linux-kernel - Could not mount sysfs when enable userns but disable netns

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Message-ID: <5871495633F38949900D2BF2DC04883E562293@G08CNEXMBPEKD02.g08.fujitsu.local>
Date:	Fri, 11 Jul 2014 07:27:53 +0000
From:	"chenhanxiao@...fujitsu.com" <chenhanxiao@...fujitsu.com>
To:	"Eric W. Biederman (ebiederm@...ssion.com)" <ebiederm@...ssion.com>,
	"Serge Hallyn (serge.hallyn@...ntu.com)" <serge.hallyn@...ntu.com>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>
CC:	"containers@...ts.linux-foundation.org" 
	<containers@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"'Daniel P. Berrange (berrange@...hat.com)'" <berrange@...hat.com>
Subject: Could not mount sysfs when enable userns but disable netns

Hello,

How to reproduce:
1. Prepare a container, enable userns and disable netns
2. use libvirt-lxc to start a container
3. libvirt could not mount sysfs then failed to start.

Then I found that
commit 7dc5dbc879bd0779924b5132a48b731a0bc04a1e says:
"Don't allow mounting sysfs unless the caller has CAP_SYS_ADMIN rights
over the net namespace."

But why should we check sysfs mouont permission over net namespace?
We've already checked CAP_SYS_ADMIN though.

What the relationship between sysfs and net namespace,
or this check is a little redundant?

Any insights on this?

Thanks,
- Chen

PS: codes below could be a workaround

@@ -34,7 +35,8 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
                if (!capable(CAP_SYS_ADMIN) && !fs_fully_visible(fs_type))
                        return ERR_PTR(-EPERM);
 
-               if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
+               if (current->nsproxy->net_ns != &init_net &&
+                    !kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
                        return ERR_PTR(-EPERM);
        }