Re: [PATCH 3/3] Enable security.selinux in user namespaces

[Stefan Berger]

On 06/23/2017 04:30 PM, Stephen Smalley wrote:
> On Thu, 2017-06-22 at 14:59 -0400, Stefan Berger wrote:
> > Before the current modifications, SELinux extended attributes were
> > visible inside the user namespace but changes in patch 1 hid them.
> > This patch enables security.selinux in user namespaces and allows
> > them to be written to in the same way as security.capability.
> >
> > Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx>
> > ---
> >   fs/xattr.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/fs/xattr.c b/fs/xattr.c
> > index 045be85..37686ee 100644
> > --- a/fs/xattr.c
> > +++ b/fs/xattr.c
> > @@ -138,6 +138,7 @@ xattr_permission(struct inode *inode, const char
> > *name, int mask)
> >    */
> >   static const char *const userns_xattrs[] = {
> >   	XATTR_NAME_CAPS,
> > +	XATTR_NAME_SELINUX,
> >   	NULL
> >   };
> >   
> (cc SELinux maintainers, curiously omitted from these patches)
>
> I don't think this works for SELinux. You don't deal with actually
> supporting multiple security.selinux attributes within SELinux itself
> (and I'm not asking you to do so), and without such support, this can't
> operate as intended. With these patches applied, IIUC, a setxattr() of
> security.selinux within a userns will end up setting only security.seli
> nux@uid=1000 on disk, but will then tell SELinux to update its in-core
> security label to the new value (via security_inode_post_setxattr).
> Meanwhile, on a subsequent getxattr(), you'll call
> security_inode_getsecurity() with the security.selinux@uid=1000 name,
> which will always fail because SELinux doesn't know anything about your
> new scheme, and then you'll call the filesystem handler and returns its
> value, which is no longer connected in any way to the actual label
> being used by SELinux.  Also, SELinux itself makes calls to
> __vfs_getxattr() and __vfs_setxattr_noperm(), and I don't think your
> name remapping is correct in those cases.
>
> You also can't hide security.selinux within user namespaces.  Today
> userspace can get and set security.selinux attributes within user
> namespaces (if allowed by policy), and further can specify the label to
> use for new files via /proc/self/attr/fscreate, which unsurprisingly
> isn't addressed by your changes.  Changing that would be a userspace
> break.

I modified the 1st patch now in such a way that only security.capability 
is rewritten, security.selinux and all other ones remain untouched.

https://github.com/stefanberger/linux/commits/xattr_for_userns.v2

   Stefan