Re: [PATCH v2] Add a "nosymlinks" mount option.

[Mattias Nissler]

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.

On Mon, Oct 24, 2016 at 7:09 AM, Mattias Nissler <mnissler@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>> ---
>>  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
>>