Re: Xen, systemd, and selinux

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

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On Mon, Jun 06, 2016 at 05:41:35PM +0100, George Dunlap wrote:
> Hey Michael,

CC-ing Olif and Daniel De Graaf,
> 
> Not sure if you know, I've been maintaining the Xen4CentOS packages; I
> suspect given the similarities between our systems we're solving the
> same issues; particularly with the systemd/selinux combination.
> 
> I've just ported my patchqueue up to 4.7-rc4, and it looks like the
> SELinux rules for xenstored -- at least the ones that come with CentOS
> 7 -- are outdated; they allow xenstored to open /proc/xen/privcmd
> (which is deprecated), but not /dev/xen/privcmd.
> 
> Do you know where the "upstream" for these rules are, and how to get
> them changed in a way that will trickle down eventually to CentOS?

You open a bug against selinux policies. For example see:
https://bugzilla.redhat.com/show_bug.cgi?id=1322625
https://bugzilla.redhat.com/show_bug.cgi?id=1334511
And (which is exactly what you are hitting):

Bug 1334115 - SELinux is preventing xenconsoled from 'ioctl' accesses on the chr_file /dev/xen/privcmd. (edit)

Since not all of them went in F24 one way you can work around
this is to have a new 'xen-tools-selinux' package that will
install the new SELinux policies.

However I have to confess I hadn't managed to fix the /dev/xen/privcmd.
It still pops up occasionaly. The fix 1334115 is:

dev/xen/blktap.*      -c      gen_context(system_u:object_r:xen_device_t,s0)
 /dev/xen/evtchn                -c      gen_context(system_u:object_r:xen_device_t,s0)
 /dev/xen/gntdev                -c      gen_context(system_u:object_r:xen_device_t,s0)
 /dev/xen/gntalloc      -c      gen_context(system_u:object_r:xen_device_t,s0)
+/dev/xen/privcmd       -c      gen_context(system_u:object_r:xen_device_t,s0)


Which I tried to replicate (see xen.fc)

For OL7 I did:

semanage fcontext -a -t xen_device_t /dev/xen/privcmd
restorecon -Rv /dev/xen/privcmd 

in the %post section of the spec file.

However oddly enough it does not always  work and I am not sure
what is up with that.

Also for OL7 I needed to do a bunch of other policies (see attached)
to get all of them SELinux complains out.

This is what I did in the %build:
                                                                               
%if 0%{?el7}                                                                       
make -f /usr/share/selinux/devel/Makefile xenstored_policy.pp                      
make -f /usr/share/selinux/devel/Makefile xenconsoled_policy.pp                    
make -f /usr/share/selinux/devel/Makefile xen.fc                                   
%endif      

and in %install:

%global modulenames xenstored_policy xenconsoled_policy                         
# Usage: _format var format                                                     
#   Expand 'modulenames' into various formats as needed                         
#   Format must contain '$x' somewhere to do anything useful                    
%global _format() export %1=""; for x in %{modulenames}; do %1+=%2; %1+=" "; done;
                                                                                

                                                                        
%_format MODULES $x.pp                                                          
install -d %{buildroot}%{_datadir}/selinux/packages                             
install -m 0644 $MODULES %{buildroot}%{_datadir}/selinux/packages               
install -m 0644 xen.fc %{buildroot}%{_datadir}/selinux/packages       


And in the %post:

%post tools-selinux                                                             
%_format MODULES %{_datadir}/selinux/packages/$x.pp                             
%{_sbindir}/semodule -s %{selinuxtype} -i $MODULES                              
%{_sbindir}/semanage fcontext -a -t xen_device_t /dev/xen/privcmd               
if %{_sbindir}/selinuxenabled ; then                                            
        %{_sbindir}/load_policy                                                 
    %{_sbindir}/restorecon -Rv /dev/xen/privcmd                                 
fi                                                                              
%endif                                                                 

CC-ing Olif.

> 
> As of 4.7-rc4, libxc will first try to open /dev/xen/privcmd, then
> *if* it fails with a certain set of error codes, it tries
> /proc/xen/privcmd instead.  Unfortunately, EACCES (the failure you get
> from SELinux denials) is not one of those error codes.  If you just
> add that error code in to the list of acceptable error codes, then
> things work for me.
> 
> Thanks,
>  -George
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

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module xenstored_policy 1.0;

require {
	type xenstored_t;
	type device_t;
	type sysctl_fs_t;
	type initrc_t;
	class unix_stream_socket accept;
	class dir search;
	class file { read open };
	class chr_file { read write open };
}

#============= xenstored_t ==============
allow xenstored_t device_t:chr_file { read write open };
allow xenstored_t initrc_t:unix_stream_socket accept;
allow xenstored_t sysctl_fs_t:dir search;
allow xenstored_t sysctl_fs_t:file { read open };
#============= xenstored_t ==============
# src="xenstored_t" tgt="device_t" class="chr_file", perms="{ read write }"
# comm="oxenstored" exe="" path=""
allow xenstored_t device_t:chr_file { read write };
# src="xenstored_t" tgt="sysctl_fs_t" class="dir", perms="search"
# comm="oxenstored" exe="" path=""
allow xenstored_t sysctl_fs_t:dir search;

[-- Attachment #3: xenstored_policy.te --]
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module xenstored_policy 1.0;

require {
	type xenstored_t;
	type device_t;
	type sysctl_fs_t;
	type initrc_t;
	class unix_stream_socket accept;
	class dir search;
	class file { read open };
	class chr_file { read write open };
}

#============= xenstored_t ==============
allow xenstored_t device_t:chr_file { read write open };
allow xenstored_t initrc_t:unix_stream_socket accept;
allow xenstored_t sysctl_fs_t:dir search;
allow xenstored_t sysctl_fs_t:file { read open };
#============= xenstored_t ==============
# src="xenstored_t" tgt="device_t" class="chr_file", perms="{ read write }"
# comm="oxenstored" exe="" path=""
allow xenstored_t device_t:chr_file { read write };
# src="xenstored_t" tgt="sysctl_fs_t" class="dir", perms="search"
# comm="oxenstored" exe="" path=""
allow xenstored_t sysctl_fs_t:dir search;

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/dev/xen/privcmd	--	gen_context(system_u:object_r:xen_device_t:s0)

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