From nobody Sun Dec 28 06:45:26 2025 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4C2FEC4332F for ; Tue, 12 Dec 2023 09:50:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1346308AbjLLJug (ORCPT ); Tue, 12 Dec 2023 04:50:36 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39392 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235029AbjLLJuX (ORCPT ); Tue, 12 Dec 2023 04:50:23 -0500 X-Greylist: delayed 1039 seconds by postgrey-1.37 at lindbergh.monkeyblade.net; Tue, 12 Dec 2023 01:50:28 PST Received: from szxga04-in.huawei.com (szxga04-in.huawei.com [45.249.212.190]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D8017112; Tue, 12 Dec 2023 01:50:28 -0800 (PST) Received: from mail.maildlp.com (unknown [172.19.162.112]) by szxga04-in.huawei.com (SkyGuard) with ESMTP id 4SqD1T0SBpz1wnf7; Tue, 12 Dec 2023 17:33:01 +0800 (CST) Received: from dggpeml500021.china.huawei.com (unknown [7.185.36.21]) by mail.maildlp.com (Postfix) with ESMTPS id 03A0E1406C8; Tue, 12 Dec 2023 17:33:07 +0800 (CST) Received: from huawei.com (10.175.127.227) by dggpeml500021.china.huawei.com (7.185.36.21) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.35; Tue, 12 Dec 2023 17:33:06 +0800 From: Baokun Li To: , CC: , , , , , , , , , , , , , Subject: [RFC PATCH] mm/filemap: avoid buffered read/write race to read inconsistent data Date: Tue, 12 Dec 2023 17:36:34 +0800 Message-ID: <20231212093634.2464108-1-libaokun1@huawei.com> X-Mailer: git-send-email 2.31.1 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Originating-IP: [10.175.127.227] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To dggpeml500021.china.huawei.com (7.185.36.21) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" The following concurrency may cause the data read to be inconsistent with the data on disk: cpu1 cpu2 ------------------------------|------------------------------ // Buffered write 2048 from 0 ext4_buffered_write_iter generic_perform_write copy_page_from_iter_atomic ext4_da_write_end ext4_da_do_write_end block_write_end __block_commit_write folio_mark_uptodate // Buffered read 4096 from 0 smp_wmb() ext4_file_read_iter set_bit(PG_uptodate, folio_flags) generic_file_read_iter i_size_write // 2048 filemap_read unlock_page(page) filemap_get_pages filemap_get_read_batch folio_test_uptodate(folio) ret =3D test_bit(PG_uptodate, folio_flags) if (ret) smp_rmb(); // Ensure that the data in page 0-2048 is up-to-date. // New buffered write 2048 from 2048 ext4_buffered_write_iter generic_perform_write copy_page_from_iter_atomic ext4_da_write_end ext4_da_do_write_end block_write_end __block_commit_write folio_mark_uptodate smp_wmb() set_bit(PG_uptodate, folio_flags) i_size_write // 4096 unlock_page(page) isize =3D i_size_read(inode) // 4096 // Read the latest isize 4096, but without smp_rmb(), there may be // Load-Load disorder resulting in the data in the 2048-4096 range // in the page is not up-to-date. copy_page_to_iter // copyout 4096 In the concurrency above, we read the updated i_size, but there is no read barrier to ensure that the data in the page is the same as the i_size at this point, so we may copy the unsynchronized page out. Hence adding the missing read memory barrier to fix this. This is a Load-Load reordering issue, which only occurs on some weak mem-ordering architectures (e.g. ARM64, ALPHA), but not on strong mem-ordering architectures (e.g. X86). And theoretically the problem doesn't only happen on ext4, filesystems that call filemap_read() but don't hold inode lock (e.g. btrfs, f2fs, ubifs ...) will have this problem, while filesystems with inode lock (e.g. xfs, nfs) won't have this problem. Cc: stable@kernel.org Signed-off-by: Baokun Li --- mm/filemap.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/mm/filemap.c b/mm/filemap.c index 71f00539ac00..6324e2ac3e74 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2607,6 +2607,9 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_i= ter *iter, goto put_folios; end_offset =3D min_t(loff_t, isize, iocb->ki_pos + iter->count); =20 + /* Ensure that the page cache within isize is updated. */ + smp_rmb(); + /* * Once we start copying data, we don't want to be touching any * cachelines that might be contended: --=20 2.31.1