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>

diff --git a/docs/misc/xenstore.txt b/docs/misc/xenstore.txt
index cb8009cb686d88fcee3b3d2da2ee8cd9c3cee208..2081f20f55e449d85df477c077f191d61f6d7420 100644 (file)
--- a/docs/misc/xenstore.txt
+++ b/docs/misc/xenstore.txt
@@ -170,6 +170,9 @@ SET_PERMS           <path>|<perm-as-string>|+?
                n<domid>        no access
        See https://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

diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 06e937de6617b713ddba57e6ade224558d870438..1db9d0cc31824538ff0ee63a2dde47ab9ea85504 100644 (file)
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -464,8 +464,8 @@ 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;
@@ -1238,22 +1238,29 @@ static int do_set_perms(struct connection *conn, struct buffered_data *in)
        if (perms.num < 2)
                return EINVAL;
 
-       /* First arg is node name. */
-       /* 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);
+       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;
+
        /* Unprivileged domains may not change the owner. */
        if (domain_is_unprivileged(conn) &&
            perms.p[0].id != node->perms.p[0].id)

diff --git a/tools/xenstore/xenstored_core.h b/tools/xenstore/xenstored_core.h
index a291f15ce78320cd4de17f36aa936075103d131b..3f958c29abd5e99b9edea2d5f9938aa8049fa545 100644 (file)
--- a/tools/xenstore/xenstored_core.h
+++ b/tools/xenstore/xenstored_core.h
@@ -162,6 +162,8 @@ 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);

diff --git a/tools/xenstore/xenstored_domain.c b/tools/xenstore/xenstored_domain.c
index 76bdd46c8d910efffa82cc80b2f74f69130bb59d..e1106d90b62afee8a5eb68b45d613cd667b9a467 100644 (file)
--- a/tools/xenstore/xenstored_domain.c
+++ b/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;
@@ -576,6 +579,59 @@ 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;
@@ -597,6 +653,10 @@ 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; 
 }
 

diff --git a/tools/xenstore/xenstored_domain.h b/tools/xenstore/xenstored_domain.h
index 56ae01597475672a285a31eb7d301d36ccce96a9..259183962a9c782f1340ff4523df01265930fab6 100644 (file)
--- a/tools/xenstore/xenstored_domain.h
+++ b/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 */

diff --git a/tools/xenstore/xenstored_watch.c b/tools/xenstore/xenstored_watch.c
index 3836675459fa11c3e210b4a4e80ca3875754d661..f4e289362eb6b3e2b72986247052fa12970860dc 100644 (file)
--- a/tools/xenstore/xenstored_watch.c
+++ b/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))