From patchwork Sun Nov 15 10:36:42 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Christian Brauner X-Patchwork-Id: 11906035 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 825B31668 for ; Sun, 15 Nov 2020 10:38:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6A7B523A32 for ; Sun, 15 Nov 2020 10:38:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726939AbgKOKi2 (ORCPT ); Sun, 15 Nov 2020 05:38:28 -0500 Received: from youngberry.canonical.com ([91.189.89.112]:58639 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726647AbgKOKiZ (ORCPT ); Sun, 15 Nov 2020 05:38:25 -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 1keFQK-0000Kt-H5; Sun, 15 Nov 2020 10:38:20 +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 , Jann Horn , Seth Forshee , =?utf-8?q?St=C3=A9phane_Graber?= , Aleksa Sarai , Lennart Poettering , "Eric W. Biederman" , smbarber@chromium.org, Phil Estes , Serge Hallyn , Kees Cook , Todd Kjos , Jonathan Corbet , containers@lists.linux-foundation.org, linux-security-module@vger.kernel.org, linux-api@vger.kernel.org, linux-ext4@vger.kernel.org, linux-audit@redhat.com, linux-integrity@vger.kernel.org, selinux@vger.kernel.org, Christian Brauner , Christoph Hellwig Subject: [PATCH v2 03/39] namespace: only take read lock in do_reconfigure_mnt() Date: Sun, 15 Nov 2020 11:36:42 +0100 Message-Id: <20201115103718.298186-4-christian.brauner@ubuntu.com> X-Mailer: git-send-email 2.29.2 In-Reply-To: <20201115103718.298186-1-christian.brauner@ubuntu.com> References: <20201115103718.298186-1-christian.brauner@ubuntu.com> MIME-Version: 1.0 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. So I think 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]. Afaict, 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") Cc: Christoph Hellwig Cc: David Howells Cc: Al Viro Cc: linux-fsdevel@vger.kernel.org Signed-off-by: Christian Brauner --- /* v2 */ unchanged --- fs/namespace.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/fs/namespace.c b/fs/namespace.c index 798bbf4f48ad..8497d149ecaa 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -2512,10 +2512,6 @@ 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; @@ -2565,13 +2561,17 @@ 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);