From patchwork Thu Jan 21 13:19:50 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Christian Brauner X-Patchwork-Id: 12036559 X-Patchwork-Delegate: paul@paul-moore.com Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E86C1C433DB for ; Thu, 21 Jan 2021 15:03:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B4F18239EF for ; Thu, 21 Jan 2021 15:03:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728643AbhAUN3n (ORCPT ); Thu, 21 Jan 2021 08:29:43 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:54822 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731366AbhAUN2r (ORCPT ); Thu, 21 Jan 2021 08:28:47 -0500 Received: from ip5f5af0a0.dynamic.kabel-deutschland.de ([95.90.240.160] helo=wittgenstein.fritz.box) by youngberry.canonical.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.86_2) (envelope-from ) id 1l2Zux-0005g7-UT; Thu, 21 Jan 2021 13:22:32 +0000 From: Christian Brauner To: Alexander Viro , Christoph Hellwig , linux-fsdevel@vger.kernel.org Cc: John Johansen , James Morris , Mimi Zohar , Dmitry Kasatkin , Stephen Smalley , Casey Schaufler , Arnd Bergmann , Andreas Dilger , OGAWA Hirofumi , Geoffrey Thomas , Mrunal Patel , Josh Triplett , Andy Lutomirski , Theodore Tso , Alban Crequy , Tycho Andersen , David Howells , James Bottomley , Seth Forshee , =?utf-8?q?St=C3=A9phane_Graber?= , Linus Torvalds , Aleksa Sarai , Lennart Poettering , "Eric W. Biederman" , smbarber@chromium.org, Phil Estes , Serge Hallyn , Kees Cook , Todd Kjos , Paul Moore , Jonathan Corbet , containers@lists.linux-foundation.org, linux-security-module@vger.kernel.org, linux-api@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, linux-integrity@vger.kernel.org, selinux@vger.kernel.org, Christian Brauner Subject: [PATCH v6 31/40] namespace: only take read lock in do_reconfigure_mnt() Date: Thu, 21 Jan 2021 14:19:50 +0100 Message-Id: <20210121131959.646623-32-christian.brauner@ubuntu.com> X-Mailer: git-send-email 2.30.0 In-Reply-To: <20210121131959.646623-1-christian.brauner@ubuntu.com> References: <20210121131959.646623-1-christian.brauner@ubuntu.com> MIME-Version: 1.0 X-Patch-Hashes: v=1; h=sha256; i=I7aJzwt0DpkJRJifO843fvdrR3C7hhJVdcz6GAHs5To=; m=kYGZs6uxDqNnDWq6amn5PljPB3N7RA7sHRkNgwMp1lA=; p=O0TWEIwrbg8QrF14GljNCoXe8MA31Ulu2rf6pUWyHAo=; g=f6e4b4a5508c730a9bc5fc9fed06d285573d486d X-Patch-Sig: m=pgp; i=christian.brauner@ubuntu.com; s=0x0x91C61BC06578DCA2; b=iHUEABYKAB0WIQRAhzRXHqcMeLMyaSiRxhvAZXjcogUCYAl9pgAKCRCRxhvAZXjcoheiAQCNq6g R7W31jEDQ1qTDxON63E9c5BpR/hnvmNsehDL4bgEA04lytDQ5sUwwJM8ZlP0XfLmigSZ09W7wckQz f0PzmgI= Precedence: bulk List-ID: X-Mailing-List: selinux@vger.kernel.org do_reconfigure_mnt() used to take the down_write(&sb->s_umount) lock which seems unnecessary since we're not changing the superblock. We're only checking whether it is already read-only. Setting other mount attributes is protected by lock_mount_hash() afaict and not by s_umount. The history of down_write(&sb->s_umount) lock being taken when setting mount attributes dates back to the introduction of MNT_READONLY in [2]. This introduced the concept of having read-only mounts in contrast to just having a read-only superblock. When it got introduced it was simply plumbed into do_remount() which already took down_write(&sb->s_umount) because it was only used to actually change the superblock before [2]. Afaict, it would've already been possible back then to only use down_read(&sb->s_umount) for MS_BIND | MS_REMOUNT since actual mount options were protected by the vfsmount lock already. But that would've meant special casing the locking for MS_BIND | MS_REMOUNT in do_remount() which people might not have considered worth it. Then in [1] MS_BIND | MS_REMOUNT mount option changes were split out of do_remount() into do_reconfigure_mnt() but the down_write(&sb->s_umount) lock was simply copied over. Now that we have this be a separate helper only take the down_read(&sb->s_umount) lock since we're only interested in checking whether the super block is currently read-only and blocking any writers from changing it. Essentially, checking that the super block is read-only has the advantage that we can avoid having to go into the slowpath and through MNT_WRITE_HOLD and can simply set the read-only flag on the mount in set_mount_attributes(). [1]: commit 43f5e655eff7 ("vfs: Separate changing mount flags full remount") [2]: commit 2e4b7fcd9260 ("[PATCH] r/o bind mounts: honor mount writer counts at remount") Link: https://lore.kernel.org/r/20210112220124.837960-4-christian.brauner@ubuntu.com Cc: David Howells Cc: Al Viro Cc: linux-fsdevel@vger.kernel.org Reviewed-by: Christoph Hellwig Signed-off-by: Christian Brauner --- /* v2 */ unchanged /* v3 */ unchanged /* v4 */ unchanged /* v5 */ unchanged base-commit: 7c53f6b671f4aba70ff15e1b05148b10d58c2837 /* v6 */ unchanged base-commit: 19c329f6808995b142b3966301f217c831e7cf31 --- fs/namespace.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index 5fceb2854395..367f1c7cb6db 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -2527,10 +2527,6 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags) return 0; } -/* - * Update the user-settable attributes on a mount. The caller must hold - * sb->s_umount for writing. - */ static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags) { mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK; @@ -2580,13 +2576,17 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags) if (!can_change_locked_flags(mnt, mnt_flags)) return -EPERM; - down_write(&sb->s_umount); + /* + * We're only checking whether the superblock is read-only not + * changing it, so only take down_read(&sb->s_umount). + */ + down_read(&sb->s_umount); lock_mount_hash(); ret = change_mount_ro_state(mnt, mnt_flags); if (ret == 0) set_mount_attributes(mnt, mnt_flags); unlock_mount_hash(); - up_write(&sb->s_umount); + up_read(&sb->s_umount); mnt_warn_timestamp_expiry(path, &mnt->mnt);