Re: [PATCH 7/7] selinux: Fix domain transitions for typical commands pasta might run

["Richard W.M. Jones" <rjones@redhat.com>]

On Wed, Aug 16, 2023 at 08:00:38AM +0200, Stefano Brivio wrote:
> ...now it gets ugly. If we use pasta without an existing target
> namespace, and run commands directly or spawn a shell, and keep
> the pasta_t domain when we do, they won't be able to do much: a
> shell might even start, but it's not going to be usable, or to
> even display a prompt.
> 
> Ideally, pasta should behave like a shell when it spawns a command:
> start as unconfined_t and automatically transition to whatever
> domain is associated in the specific policy for that command. But
> we can't run as unconfined_t, of course.
> 
> It would seem natural to switch to unconfined_t "just before", so
> that the default transitions happen. But transitions can only happen
> when we execvp(), and that's one single transition -- not two.
> 
> That is, this approach would work for:
> 
>   pasta -- sh -c 'ip address show'
> 
> but not for:
> 
>   pasta -- ip address show
> 
> If we configure a transition to unconfined_t when we run ip(8), we'll
> really try to start that as unconfined_t -- but unconfined_t isn't
> allowed as entrypoint for ip(8) itself, and execvp() will fail.
> 
> However, there aren't many different types of binaries pasta might
> commonly run -- for example, we're unlikely to see pasta used to run
> a mount(8) command.
> 
> Explicitly set up domain transition for common stuff -- switching to
> unconfined_t for bin_t and shells works just fine, ip(8), ping(8),
> arping(8) and similar need a different treatment.
> 
> While at it, allow commands we spawn to inherit resource limits and
> signal masks, because that's what happens by default, and don't
> require AT_SECURE sanitisation of the environment (because that
> won't happen by default). Slightly unrelated: we also need to
> explicitly allow pasta_t to use TTYs, not just PTYs, otherwise
> we can't keep stdin and stdout open for shells.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>  contrib/selinux/pasta.te | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/contrib/selinux/pasta.te b/contrib/selinux/pasta.te
> index 31e82dc..c37a847 100644
> --- a/contrib/selinux/pasta.te
> +++ b/contrib/selinux/pasta.te
> @@ -51,6 +51,7 @@ require {
>  	type tun_tap_device_t;
>  	type sysctl_net_t;
>  	class tun_socket create;
> +	type user_tty_device_t;
>  
>  	attribute port_type;
>  	type port_t;
> @@ -77,6 +78,11 @@ require {
>  	type kernel_t;
>  	class process setpgid;
>  	type shell_exec_t;
> +	type ifconfig_exec_t;
> +	type netutils_exec_t;
> +	type ping_exec_t;
> +	type ifconfig_t;
> +	type ping_t;
>  	type init_t;
>  
>  	class capability { sys_tty_config setuid setgid };
> @@ -111,7 +117,12 @@ allow pasta_t self:user_namespace create;
>  auth_read_passwd_file(pasta_t)
>  sssd_search_lib(pasta_t)
>  
> -allow pasta_t bin_t:file { execute execute_no_trans map };
> +domain_auto_trans(pasta_t, bin_t, unconfined_t);
> +domain_auto_trans(pasta_t, shell_exec_t, unconfined_t);
> +domain_auto_trans(pasta_t, ifconfig_exec_t, ifconfig_t);
> +domain_auto_trans(pasta_t, netutils_exec_t, netutils_t);
> +domain_auto_trans(pasta_t, ping_exec_t, ping_t);
> +
>  allow pasta_t nsfs_t:file { open read };
>  
>  allow pasta_t user_home_t:dir getattr;
> @@ -192,3 +203,8 @@ allow pasta_t nsfs_t:file read;
>  allow pasta_t net_conf_t:lnk_file read;
>  allow pasta_t proc_net_t:lnk_file read;
>  
> +allow pasta_t unconfined_t:process { noatsecure rlimitinh siginh };
> +allow pasta_t ifconfig_t:process { noatsecure rlimitinh siginh };
> +allow pasta_t netutils_t:process { noatsecure rlimitinh siginh };
> +allow pasta_t ping_t:process { noatsecure rlimitinh siginh };
> +allow pasta_t user_tty_device_t:chr_file { append read write };

No idea about this one, probably best to ask someone from the SELinux
group at Red Hat about it ...

Rich.

-- 
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