linux-kernel - Re: [PATCH 1/4] kernel: add a helper to get an owning user namespace for a namespace

Open Source and information security mailing list archives

Date:   Wed, 31 Aug 2016 13:38:35 -0700
From:   Andrey Vagin <avagin@...nvz.org>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Serge Hallyn <serge.hallyn@...onical.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 1/4] kernel: add a helper to get an owning user namespace
 for a namespace

On Tue, Aug 30, 2016 at 7:56 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> On Fri, Aug 26, 2016 at 04:08:08PM -0700, Andrei Vagin wrote:
>> +struct ns_common *ns_get_owner(struct ns_common *ns)
>> +{
>> +     struct user_namespace *my_user_ns = current_user_ns();
>> +     struct user_namespace *owner, *p;
>> +
>> +     /* See if the owner is in the current user namespace */
>> +     owner = p = ns->ops->get_owner(ns);
>> +     for (;;) {
>> +             if (!p)
>> +                     return ERR_PTR(-EPERM);
>> +             if (p == my_user_ns)
>> +                     break;
>> +             p = p->parent;
>> +     }
>> +
>> +     return &get_user_ns(owner)->ns;
>
> get_user_ns() bumps the owner's refcount.  I don't see where
> this is being dropped, especially when ns_ioctl() uses it in
> the next patch.

It is dropped in __ns_get_path if a namespace has a dentry, otherwise
it is dropped from nsfs_evict.

static void *__ns_get_path(struct path *path, struct ns_common *ns)
        |                return -EPERM;
...
        ns->ops->put(ns);                                                      |
got_it:
        |        /* See if the owner is in the current user namespace
*/
        path->mnt = mnt;
        |        owner = p = ns->ops->get_owner(ns);
        path->dentry = dentry;
        |        for (;;) {
        return NULL;
...

static void nsfs_evict(struct inode *inode)                                    |
{
        |        if (!ns_capable(user_ns, CAP_SYS_ADMIN))
        struct ns_common *ns = inode->i_private;
        |                return -EPERM;
        clear_inode(inode);                                                    |
        ns->ops->put(ns);
        |        cred = prepare_creds();
}

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