Re: [RFC 20/20] ima: Setup securityfs_ns for IMA namespace

[Stefan Berger]

On 12/1/21 16:11, James Bottomley wrote:
> On Wed, 2021-12-01 at 15:25 -0500, Stefan Berger wrote:
> > On 12/1/21 14:21, James Bottomley wrote:
> > > On Wed, 2021-12-01 at 13:11 -0500, Stefan Berger wrote:
> > > > On 12/1/21 12:56, James Bottomley wrote:
> > > [...]
> > > > I tried this with runc and a user namespace active mapping uid
> > > > 1000 on the host to uid 0 in the container. There I run into the
> > > > problem that  all of the files and directories without the above
> > > > work-around are mapped to 'nobody', just like all the files in
> > > > sysfs in this case are also mapped to nobody. This code resolved
> > > > the issue.
> > > So I applied your patches with the permission shift commented out
> > > and instrumented inode_alloc() to see where it might be failing and
> > > I actually find it all works as expected for me:
> > >
> > > ejb@testdeb:~> unshare -r --user --mount --ima
> > > root@testdeb:~# mount -t securityfs_ns none /sys/kernel/security
> > > root@testdeb:~# ls -l /sys/kernel/security/ima/
> > > total 0
> > > -r--r----- 1 root root 0 Dec  1 19:11 ascii_runtime_measurements
> > > -r--r----- 1 root root 0 Dec  1 19:11 binary_runtime_measurements
> > > -rw------- 1 root root 0 Dec  1 19:11 policy
> > > -r--r----- 1 root root 0 Dec  1 19:11 runtime_measurements_count
> > > -r--r----- 1 root root 0 Dec  1 19:11 violations
> > >
> > > I think your problem is something to do with how runc is installing
> > > the uid/gid mappings.  If it's installing them after the
> > > security_ns inodes are created then they get the -1 value (because
> > > no mappings exist in s_user_ns).  I can even demonstrate this by
> > > forcing unshare to enter the IMA namespace before writing the
> > > mapping values and I'll see "nobody nogroup" above like you do.
> > I am surprised you get this mapping even after commenting the
> > permission adjustments... it doesn't work for me when I comment them
> > out:
> >
> > [stefanb@ima-ns-dev rootfs]$ unshare -r --user --mount
> > [root@ima-ns-dev rootfs]# mount -t securityfs_ns none
> > /sys/kernel/security/
> > [root@ima-ns-dev rootfs]# cd /sys/kernel/security/ima/
> > [root@ima-ns-dev ima]# ls -l
> > total 0
> > -r--r-----. 1 nobody nobody 0 Dec  1 15:20 ascii_runtime_measurements
> > -r--r-----. 1 nobody nobody 0 Dec  1 15:20
> > binary_runtime_measurements
> > -rw-------. 1 nobody nobody 0 Dec  1 15:20 policy
> > -r--r-----. 1 nobody nobody 0 Dec  1 15:20 runtime_measurements_count
> > -r--r-----. 1 nobody nobody 0 Dec  1 15:20 violations
> > [root@ima-ns-dev ima]# cat /proc/self/uid_map
> >            0       1000          1
> > [root@ima-ns-dev ima]# cat /proc/self/gid_map
> >            0       1000          1
> >
> > The initialization of securityfs and setup of files and directories
> > happens at the same time as the IMA namespace is created. At this
> > time there are no user mappings available, so that's why I need to
> > make the adjustments 'late'.
> There is one other possible difference:  To get the correct s_user_ns

I am currently wondering why I cannot re-create your setup while 
disabling the remapping...




> on the securityfs_ns mount, the mount namespace itself has to be owned
> by the user namespace ... is runc doing that correctly?  I always

Following an strace of 'runc create' I see an unshare(CLONE_NEWUSER) by 
a process before it does an 
unshare(CLONE_NEWNS|CLONE_NEWUTS|CLONE_NEWIPC|CLONE_NEWPID|CLONE_NEWNET), 
so this seems to be doing it in the order you suggest.

Also, runc seems to have its own set of struggles. I am not sure we 
would be able to ask them to accommodate us to do it 'correctly' - it 
doesn't sound so 'easy' for them either to get everything under the hood:

https://github.com/opencontainers/runc/blob/master/libcontainer/nsenter/nsexec.c#L919

     * In order for this unsharing code to be more extensible we need 
to split
     * up unshare(CLONE_NEWUSER) and clone() in various ways. The ideal 
case
     * would be if we did clone(CLONE_NEWUSER) and the other namespaces
     * separately, but because of SELinux issues we cannot really do 
that. But

[...]

     * However, if we unshare(2) the user namespace *before* we 
clone(2), then
     * all hell breaks loose.

sounds like fun

So, I am not quite sure whether I am working around an issue of runc but 
for that I would like to first be able to re-create your successful 
setup to see what's different.

   Stefan


> forget this detail because unshare does it correctly automatically but
> it means you must unshare the user namespace first and then unshare the
> mount namespace (or do it in the same sys call because the kernel will
> get the correct order).
>
> James