Stephen Smalley <stephen.smalley.work@xxxxxxxxx> writes:
> On Wed, Jan 13, 2021 at 4:52 PM Petr Lautrbach <plautrba@xxxxxxxxxx> wrote:
>>
>> Hi,
>>
>> we have few tests which uses `umount /sys/fs/selinux` trick to check how
>> userspace works in SELinux "disabled" environment. But it's not possible
>> with the current master:
>>
>> # umount /sys/fs/selinux
>> umount: /sys/fs/selinux: target is busy.
>>
>> # lsof /sys/fs/selinux
>> COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
>> systemd 1 root mem REG 0,21 0 19 /sys/fs/selinux/status
>> systemd 1 root 55r REG 0,21 0 19 /sys/fs/selinux/status
>> systemd-u 875 root mem REG 0,21 0 19 /sys/fs/selinux/status
>> systemd-u 875 root 6r REG 0,21 0 19 /sys/fs/selinux/status
>> dbus-brok 1116 dbus mem REG 0,21 0 19 /sys/fs/selinux/status
>> dbus-brok 1116 dbus 5r REG 0,21 0 19 /sys/fs/selinux/status
>> systemd-l 1134 root mem REG 0,21 0 19 /sys/fs/selinux/status
>> systemd-l 1134 root 4r REG 0,21 0 19 /sys/fs/selinux/status
>> systemd 1643 root mem REG 0,21 0 19 /sys/fs/selinux/status
>> systemd 1643 root 28r REG 0,21 0 19 /sys/fs/selinux/status
>> (sd-pam) 1645 root mem REG 0,21 0 19 /sys/fs/selinux/status
>> ...
>> sshd 218874 root mem REG 0,21 0 19 /sys/fs/selinux/status
>> sshd 218874 root 3r REG 0,21 0 19 /sys/fs/selinux/status
>> sshd 218880 plautrba mem REG 0,21 0 19 /sys/fs/selinux/status
>> sshd 218880 plautrba 3r REG 0,21 0 19 /sys/fs/selinux/status
>>
>> It seems to be caused by commit 05bdc03130d7 ("libselinux: use kernel
>> status page by default") which replaced avc_netlink_open() in
>> avc_init_internal() with selinux_status_open()
>>
>> In case of sshd process, /sys/fs/selinux/status seems to be mapped by
>> selinux_check_access() which is called from pam_selinux and it's left
>> open as there's no selinux_status_close() in selinux_check_access().
>> The similar situations probably happen in systemd and dbus.
>>
>> So is it expected? Is it a bug? Do we need to change other components so
>> that they would call selinux_status_close() when they use check access?
>
> What if we just close the fd after mmap and not keep it open? I don't
> see any use of selinux_status_fd beyond assignment and closing.
> Tearing down the mapping and re-creating it on every access check
> would defeat the purpose.
Thanks for this hint! I've checked mmap(2) and it's there: After the
mmap() call has returned, the file descriptor, fd, can be closed
immediately without invalidating the mapping.
I'll try it tomorrow.
