From patchwork Wed Mar 4 17:34:46 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ross Zwisler X-Patchwork-Id: 11420533 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 8D99214BC for ; Wed, 4 Mar 2020 17:35:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 665B624673 for ; Wed, 4 Mar 2020 17:35:23 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="WGELEpOD" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730006AbgCDRfW (ORCPT ); Wed, 4 Mar 2020 12:35:22 -0500 Received: from mail-io1-f66.google.com ([209.85.166.66]:45894 "EHLO mail-io1-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727308AbgCDRfW (ORCPT ); Wed, 4 Mar 2020 12:35:22 -0500 Received: by mail-io1-f66.google.com with SMTP id w9so3278693iob.12 for ; Wed, 04 Mar 2020 09:35:20 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=mZTD9KMsKfztS9z4ihucA5Dr5NrKiyOZlSRfhuTSHZ0=; b=WGELEpODqQ2xvAWmt7hf8UU7aiWEjaLDxusURIHdxWcZTSn/wjRxcXpmKmnV6raulL RBUBaoZkOVap9hzzhM77/lwCokBIrGvyblmYg95FCMps5W8l3LD6byUevYLxJfU/rGEJ hBo2gByRPNaW0YUmr3fqqJM2AOXKVaa9/pgYE= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=mZTD9KMsKfztS9z4ihucA5Dr5NrKiyOZlSRfhuTSHZ0=; b=C7rlkL74Z1QFbitp0fy19SmANtRmFmELIo7/ySskCwVd1YUPz01ziSshhaXUAYiXCb ZDK+yHxE/+/60GRwttTi5mY8N+L4NdKtVdF5EBErsF0c/M5yi/5lIrwzY4MMwLDCvCwT BR8R6NdEZcGgGJbBkX4cT4RSZM97B2rSgiaVjbXhsxV/Rnc4OlBqExo3kKiL1nAwv95A Oj3BQ/qAUZKkF+wOFKkKuSFGTYJblEnpHToM5hMc1QS9Zwk0iMY3CZWsEsGqlL7YvTQ0 qT6KpehzXNqb0XqXXOfN45F2hmCmcvownYYXQL/Zu5nPkWoWckf7Tad/qUwC8BgEOGmj Qyxw== X-Gm-Message-State: ANhLgQ1GiOHb9wgWxPOSqYjWKyIA6aMPqflTjRLw7zArA6PbiA6DRcZF NggRM2RZtXDEIMHdy3cmh2rShA== X-Google-Smtp-Source: ADFU+vsvhwY1ciSk27AccTZgx6m46/Y31Yyx/ZPc8zFT1PN27HXTw0DnGPGKT4cgsMyGVOy1irPzZg== X-Received: by 2002:a02:a1c9:: with SMTP id o9mr3798084jah.33.1583343319866; Wed, 04 Mar 2020 09:35:19 -0800 (PST) Received: from localhost ([2620:15c:183:200:855f:8919:84a7:4794]) by smtp.gmail.com with ESMTPSA id q83sm5807706ilb.31.2020.03.04.09.35.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Mar 2020 09:35:19 -0800 (PST) From: Ross Zwisler X-Google-Original-From: Ross Zwisler To: Alexander Viro , Aleksa Sarai , Andrew Morton Cc: Mattias Nissler , David Howells , Matthew Wilcox , Raul Rangel , linux-fsdevel@vger.kernel.org, Benjamin Gordon , Micah Morton , Dmitry Torokhov , Ross Zwisler , Jesse Barnes , linux-kernel@vger.kernel.org Subject: [PATCH RESEND v6] Add a "nosymfollow" mount option. Date: Wed, 4 Mar 2020 10:34:46 -0700 Message-Id: <20200304173446.122990-1-zwisler@google.com> X-Mailer: git-send-email 2.25.1.481.gfbce0eb801-goog MIME-Version: 1.0 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org From: Mattias Nissler For mounts that have the new "nosymfollow" option, don't follow symlinks when resolving paths. The new option is similar in spirit to the existing "nodev", "noexec", and "nosuid" options, as well as to the LOOKUP_NO_SYMLINKS resolve flag in the openat2(2) syscall. Various BSD variants have been supporting the "nosymfollow" mount option for a long time with equivalent implementations. Note that symlinks may still be created on file systems mounted with the "nosymfollow" option present. readlink() remains functional, so user space code that is aware of symlinks can still choose to follow them explicitly. Setting the "nosymfollow" mount option helps prevent privileged writers from modifying files unintentionally in case there is an unexpected link along the accessed path. The "nosymfollow" option is thus useful as a defensive measure for systems that need to deal with untrusted file systems in privileged contexts. More information on the history and motivation for this patch can be found here: https://sites.google.com/a/chromium.org/dev/chromium-os/chromiumos-design-docs/hardening-against-malicious-stateful-data#TOC-Restricting-symlink-traversal Signed-off-by: Mattias Nissler Signed-off-by: Ross Zwisler --- Resending v6 which was previously posted here [0]. Aleksa, if I've addressed all of your feedback, would you mind adding your Reviewed-by? Andrew, would you please consider merging this? This patch adds a security measure which we feel is necessary to have in Chrome OS, and which would be beneficial to have in other Linux setups as well. Lots of details on exactly what we're protecting against exist in the write-up that I linked to in the commit message. Changes since v5 [1]: * Redefined MS_NOSYMFOLLOW to use a lower unused bit value (256) so it doesn't collide with MS_SUBMOUNT. * Updated the mount code in util-linux [2] to use the newly defined flag value. * Updated man pages for mount(8) [2], as well as mount(2) and statfs(2) [3]. [0]: https://patchwork.kernel.org/patch/11405065/ [1]: https://patchwork.kernel.org/patch/11365291/ [2]: https://github.com/rzwisler/util-linux/commit/7f8771acd85edb70d97921c026c55e1e724d4e15 [3]: https://github.com/rzwisler/man-pages/commit/b8fe8079f64b5068940c0144586e580399a71668 --- fs/namei.c | 3 ++- fs/namespace.c | 2 ++ fs/proc_namespace.c | 1 + fs/statfs.c | 2 ++ include/linux/mount.h | 3 ++- include/linux/statfs.h | 1 + include/uapi/linux/mount.h | 1 + 7 files changed, 11 insertions(+), 2 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index db6565c998259..026a774d28c3d 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1122,7 +1122,8 @@ const char *get_link(struct nameidata *nd) int error; const char *res; - if (unlikely(nd->flags & LOOKUP_NO_SYMLINKS)) + if (unlikely(nd->flags & LOOKUP_NO_SYMLINKS) || + unlikely(nd->path.mnt->mnt_flags & MNT_NOSYMFOLLOW)) return ERR_PTR(-ELOOP); if (!(nd->flags & LOOKUP_RCU)) { diff --git a/fs/namespace.c b/fs/namespace.c index 85b5f7bea82e7..9b843b66d39e4 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -3074,6 +3074,8 @@ long do_mount(const char *dev_name, const char __user *dir_name, mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME); if (flags & MS_RDONLY) mnt_flags |= MNT_READONLY; + if (flags & MS_NOSYMFOLLOW) + mnt_flags |= MNT_NOSYMFOLLOW; /* The default atime for remount is preservation */ if ((flags & MS_REMOUNT) && diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c index 273ee82d8aa97..91a552f617406 100644 --- a/fs/proc_namespace.c +++ b/fs/proc_namespace.c @@ -70,6 +70,7 @@ static void show_mnt_opts(struct seq_file *m, struct vfsmount *mnt) { MNT_NOATIME, ",noatime" }, { MNT_NODIRATIME, ",nodiratime" }, { MNT_RELATIME, ",relatime" }, + { MNT_NOSYMFOLLOW, ",nosymfollow" }, { 0, NULL } }; const struct proc_fs_info *fs_infop; diff --git a/fs/statfs.c b/fs/statfs.c index 2616424012ea7..59f33752c1311 100644 --- a/fs/statfs.c +++ b/fs/statfs.c @@ -29,6 +29,8 @@ static int flags_by_mnt(int mnt_flags) flags |= ST_NODIRATIME; if (mnt_flags & MNT_RELATIME) flags |= ST_RELATIME; + if (mnt_flags & MNT_NOSYMFOLLOW) + flags |= ST_NOSYMFOLLOW; return flags; } diff --git a/include/linux/mount.h b/include/linux/mount.h index bf8cc4108b8f9..ff2d132c21f5d 100644 --- a/include/linux/mount.h +++ b/include/linux/mount.h @@ -30,6 +30,7 @@ struct fs_context; #define MNT_NODIRATIME 0x10 #define MNT_RELATIME 0x20 #define MNT_READONLY 0x40 /* does the user want this to be r/o? */ +#define MNT_NOSYMFOLLOW 0x80 #define MNT_SHRINKABLE 0x100 #define MNT_WRITE_HOLD 0x200 @@ -46,7 +47,7 @@ struct fs_context; #define MNT_SHARED_MASK (MNT_UNBINDABLE) #define MNT_USER_SETTABLE_MASK (MNT_NOSUID | MNT_NODEV | MNT_NOEXEC \ | MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME \ - | MNT_READONLY) + | MNT_READONLY | MNT_NOSYMFOLLOW) #define MNT_ATIME_MASK (MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME ) #define MNT_INTERNAL_FLAGS (MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL | \ diff --git a/include/linux/statfs.h b/include/linux/statfs.h index 9bc69edb8f188..fac4356ea1bfc 100644 --- a/include/linux/statfs.h +++ b/include/linux/statfs.h @@ -40,6 +40,7 @@ struct kstatfs { #define ST_NOATIME 0x0400 /* do not update access times */ #define ST_NODIRATIME 0x0800 /* do not update directory access times */ #define ST_RELATIME 0x1000 /* update atime relative to mtime/ctime */ +#define ST_NOSYMFOLLOW 0x2000 /* do not follow symlinks */ struct dentry; extern int vfs_get_fsid(struct dentry *dentry, __kernel_fsid_t *fsid); diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h index 96a0240f23fed..dd8306ea336c1 100644 --- a/include/uapi/linux/mount.h +++ b/include/uapi/linux/mount.h @@ -16,6 +16,7 @@ #define MS_REMOUNT 32 /* Alter flags of a mounted FS */ #define MS_MANDLOCK 64 /* Allow mandatory locks on an FS */ #define MS_DIRSYNC 128 /* Directory modifications are synchronous */ +#define MS_NOSYMFOLLOW 256 /* Do not follow symlinks */ #define MS_NOATIME 1024 /* Do not update access times. */ #define MS_NODIRATIME 2048 /* Do not update directory access times */ #define MS_BIND 4096