Re: [PATCH v2] landlock: Add abstract unix socket connect restrictions

[Tahera Fahimi]

On Tue, Apr 02, 2024 at 11:53:09AM +0200, Mickaël Salaün wrote:
> Thanks for this patch.  Please CC the netdev mailing list too, they may
> be interested by this feature. I also added a few folks that previously
> showed their interest for this feature.
> 
> On Thu, Mar 28, 2024 at 05:12:13PM -0600, TaheraFahimi wrote:
> > Abstract unix sockets are used for local interprocess communication without
> > relying on filesystem. Since landlock has no restriction for connecting to
> > a UNIX socket in the abstract namespace, a sandboxed process can connect to
> > a socket outside the sandboxed environment. Access to such sockets should
> > be scoped the same way ptrace access is limited.
> 
> This is good but it would be better to explain that Landlock doesn't
> currently control abstract unix sockets and that it would make sense for
> a sandbox.
> 
> 
> > 
> > For a landlocked process to be allowed to connect to a target process, it
> > must have a subset of the target process’s rules (the connecting socket
> > must be in a sub-domain of the listening socket). This patch adds a new
> > LSM hook for connect function in unix socket with the related access rights.
> 
> Because of compatibility reasons, and because Landlock should be
> flexible, we need to extend the user space interface.  As explained in
> the GitHub issue, we need to add a new "scoped" field to the
> landlock_ruleset_attr struct. This field will optionally contain a
> LANDLOCK_RULESET_SCOPED_ABSTRACT_UNIX_SOCKET flag to specify that this
> ruleset will deny any connection from within the sandbox to its parents
> (i.e. any parent sandbox or not-sandboxed processes).
Thanks for the feedback. Here is what I understood, please correct me if
I am wrong. First, I should add another field to the
landlock_ruleset_attr (a field like handled_access_net, but for the unix
sockets) with a flag LANDLOCK_ACCESS_UNIX_CONNECT (it is a flag like
LANDLOCK_ACCESS_NET_CONNECT_TCP but fot the unix sockets connect).

> > 
> > Link to first draft:
> > 	https://lore.kernel.org/outreachy/20240328.ShoR4Iecei8o@xxxxxxxxxxx/
> 
> You can move this sentence in the below changelog.
> 
> > 
> 
> You can add this:
> 
> Closes: https://github.com/landlock-lsm/linux/issues/7
> 
> > Signed-off-by: Tahera Fahimi <fahimitahera@xxxxxxxxx>
> 
> Your Git (or email app) configuration doesn't use the same name as here.
> 
> Please run ./scripts/checkpatch.pl on this patch and fix the warnings.
> 
> > 
> > ----
> > Changes in v2:
> > - Remove wrapper functions, noted by Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> > ---
> >  security/landlock/task.c | 40 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 40 insertions(+)
> > 
> > diff --git a/security/landlock/task.c b/security/landlock/task.c
> > index 849f5123610b..67528f87b7de 100644
> > --- a/security/landlock/task.c
> > +++ b/security/landlock/task.c
> > @@ -13,6 +13,7 @@
> >  #include <linux/lsm_hooks.h>
> >  #include <linux/rcupdate.h>
> >  #include <linux/sched.h>
> > +#include <net/sock.h>
> >  
> >  #include "common.h"
> >  #include "cred.h"
> > @@ -108,9 +109,48 @@ static int hook_ptrace_traceme(struct task_struct *const parent)
> >  	return task_ptrace(parent, current);
> >  }
> >  
> > +static bool unix_sock_is_scoped(struct sock *const sock,
> 
> For consistency with task_is_scoped(), you can rename this to
> sock_is_scoped().
> 
> > +				struct sock *const other)
> > +{
> > +	bool is_scoped = true;
> > +
> > +	/* get the ruleset of connecting sock*/
> 
> These comments don't help more than the following line, you can remove
> them.
> 
> > +	const struct landlock_ruleset *const dom_sock =
> 
> According to the name it looks like the domain of the socket but it is
> just the domain of the current task. Just "dom" would be clearer and
> more consistent with security/landlock/fs.c
> 
> > +		landlock_get_current_domain();
> > +
> > +	if (!dom_sock)
> > +		return true;
> > +
> > +	/* get credential of listening sock*/
> > +	const struct cred *cred_other = get_cred(other->sk_peer_cred);
> 
> We have a get but not a put call, so the credentials will never be
> freed.  The put call must be called before any return, so you
> probably need to follow the goto/error pattern.
> 
> In the context of these LSM hooks, only unix_listen() sets the "other"
> socket credential, and unix_listen() is guarded by unix_state_lock()
> which locks unix_sk(s)->lock .  When security_unix_stream_connect() or
> security_unix_may_send() are called, unix_sk(s)->lock is locked as well,
> which protects the credentials against race-conditions (TOCTOU:
> time-of-check to time-of-use).  We should then make that explicit with
> this assertion (which also documents it):
> 
> lockdep_assert_held(&unix_sk(other)->lock);
> 
> In theory it is then not required to call get_cred().  However, because
> the performance impact should be negligible and to avoid a potential
> use-after-free (not possible in theory with the current code), it would
> be safer to still call get/put.  It would be worse to have a
> use-after-free rather than an access control issue.
> 
> Another thing to keep in mind is that for this hook to be
> race-condition-free, the credential must not change anyway.  A comment
> should highlight that.
> 
> > +
> > +	if (!cred_other)
> > +		return true;
> > +
> > +	/* retrieve the landlock_rulesets */
> > +	const struct landlock_ruleset *dom_parent;
> 
> All declarations should be at the top of functions.
> 
> > +
> > +	rcu_read_lock();
> 
> No need for this RCU lock because the lock is managed by
> unix_state_lock() in this case.
> 
> > +	dom_parent = landlock_cred(cred_other)->domain;
> > +	is_scoped = domain_scope_le(dom_parent, dom_sock);
> > +	rcu_read_unlock();
> > +
> > +	return is_scoped;
> > +}
> > +
> > +static int hook_unix_stream_connect(struct sock *const sock,
> > +				    struct sock *const other,
> > +				    struct sock *const newsk)
> > +{
> > +	if (unix_sock_is_scoped(sock, other))
> > +		return 0;
> > +	return -EPERM;
> > +}
> > +
> >  static struct security_hook_list landlock_hooks[] __ro_after_init = {
> >  	LSM_HOOK_INIT(ptrace_access_check, hook_ptrace_access_check),
> >  	LSM_HOOK_INIT(ptrace_traceme, hook_ptrace_traceme),
> > +	LSM_HOOK_INIT(unix_stream_connect, hook_unix_stream_connect),
> 
> Please add a hook for security_unix_may_send() too, it should be quite
> similar, and simplify the patch's subject accordingly.
> 
> You now need to add tests (in a dedicated patch) extending
> tools/testing/selftests/landlock/ptrace_test.c (I'll rename the file
> later).
> 
> These tests should also check with unnamed and named unix sockets.  I
> guess the current code doesn't differentiate them and control all kind
> of unix sockets.  Because they must explicitly be passed, sockets
> created with socketpair(2) (i.e. unnamed socket) should never be denied.
> 
> >  };
> >  
> >  __init void landlock_add_task_hooks(void)
> > -- 
> > 2.34.1
> > 
> >