Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933894AbcKQRZO (ORCPT ); Thu, 17 Nov 2016 12:25:14 -0500 Received: from mail-vk0-f48.google.com ([209.85.213.48]:36390 "EHLO mail-vk0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S938892AbcKQRZD (ORCPT ); Thu, 17 Nov 2016 12:25:03 -0500 MIME-Version: 1.0 In-Reply-To: <1994a9fa-4dd5-1042-7d5d-e9e8c6680bad@gmail.com> References: <1476880312-64786-1-git-send-email-mnissler@chromium.org> <1994a9fa-4dd5-1042-7d5d-e9e8c6680bad@gmail.com> From: Mattias Nissler Date: Thu, 17 Nov 2016 14:11:32 +0100 X-Google-Sender-Auth: P-jO8QWg4fA1qH6BQ1lfXPAoUKY Message-ID: Subject: Re: [PATCH v2] Add a "nosymlinks" mount option. To: "Austin S. Hemmelgarn" Cc: linux-fsdevel@vger.kernel.org, Alexander Viro , linux-kernel@vger.kernel.org, Colin Walters Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7846 Lines: 189 On Thu, Nov 17, 2016 at 1:43 PM, Austin S. Hemmelgarn wrote: > On 2016-11-16 16:18, Mattias Nissler wrote: >> >> I understand that silence suggests there's little interest, but here's >> some new information I discovered today that may justify to reconsider >> the patch: >> >> The BSDs already have exactly what I propose, the mount option is >> called "nosymfollow" there: >> >> https://github.com/freebsd/freebsd/blob/a41f4cc9a57cd74604ae7b051eec2f48865f18d6/sys/kern/vfs_lookup.c#L939 >> >> There's also some evidence on the net that people have been using said >> nosymfollow mount option to mitigate symlink attacks. > > I will comment that I do have interest in this (to the point that if this > doesn't go into mainline, I'll probably add it to my local set of patches), > though I'm not a maintainer or reviewer for the relevant area, so there's > not much I can do except say that there is interest from at least one person > who didn't work on the patch. > > That said, it would be nice if we matched the BSD option name (we do on > pretty much every other core VFS level mount option). Agreed. I'm happy to send an updated patch with adjusted naming. > >> >> On Mon, Oct 24, 2016 at 7:09 AM, Mattias Nissler >> wrote: >>> >>> Friendly ping - does this version of the patch have any chance on >>> getting included in mainline? >>> >>> On Wed, Oct 19, 2016 at 2:31 PM, Mattias Nissler >>> wrote: >>>> >>>> For mounts that have the new "nosymlinks" option, don't follow >>>> symlinks when resolving paths. The new option is similar in spirit to >>>> the existing "nodev", "noexec", and "nosuid" options. >>>> >>>> Note that symlinks may still be created on mounts where the >>>> "nosymlinks" option is present. readlink() remains functional, so user >>>> space code that is aware of symlinks can still choose to follow them >>>> explicitly. >>>> >>>> Setting the "nosymlinks" mount option helps prevent privileged writers >>>> from modifying files unintentionally in case there is an unexpected >>>> link along the accessed path. The "nosymlinks" option is thus useful >>>> as a defensive measure for systems that need to deal with untrusted >>>> file systems in privileged contexts. >>>> >>>> Signed-off-by: Mattias Nissler >>>> --- >>>> fs/namei.c | 3 +++ >>>> fs/namespace.c | 9 ++++++--- >>>> fs/proc_namespace.c | 1 + >>>> fs/statfs.c | 2 ++ >>>> include/linux/mount.h | 3 ++- >>>> include/linux/statfs.h | 1 + >>>> include/uapi/linux/fs.h | 1 + >>>> 7 files changed, 16 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/fs/namei.c b/fs/namei.c >>>> index 5b4eed2..4cddcf3 100644 >>>> --- a/fs/namei.c >>>> +++ b/fs/namei.c >>>> @@ -1021,6 +1021,9 @@ const char *get_link(struct nameidata *nd) >>>> touch_atime(&last->link); >>>> } >>>> >>>> + if (nd->path.mnt->mnt_flags & MNT_NOSYMLINKS) >>>> + return ERR_PTR(-EACCES); >>>> + >>>> error = security_inode_follow_link(dentry, inode, >>>> nd->flags & LOOKUP_RCU); >>>> if (unlikely(error)) >>>> diff --git a/fs/namespace.c b/fs/namespace.c >>>> index e6c234b..deec84e 100644 >>>> --- a/fs/namespace.c >>>> +++ b/fs/namespace.c >>>> @@ -2732,6 +2732,8 @@ 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_NOSYMLINKS) >>>> + mnt_flags |= MNT_NOSYMLINKS; >>>> >>>> /* The default atime for remount is preservation */ >>>> if ((flags & MS_REMOUNT) && >>>> @@ -2741,9 +2743,10 @@ long do_mount(const char *dev_name, const char >>>> __user *dir_name, >>>> mnt_flags |= path.mnt->mnt_flags & MNT_ATIME_MASK; >>>> } >>>> >>>> - flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | >>>> MS_BORN | >>>> - MS_NOATIME | MS_NODIRATIME | MS_RELATIME| >>>> MS_KERNMOUNT | >>>> - MS_STRICTATIME | MS_NOREMOTELOCK); >>>> + flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_NOSYMLINKS | >>>> + MS_ACTIVE | MS_BORN | MS_NOATIME | MS_NODIRATIME | >>>> + MS_RELATIME | MS_KERNMOUNT | MS_STRICTATIME | >>>> + MS_NOREMOTELOCK); >>>> >>>> if (flags & MS_REMOUNT) >>>> retval = do_remount(&path, flags & ~MS_REMOUNT, >>>> mnt_flags, >>>> diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c >>>> index 3f1190d..a1949d9 100644 >>>> --- a/fs/proc_namespace.c >>>> +++ b/fs/proc_namespace.c >>>> @@ -67,6 +67,7 @@ static void show_mnt_opts(struct seq_file *m, struct >>>> vfsmount *mnt) >>>> { MNT_NOATIME, ",noatime" }, >>>> { MNT_NODIRATIME, ",nodiratime" }, >>>> { MNT_RELATIME, ",relatime" }, >>>> + { MNT_NOSYMLINKS, ",nosymlinks" }, >>>> { 0, NULL } >>>> }; >>>> const struct proc_fs_info *fs_infop; >>>> diff --git a/fs/statfs.c b/fs/statfs.c >>>> index 083dc0a..7ff7c32 100644 >>>> --- a/fs/statfs.c >>>> +++ b/fs/statfs.c >>>> @@ -27,6 +27,8 @@ static int flags_by_mnt(int mnt_flags) >>>> flags |= ST_NODIRATIME; >>>> if (mnt_flags & MNT_RELATIME) >>>> flags |= ST_RELATIME; >>>> + if (mnt_flags & MNT_NOSYMLINKS) >>>> + flags |= ST_NOSYMLINKS; >>>> return flags; >>>> } >>>> >>>> diff --git a/include/linux/mount.h b/include/linux/mount.h >>>> index 1172cce..5e302f4 100644 >>>> --- a/include/linux/mount.h >>>> +++ b/include/linux/mount.h >>>> @@ -28,6 +28,7 @@ struct mnt_namespace; >>>> #define MNT_NODIRATIME 0x10 >>>> #define MNT_RELATIME 0x20 >>>> #define MNT_READONLY 0x40 /* does the user want this to be r/o? */ >>>> +#define MNT_NOSYMLINKS 0x80 >>>> >>>> #define MNT_SHRINKABLE 0x100 >>>> #define MNT_WRITE_HOLD 0x200 >>>> @@ -44,7 +45,7 @@ struct mnt_namespace; >>>> #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_NOSYMLINKS) >>>> #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 0166d32..994b059 100644 >>>> --- a/include/linux/statfs.h >>>> +++ b/include/linux/statfs.h >>>> @@ -39,5 +39,6 @@ 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_NOSYMLINKS 0x2000 /* do not follow symbolic links */ >>>> >>>> #endif >>>> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h >>>> index acb2b61..06193d8 100644 >>>> --- a/include/uapi/linux/fs.h >>>> +++ b/include/uapi/linux/fs.h >>>> @@ -130,6 +130,7 @@ struct inodes_stat_t { >>>> #define MS_I_VERSION (1<<23) /* Update inode I_version field */ >>>> #define MS_STRICTATIME (1<<24) /* Always perform atime updates */ >>>> #define MS_LAZYTIME (1<<25) /* Update the on-disk [acm]times lazily >>>> */ >>>> +#define MS_NOSYMLINKS (1<<26) /* Do not follow symbolic links */ >>>> >>>> /* These sb flags are internal to the kernel */ >>>> #define MS_NOREMOTELOCK (1<<27) >>>> -- >>>> 2.8.0.rc3.226.g39d4020 >>>> >