Re: [PATCH] mnt: allow to add a mount into an existing group

Andrey Vagin [off-list ref] writes:

On Tue, Jan 24, 2017 at 02:03:23PM +1300, Eric W. Biederman wrote:

quoted

Andrei Vagin [off-list ref] writes:

quoted

Now a shared group can be only inherited from a source mount.
This patch adds an ability to add a mount into an existing shared
group.

This sounds like a lot of the discussion on bind mounts accross
namespaces.  I am going to stay out of this for a bit until
we resolve my latest patch.

Hi Eric,

Your patches about shadow/side mounts were committed, can we resume
the discussion about this patch?

We can.

Although personally I would rather get back to figuring out how
we can reduce the horrible time complexity of the worst case
for umount -l.

As for security, a mount can be added to a shared group, only if a
caller has CAP_SYS_ADMIN in namespaces of both mounts, so an
unprivileged user can't create a shared mount with a parent mount
namespace. If a user has CAP_SYS_ADMIN, I don't see a reason to
restrict him to create shared mounts between namespaces, even if they
are in different user-namespaces.

Can they create loops in mount propagation trees that we can not create
today?  It feels like that would be very simple to do with an interface
like this.

A loop in a mount propagation tree would be an absolute disaster.

I remember Al Viro had some very firm ideas about bind mounts from
foreign namespaces.  That I have never take the time to understand.
I suspect whatever objections he had will apply in this case.  Or else
this code might be made unnecessary by allowing bind mounts between
mount namespaces.

Now I look at volume drivers in container services (like docker and
kubernetes) and I think this functionality can be useful for them too.
Now it is impossible to run a plugin driver in unprivileged containers
(with sub-userns), because a container has to have a shared mount with
a mount namespace where the service is running. The idea of these
plugins is that a service requests a volume mount from a plugin and
then starts a container with this volume, so the service need to have
a way to get a mount from a service.

Interesting.  I personally think the checkpoint/restart use case
is more compelling.

quoted

Eric

On Thu, Apr 27, 2017 at 10:18 PM, Andrei Vagin [off-list ref] wrote:

quoted

Now a shared group can be only inherited from a source mount.
This patch adds an ability to add a mount into an existing shared
group.

mount(source, target, NULL, MS_SET_GROUP, NULL)

mount() with the MS_SET_GROUP flag adds the "target" mount into a group
of the "source" mount. The calling process has to have the CAP_SYS_ADMIN
capability in namespaces of these mounts. The source and the target
mounts have to have the same super block.

This new functionality together with "mnt: Tuck mounts under others
instead of creating shadow/side mounts." allows CRIU to dump and restore
any set of mount namespaces.

Currently we have a lot of issues about dumping and restoring mount
namespaces. The bigest problem is that we can't construct mount trees
directly due to several reasons:
* groups can't be set, they can be only inherited
* file systems has to be mounted from the specified user namespaces
* the mount() syscall doesn't just create one mount -- the mount is
  also propagated to all members of a parent group
* umount() doesn't detach mounts from all members of a group
  (mounts with children are not umounted)
* mounts are propagated underneath of existing mounts
* mount() doesn't allow to make bind-mounts between two namespaces
* processes can have opened file descriptors to overmounted files

All these operations are non-trivial, making the task of restoring
a mount namespace practically unsolvable for reasonable time. The
proposed change allows to restore a mount namespace in a direct
manner, without any super complex logic.

