Re: [PATCH v3 5/5] kernfs: initialize security of newly created nodes

[Ondrej Mosnacek <omosnace@redhat.com>]

On Thu, Jan 31, 2019 at 5:39 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> On 1/31/2019 2:20 AM, Ondrej Mosnacek wrote:
> > Hi Tejun,
> >
> > On Wed, Jan 30, 2019 at 6:09 PM Tejun Heo <tj@kernel.org> wrote:
> >> Hello,
> >>
> >> On Wed, Jan 30, 2019 at 12:41:50PM +0100, Ondrej Mosnacek wrote:
> >>> @@ -673,6 +698,12 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
> >>>                       goto err_out3;
> >>>       }
> >>>
> >>> +     if (parent) {
> >>> +             ret = kernfs_node_init_security(parent, kn);
> >>> +             if (ret)
> >>> +                     goto err_out3;
> >>> +     }
> >> So, doing this unconditionally isn't a good idea.  kernfs doesn't use
> >> the usual dentry/inode because there are machines with 6, even 7 digit
> >> number of kernfs nodes and some of them even failed to boot due to
> >> memory shortage.  Please don't blow it up by default.
> > Hm, I see... basically the only thing that gets allocated in
> > kernfs_node_init_security() by default (at least under SELinux/ no
> > LSM) is the kernfs_iattrs structures, so I assume you are pointing at
> > that. I think this can be easily fixed, if we again use the assumption
> > that whenever the parent node has only default attributes
> > (parent->iattrs == NULL), then the child node should also have just
> > default attributes (and so we don't need to call kernfs_iattrs() on it
> > nor call the security hook). Something along these lines:
> >
> > [...]
> > +static int kernfs_node_init_security(struct kernfs_node *parent,
> > +                                    struct kernfs_node *kn)
> > +{
> > +       struct kernfs_iattrs *attrs, *pattrs;
> > +       struct qstr q;
> > +
> > +       pattrs = parent->iattrs;
> > +       if (!pattrs)
> > +               return 0;
> > +
> > +       attrs = kernfs_iattrs(kn);
> > +       if (!attrs)
> > +               return -ENOMEM;
> > +
> > +       q.name = kn->name;
> > +       q.hash_len = hashlen_string(parent, kn->name);
> > [...]
> >
> > Technically this might make some LSMs unhappy, if they want to set
> > some non-default context even if parent is all default,
>
> The only possibility I see as a potential problem is a kernfs
> mounted with the smackfstransmute=Something option. This sets
> the security.SMACK64 to "Something" and the security.SMACK64TRANSMUTE
> to true on the root node. But that doesn't seem like a rational
> thing to do for a kernfs based filesystem.

Actually... I am now experimenting with a slightly different
kernfs_node_init_security() implementation that should allow for
calling the hook every time and only allocating kernfs_iattrs when it
detects that the hook actually did add some security xattrs. It is
somewhat more hacky and complex, but it should provide the best
possible compromise. I will post it for review soon.

>
> > but this is
> > already impossible now and in this case I think we have no better
> > choice than sacrificing a bit of flexibility for memory efficiency,
> > which is apparently critical here.
> >
> > Tejun, Casey, would the above modification be fine with you?
>
> I *think so*, but I can't say 100% that I really understand the
> entire issue.
>
> >
> > --
> > Ondrej Mosnacek <omosnace at redhat dot com>
> > Associate Software Engineer, Security Technologies
> > Red Hat, Inc.
> >

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.