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

[Casey Schaufler]

On 1/31/2019 2:20 AM, Ondrej Mosnacek wrote:
> Hi Tejun,
>
> On Wed, Jan 30, 2019 at 6:09 PM Tejun Heo <tj at 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.

> 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.
>