Re: 4.2: Can't mount sysfs in a mount ns & user ns

[ebiederm@xmission.com (Eric W. Biederman)]

Lubomir Rintel <lkundrak@v3.sk> writes:

> Hello,
>
> On Thu, 2015-08-13 at 10:20 -0500, Eric W. Biederman wrote:
>> Lubomir Rintel <lkundrak@v3.sk> writes:
>> 
>> > Hi,
>> > 
>> > 4.0.6-300.fc22.x86_64:
>> > [lkundrak@fedora22-1 ~]$ unshare -r --mount --net
>> > [root@fedora22-1 ~]# mount --make-slave /sys
>> > [root@fedora22-1 ~]# mount -t sysfs sysfs /sys
>> > [root@fedora22-1 ~]# 
>> > 
>> > 4.2.0-0.rc6.git0.1.fc24.x86_64:
>> > [lkundrak@fedora23-1 ~]$ unshare -r --mount --net
>> > [root@fedora23-1 ~]# mount --make-slave /sys
>> > [root@fedora23-1 ~]# mount -t sysfs sysfs /sys
>> > mount: permission denied
>> > [root@fedora23-1 ~]#
>> > 
>> > we use this in NetworkManager test suite, to ensure the devices we 
>> > see
>> > via GUdev are the same as we see via rtnetlink.
>> > 
>> > I'm wondering if this is a bug or an intended change?
>> 
>> There was an intentional tightening up of the permissions required to
>> mount sysfs to prevent people in jails from gaining access to things
>> they would not ordinarily have access to.  The change was not 
>> expected
>> to affect anyones legitimate use case.
>> 
>> What are the mount flags of the previous mount of sysfs?
>> What is mounted on top of sysfs?
>> 
>> Or in short can I see /proc/self/mounts for the failing scenario?
>
> Looks like this:
>
> sysfs /sys sysfs rw,seclabel,nosuid,nodev,noexec,relatime 0 0
> proc /proc proc rw,relatime 0 0
> devtmpfs /dev devtmpfs rw,seclabel,nosuid,size=882904k,nr_inodes=220726,mode=755 0 0
> securityfs /sys/kernel/security securityfs rw,nosuid,nodev,noexec,relatime 0 0
> tmpfs /dev/shm tmpfs rw,seclabel,nosuid,nodev 0 0
> devpts /dev/pts devpts rw,seclabel,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
> tmpfs /run tmpfs rw,seclabel,nosuid,nodev,mode=755 0 0
> tmpfs /sys/fs/cgroup tmpfs ro,seclabel,nosuid,nodev,noexec,mode=755 0 0
> cgroup /sys/fs/cgroup/systemd cgroup rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/usr/lib/systemd/systemd-cgroups-agent,name=systemd 0 0
> pstore /sys/fs/pstore pstore rw,seclabel,nosuid,nodev,noexec,relatime 0 0
> kdbusfs /sys/fs/kdbus kdbusfs rw,nosuid,nodev,noexec,relatime 0 0
          ^^^^^^^^^^^^^  This directory is probably not created with sysfs_create_mount_point
                         So I suspect this is your culprit.
> cgroup /sys/fs/cgroup/hugetlb cgroup rw,nosuid,nodev,noexec,relatime,hugetlb 0 0
> cgroup /sys/fs/cgroup/net_cls,net_prio cgroup rw,nosuid,nodev,noexec,relatime,net_cls,net_prio 0 0
> cgroup /sys/fs/cgroup/devices cgroup rw,nosuid,nodev,noexec,relatime,devices 0 0
> cgroup /sys/fs/cgroup/cpu,cpuacct cgroup rw,nosuid,nodev,noexec,relatime,cpu,cpuacct 0 0
> cgroup /sys/fs/cgroup/perf_event cgroup rw,nosuid,nodev,noexec,relatime,perf_event 0 0
> cgroup /sys/fs/cgroup/cpuset cgroup rw,nosuid,nodev,noexec,relatime,cpuset 0 0
> cgroup /sys/fs/cgroup/memory cgroup rw,nosuid,nodev,noexec,relatime,memory 0 0
> cgroup /sys/fs/cgroup/freezer cgroup rw,nosuid,nodev,noexec,relatime,freezer 0 0
> cgroup /sys/fs/cgroup/blkio cgroup rw,nosuid,nodev,noexec,relatime,blkio 0 0
> configfs /sys/kernel/config configfs rw,relatime 0 0
> /dev/vda3 / btrfs rw,seclabel,relatime,space_cache,subvolid=5,subvol=/ 0 0
> selinuxfs /sys/fs/selinux selinuxfs rw,relatime 0 0
> systemd-1 /proc/sys/fs/binfmt_misc autofs rw,relatime,fd=36,pgrp=1,timeout=0,minproto=5,maxproto=5,direct 0 0
> tmpfs /tmp tmpfs rw,seclabel 0 0
> mqueue /dev/mqueue mqueue rw,seclabel,relatime 0 0
> debugfs /sys/kernel/debug debugfs rw,seclabel,relatime 0 0
> hugetlbfs /dev/hugepages hugetlbfs rw,seclabel,relatime 0 0
> /dev/vda1 /boot ext4 rw,seclabel,relatime,data=ordered 0 0
> tmpfs /run/user/42 tmpfs rw,seclabel,nosuid,nodev,relatime,size=178648k,mode=700,uid=42,gid=42 0 0
> gvfsd-fuse /run/user/42/gvfs fuse.gvfsd-fuse rw,nosuid,nodev,relatime,user_id=42,group_id=42 0 0
> fusectl /sys/fs/fuse/connections fusectl rw,relatime 0 0
> tmpfs /run/user/8086 tmpfs rw,seclabel,nosuid,nodev,relatime,size=178648k,mode=700,uid=8086,gid=8086 0 0
> gvfsd-fuse /run/user/8086/gvfs fuse.gvfsd-fuse rw,nosuid,nodev,relatime,user_id=8086,group_id=8086 0 0
>
>> Without a little more detail I can't see if there is a possible 
>> security
>> violation in your code or if this is something I can fix.
>> 
>> Eric
>
> Thanks for the response

It looks like this is a kdbus thing.  I don't see anything else that
should be causing problems.  Please try again with kdbus disabled and
see what happens, and when it works please let the kdbus guys know that
the need to use sysfs_create_mount_point.

Eric