tools/xenstore: allow special watches for privileged callers only

The special watches "@introduceDomain" and "@releaseDomain" should be
allowed for privileged callers only, as they allow to gain information
about presence of other guests on the host. So send watch events for
those watches via privileged connections only.

In order to allow for disaggregated setups where e.g. driver domains
need to make use of those special watches add support for calling
"set permissions" for those special nodes, too.

This is part of XSA-115.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Paul Durrant <paul@xen.org>

docs/misc/xenstore.txt

@@ -170,6 +170,9 @@ SET_PERMS		<path>|<perm-as-string>|+?
 		n<domid>	no access
 	See http://wiki.xen.org/wiki/XenBus section
 	`Permissions' for details of the permissions system.
+	It is possible to set permissions for the special watch paths
+	"@introduceDomain" and "@releaseDomain" to enable receiving those
+	watches in unprivileged domains.
 
 ---------- Watches ----------
 
@@ -194,6 +197,8 @@ WATCH			<wpath>|<token>|?
 	    @releaseDomain 	occurs on any domain crash or
 				shutdown, and also on RELEASE
 				and domain destruction
+	<wspecial> events are sent to privileged callers or explicitly
+	via SET_PERMS enabled domains only.
 
 	When a watch is first set up it is triggered once straight
 	away, with <path> equal to <wpath>.  Watches may be triggered

tools/xenstore/xenstored_core.c

@@ -468,8 +468,8 @@ static int write_node(struct connection *conn, struct node *node,
 	return write_node_raw(conn, &key, node, no_quota_check);
 }
 
-static enum xs_perm_type perm_for_conn(struct connection *conn,
-				       const struct node_perms *perms)
+enum xs_perm_type perm_for_conn(struct connection *conn,
+				const struct node_perms *perms)
 {
 	unsigned int i;
 	enum xs_perm_type mask = XS_PERM_READ|XS_PERM_WRITE|XS_PERM_OWNER;
@@ -1245,22 +1245,29 @@ static int do_set_perms(struct connection *conn, struct buffered_data *in)
 	if (perms.num < 2)
 		return EINVAL;
 
+	permstr = in->buffer + strlen(in->buffer) + 1;
+	perms.num--;
+
+	perms.p = talloc_array(in, struct xs_permissions, perms.num);
+	if (!perms.p)
+		return ENOMEM;
+	if (!xs_strings_to_perms(perms.p, perms.num, permstr))
+		return errno;
+
 	/* First arg is node name. */
+	if (strstarts(in->buffer, "@")) {
+		if (set_perms_special(conn, in->buffer, &perms))
+			return errno;
+		send_ack(conn, XS_SET_PERMS);
+		return 0;
+	}
+
 	/* We must own node to do this (tools can do this too). */
 	node = get_node_canonicalized(conn, in, in->buffer, &name,
 				      XS_PERM_WRITE | XS_PERM_OWNER);
 	if (!node)
 		return errno;
 
-	permstr = in->buffer + strlen(in->buffer) + 1;
-	perms.num--;
-
-	perms.p = talloc_array(node, struct xs_permissions, perms.num);
-	if (!perms.p)
-		return ENOMEM;
-	if (!xs_strings_to_perms(perms.p, perms.num, permstr))
-		return errno;
-
 	/* Unprivileged domains may not change the owner. */
 	if (domain_is_unprivileged(conn) &&
 	    perms.p[0].id != node->perms.p[0].id)

tools/xenstore/xenstored_core.h

@@ -167,6 +167,8 @@ struct node *get_node(struct connection *conn,
 struct connection *new_connection(connwritefn_t *write, connreadfn_t *read);
 void check_store(void);
 void corrupt(struct connection *conn, const char *fmt, ...);
+enum xs_perm_type perm_for_conn(struct connection *conn,
+				const struct node_perms *perms);
 
 /* Is this a valid node name? */
 bool is_valid_nodename(const char *node);

tools/xenstore/xenstored_domain.c

@@ -41,6 +41,9 @@ static evtchn_port_t virq_port;
 
 xenevtchn_handle *xce_handle = NULL;
 
+static struct node_perms dom_release_perms;
+static struct node_perms dom_introduce_perms;
+
 struct domain
 {
 	struct list_head list;
@@ -597,6 +600,59 @@ void restore_existing_connections(void)
 {
 }
 
+static int set_dom_perms_default(struct node_perms *perms)
+{
+	perms->num = 1;
+	perms->p = talloc_array(NULL, struct xs_permissions, perms->num);
+	if (!perms->p)
+		return -1;
+	perms->p->id = 0;
+	perms->p->perms = XS_PERM_NONE;
+
+	return 0;
+}
+
+static struct node_perms *get_perms_special(const char *name)
+{
+	if (!strcmp(name, "@releaseDomain"))
+		return &dom_release_perms;
+	if (!strcmp(name, "@introduceDomain"))
+		return &dom_introduce_perms;
+	return NULL;
+}
+
+int set_perms_special(struct connection *conn, const char *name,
+		      struct node_perms *perms)
+{
+	struct node_perms *p;
+
+	p = get_perms_special(name);
+	if (!p)
+		return EINVAL;
+
+	if ((perm_for_conn(conn, p) & (XS_PERM_WRITE | XS_PERM_OWNER)) !=
+	    (XS_PERM_WRITE | XS_PERM_OWNER))
+		return EACCES;
+
+	p->num = perms->num;
+	talloc_free(p->p);
+	p->p = perms->p;
+	talloc_steal(NULL, perms->p);
+
+	return 0;
+}
+
+bool check_perms_special(const char *name, struct connection *conn)
+{
+	struct node_perms *p;
+
+	p = get_perms_special(name);
+	if (!p)
+		return false;
+
+	return perm_for_conn(conn, p) & XS_PERM_READ;
+}
+
 static int dom0_init(void) 
 { 
 	evtchn_port_t port;
@@ -618,6 +674,10 @@ static int dom0_init(void)
 
 	xenevtchn_notify(xce_handle, dom0->port);
 
+	if (set_dom_perms_default(&dom_release_perms) ||
+	    set_dom_perms_default(&dom_introduce_perms))
+		return -1;
+
 	return 0; 
 }
 

tools/xenstore/xenstored_domain.h

@@ -65,6 +65,11 @@ void domain_watch_inc(struct connection *conn);
 void domain_watch_dec(struct connection *conn);
 int domain_watch(struct connection *conn);
 
+/* Special node permission handling. */
+int set_perms_special(struct connection *conn, const char *name,
+		      struct node_perms *perms);
+bool check_perms_special(const char *name, struct connection *conn);
+
 /* Write rate limiting */
 
 #define WRL_FACTOR   1000 /* for fixed-point arithmetic */

tools/xenstore/xenstored_watch.c

@@ -133,6 +133,10 @@ void fire_watches(struct connection *conn, const void *ctx, const char *name,
 
 	/* Create an event for each watch. */
 	list_for_each_entry(i, &connections, list) {
+		/* introduce/release domain watches */
+		if (check_special_event(name) && !check_perms_special(name, i))
+			continue;
+
 		list_for_each_entry(watch, &i->watches, list) {
 			if (exact) {
 				if (streq(name, watch->node))