nov 2022 19:04:46 -0500 Message-Id: <20221114000447.1681003-2-peterx@redhat.com> X-Mailer: git-send-email 2.37.3 In-Reply-To: <20221114000447.1681003-1-peterx@redhat.com> References: <20221114000447.1681003-1-peterx@redhat.com> MIME-Version: 1.0 Content-type: text/plain X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1749427958402640158?= X-GMAIL-MSGID: =?utf-8?q?1749427958402640158?= Ives van Hoorne from codesandbox.io reported an issue regarding possible data loss of uffd-wp when applied to memfds on heavily loaded systems. The symptom is some read page got data mismatch from the snapshot child VMs. Here I can also reproduce with a Rust reproducer that was provided by Ives that keeps taking snapshot of a 256MB VM, on a 32G system when I initiate 80 instances I can trigger the issues in ten minutes. It turns out that we got some pages write-through even if uffd-wp is applied to the pte. The problem is, when removing migration entries, we didn't really worry about write bit as long as we know it's not a write migration entry. That may not be true, for some memory types (e.g. writable shmem) mk_pte can return a pte with write bit set, then to recover the migration entry to its original state we need to explicit wr-protect the pte or it'll has the write bit set if it's a read migration entry. For uffd it can cause write-through. The relevant code on uffd was introduced in the anon support, which is commit f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration", 2020-04-07). However anon shouldn't suffer from this problem because anon should already have the write bit cleared always, so that may not be a proper Fixes target, while I'm adding the Fixes to be uffd shmem support. Cc: Andrea Arcangeli Cc: stable@vger.kernel.org Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs") Reported-by: Ives van Hoorne Reviewed-by: Alistair Popple Tested-by: Ives van Hoorne Signed-off-by: Peter Xu --- mm/migrate.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/mm/migrate.c b/mm/migrate.c index dff333593a8a..8b6351c08c78 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -213,8 +213,14 @@ static bool remove_migration_pte(struct folio *folio, pte = pte_mkdirty(pte); if (is_writable_migration_entry(entry)) pte = maybe_mkwrite(pte, vma); - else if (pte_swp_uffd_wp(*pvmw.pte)) + else + /* NOTE: mk_pte can have write bit set */ + pte = pte_wrprotect(pte); + + if (pte_swp_uffd_wp(*pvmw.pte)) { + WARN_ON_ONCE(pte_write(pte)); pte = pte_mkuffd_wp(pte); + } if (folio_test_anon(folio) && !is_readable_migration_entry(entry)) rmap_flags |= RMAP_EXCLUSIVE;