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

From: Austin S. Hemmelgarn
Date: Thu Nov 17 2016 - 12:20:33 EST

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).

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