Re: [PATCH v7 10/14] securityfs: Extend securityfs with namespacing support

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, Dec 16, 2021 at 02:40:27PM +0100, Christian Brauner wrote:
> On Thu, Dec 16, 2021 at 12:43:19AM -0500, Stefan Berger wrote:
> > From: Stefan Berger <stefanb@linux.ibm.com>
> > 
> > Extend 'securityfs' for support of IMA namespacing so that each
> > IMA (user) namespace can have its own front-end for showing the currently
> > active policy, the measurement list, number of violations and so on.
> > 
> > Drop the addition dentry reference to enable simple cleanup of dentries
> > upon umount.
> > 
> > Prevent mounting of an instance of securityfs in another user namespace
> > than it belongs to. Also, prevent accesses to directories when another
> > user namespace is active than the one that the instance of securityfs
> > belongs to.
> > 
> > Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> > Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> > ---
> >  security/inode.c | 37 ++++++++++++++++++++++++++++++++++---
> >  1 file changed, 34 insertions(+), 3 deletions(-)
> > 
> > diff --git a/security/inode.c b/security/inode.c
> > index fee01ff4d831..a0d9f086e3d5 100644
> > --- a/security/inode.c
> > +++ b/security/inode.c
> > @@ -26,6 +26,29 @@
> >  static struct vfsmount *init_securityfs_mount;
> >  static int init_securityfs_mount_count;
> >  
> > +static int securityfs_permission(struct user_namespace *mnt_userns,
> > +				 struct inode *inode, int mask)
> > +{
> > +	int err;
> > +
> > +	err = generic_permission(&init_user_ns, inode, mask);
> > +	if (!err) {
> > +		if (inode->i_sb->s_user_ns != current_user_ns())
> > +			err = -EACCES;
> 
> I really think the correct semantics is to grant all callers access
> whose user namespace is the same as or an ancestor of the securityfs
> userns. It's weird to deny access to callers who are located in an
> ancestor userns.
> 
> For example, a privileged process on the host should be allowed to setns
> to the userns of an unprivileged container and inspect its securityfs

s/userns/mntns/

> instance.
> 
> We're mostly interested to block such as scenarios where two sibling
> unprivileged containers are created in the initial userns and an fd
> proxy or something funnels a file descriptor from one sibling container
> to the another one and the receiving sibling container can use readdir()
> or openat() on this fd. (I'm not even convinced that this is actually a
> problem but stricter semantics at the beginning can't hurt. We can
> always relax this later.)
>