[PATCH v6 15/17] ima: Use mac_admin_ns_capable() to check corresponding capability

[James Bottomley]

On Sat, 2021-12-11 at 13:22 -0600, Serge E. Hallyn wrote:
> On Sat, Dec 11, 2021 at 11:05:10AM -0500, James Bottomley wrote:
> > On Sat, 2021-12-11 at 09:29 -0600, Serge E. Hallyn wrote:
> > > On Fri, Dec 10, 2021 at 02:47:34PM -0500, Stefan Berger wrote:
> > > > Use mac_admin_ns_capable() to check corresponding capability to
> > > > allow
> > > > read/write IMA policy without CAP_SYS_ADMIN but with
> > > > CAP_MAC_ADMIN.
> > > > 
> > > > Signed-off-by: Denis Semakin <denis.semakin at huawei.com>
> > > > Signed-off-by: Stefan Berger <stefanb at linux.ibm.com>
> > > > ---
> > > >  include/linux/capability.h      | 6 ++++++
> > > >  security/integrity/ima/ima_fs.c | 2 +-
> > > >  2 files changed, 7 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/include/linux/capability.h
> > > > b/include/linux/capability.h
> > > > index 65efb74c3585..991579178f32 100644
> > > > --- a/include/linux/capability.h
> > > > +++ b/include/linux/capability.h
> > > > @@ -270,6 +270,12 @@ static inline bool
> > > > checkpoint_restore_ns_capable(struct user_namespace *ns)
> > > >  		ns_capable(ns, CAP_SYS_ADMIN);
> > > >  }
> > > >  
> > > > +static inline bool mac_admin_ns_capable(struct user_namespace
> > > > *ns)
> > > > +{
> > > > +	return ns_capable(ns, CAP_MAC_ADMIN) ||
> > > > +		ns_capable(ns, CAP_SYS_ADMIN);
> > > > +}
> > > > +
> > > >  /* audit system wants to get cap info from files as well */
> > > >  int get_vfs_caps_from_disk(struct user_namespace *mnt_userns,
> > > >  			   const struct dentry *dentry,
> > > > diff --git a/security/integrity/ima/ima_fs.c
> > > > b/security/integrity/ima/ima_fs.c
> > > > index a136d14f29ec..090ee85bfa3a 100644
> > > > --- a/security/integrity/ima/ima_fs.c
> > > > +++ b/security/integrity/ima/ima_fs.c
> > > > @@ -440,7 +440,7 @@ static int ima_open_policy(struct inode
> > > > *inode,
> > > > struct file *filp)
> > > >  #else
> > > >  		if ((filp->f_flags & O_ACCMODE) != O_RDONLY)
> > > >  			return -EACCES;
> > > > -		if (!capable(CAP_SYS_ADMIN))
> > > > +		if (!mac_admin_ns_capable(user_ns))
> > > 
> > > Sorry if I'm missing something.  But I'm looking at your tree's
> > > version of ima_update_policy() and failing to see where it adds
> > > extra capability checks.  Note that any unprivileged user can
> > > unshare a user namespace, map its hostuid to nsuid 0, and pass
> > > ns_capable(CAP_MAC_ADMIN).
> > > 
> > > Likewise, a host uid 0 process which does not have CAP_MAC_ADMIN
> > > can create a new user namespace, map hostuid 0 to nsuid 0, and
> > > have CAP_MAC_ADMIN against the new userns.
> > > 
> > > Somewhere you need to be checking for privilege against either
> > > the parent ns or the init_user_ns.  I'm not seeing where that's
> > > being done.  Can you point me to it?
> > 
> > I think you're thinking of my patches, which used a single ima log
> > for everything.  With Stefan's patches, each namespace gets its own
> > log and starts out with and empty "do nothing" policy.  However,
> > whatever policy was active in the parent namespace is still run for
> > activities in  the child, so any activity the parent requires to be
> > measured/audited/appraised etc still happens, but all entries are
> > made in the parent.
> 
> Oh.  Stefan had previously said that one motivation was to do less
> logging in the hostns, so I assumed that a child ns policy could make
> it so the parent ns policy was no longer applied.

Well, the default policy even for the init_user_ns is don't log
anything, so you can keep that if you don't care.  However, if you do
care you can log all the containers.  If the containers install a
logging policy, that log will disappear when the namespace is reaped,
so if the hostns has a no log policy, all the log space the child used
will be freed.

James


>   If that's not the case, and not planned, then that makes this a lot
> simpler.
>