Cc: Eric W. Biederman <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
Cc: Alexander Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Signed-off-by: Andrei Vagin <redacted>
---
 fs/namespace.c          | 66 ++++++++++++++++++++++++++++++++++++++++++++++---
 include/uapi/linux/fs.h |  6 +++++
 2 files changed, 68 insertions(+), 4 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index cc1375ef..3bf0cd2 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -2355,6 +2355,57 @@ static inline int tree_contains_unbindable(struct mount *mnt)
        return 0;
 }

+static int do_set_group(struct path *path, const char *sibling_name)
+{
+       struct mount *sibling, *mnt;
+       struct path sibling_path;
+       int err;
+
+       if (!sibling_name || !*sibling_name)
+               return -EINVAL;
+
+       err = kern_path(sibling_name, LOOKUP_FOLLOW, &sibling_path);
+       if (err)
+               return err;
+
+       sibling = real_mount(sibling_path.mnt);
+       mnt = real_mount(path->mnt);
+
+       namespace_lock();
+
+       err = -EPERM;
+       if (!sibling->mnt_ns ||
+           !ns_capable(sibling->mnt_ns->user_ns, CAP_SYS_ADMIN))
+               goto out_unlock;
+
+       err = -EINVAL;
+       if (sibling->mnt.mnt_sb != mnt->mnt.mnt_sb)
+               goto out_unlock;
+
+       if (IS_MNT_SHARED(mnt) || IS_MNT_SLAVE(mnt))
+               goto out_unlock;
+
+       if (IS_MNT_SLAVE(sibling)) {
+               struct mount *m = sibling->mnt_master;
+
+               list_add(&mnt->mnt_slave, &m->mnt_slave_list);
+               mnt->mnt_master = m;
+       }
+
+       if (IS_MNT_SHARED(sibling)) {
+               mnt->mnt_group_id = sibling->mnt_group_id;
+               list_add(&mnt->mnt_share, &sibling->mnt_share);
+               set_mnt_shared(mnt);
+       }
+
+       err = 0;
+out_unlock:
+       namespace_unlock();
+
+       path_put(&sibling_path);
+       return err;
+}
+
 static int do_move_mount(struct path *path, const char *old_name)
 {
        struct path old_path, parent_path;

@@ -2769,6 +2820,7 @@ long do_mount(const char *dev_name, const char __user *dir_name,
        struct path path;
        int retval = 0;
        int mnt_flags = 0;
+       unsigned long cmd;

        /* Discard magic */
        if ((flags & MS_MGC_MSK) == MS_MGC_VAL)

@@ -2820,19 +2872,25 @@ long do_mount(const char *dev_name, const char __user *dir_name,
                mnt_flags |= path.mnt->mnt_flags & MNT_ATIME_MASK;
        }

+       cmd = flags & (MS_REMOUNT | MS_BIND |
+                      MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE |
+                      MS_MOVE | MS_SET_GROUP);
+
        flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
                   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
                   MS_STRICTATIME | MS_NOREMOTELOCK | MS_SUBMOUNT);

-       if (flags & MS_REMOUNT)
+       if (cmd & MS_REMOUNT)
                retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
                                    data_page);
-       else if (flags & MS_BIND)
+       else if (cmd & MS_BIND)
                retval = do_loopback(&path, dev_name, flags & MS_REC);
-       else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
+       else if (cmd & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
                retval = do_change_type(&path, flags);
-       else if (flags & MS_MOVE)
+       else if (cmd & MS_MOVE)
                retval = do_move_mount(&path, dev_name);
+       else if (cmd & MS_SET_GROUP)
+               retval = do_set_group(&path, dev_name);
        else
                retval = do_new_mount(&path, type_page, flags, mnt_flags,
                                      dev_name, data_page);

diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 048a85e..33423aa 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h

@@ -131,6 +131,12 @@ struct inodes_stat_t {
 #define MS_STRICTATIME (1<<24) /* Always perform atime updates */
 #define MS_LAZYTIME    (1<<25) /* Update the on-disk [acm]times lazily */

+/*
+ * Here are commands and flags. Commands are handled in do_mount()
+ * and can intersect with kernel internal flags.
+ */
+#define MS_SET_GROUP   (1<<26) /* Add a mount into a shared group */
+
 /* These sb flags are internal to the kernel */
 #define MS_SUBMOUNT     (1<<26)
 #define MS_NOREMOTELOCK        (1<<27)
--

2.9.3