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Date: Thu, 17 Nov 2016 14:11:32 +0100
From: Mattias Nissler <mnissler@...omium.org>
To: "Austin S. Hemmelgarn" <ahferroin7@...il.com>
Cc: linux-fsdevel@...r.kernel.org,
Alexander Viro <viro@...iv.linux.org.uk>,
linux-kernel@...r.kernel.org, Colin Walters <walters@...bum.org>
Subject: Re: [PATCH v2] Add a "nosymlinks" mount option.
On Thu, Nov 17, 2016 at 1:43 PM, Austin S. Hemmelgarn
<ahferroin7@...il.com> 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 <mnissler@...omium.org>
>> 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 <mnissler@...omium.org>
>>> 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 <mnissler@...omium.org>
>>>> ---
>>>> 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
>>>>
>
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