From da677b071dadda3700d12d037f5896b166d3546d Mon Sep 17 00:00:00 2001
From: Greg Hudson <ghudson@mit.edu>
Date: Tue, 9 Nov 2021 13:00:43 -0500
Subject: [PATCH] Avoid use after free during libkrad cleanup

libkrad client requests contain a list of references to remotes, with
no back-references or reference counts.  To prevent accesses to
dangling references during cleanup, cancel all requests on all remotes
before freeing any remotes.

Remove the code for aging out unused servers.  This code was fairly
safe as all requests referencing a remote should have completed or
timed out during an hour of disuse, but in the current design we have
no way to guarantee or check that.  The set of addresses we send
RADIUS requests to will generally be small, so aging out servers is
unnecessary.

ticket: 9035 (new)
---
 src/lib/krad/client.c   | 42 ++++++++++++++---------------------------
 src/lib/krad/internal.h |  4 ++++
 src/lib/krad/remote.c   | 11 ++++++++---
 3 files changed, 26 insertions(+), 31 deletions(-)

diff --git a/src/lib/krad/client.c b/src/lib/krad/client.c
index 6365dd1c6..810940afc 100644
--- a/src/lib/krad/client.c
+++ b/src/lib/krad/client.c
@@ -64,7 +64,6 @@ struct request_st {
 
 struct server_st {
     krad_remote *serv;
-    time_t last;
     K5_LIST_ENTRY(server_st) list;
 };
 
@@ -81,15 +80,10 @@ get_server(krad_client *rc, const struct addrinfo *ai, const char *secret,
            krad_remote **out)
 {
     krb5_error_code retval;
-    time_t currtime;
     server *srv;
 
-    if (time(&currtime) == (time_t)-1)
-        return errno;
-
     K5_LIST_FOREACH(srv, &rc->servers, list) {
         if (kr_remote_equals(srv->serv, ai, secret)) {
-            srv->last = currtime;
             *out = srv->serv;
             return 0;
         }
@@ -98,7 +92,6 @@ get_server(krad_client *rc, const struct addrinfo *ai, const char *secret,
     srv = calloc(1, sizeof(server));
     if (srv == NULL)
         return ENOMEM;
-    srv->last = currtime;
 
     retval = kr_remote_new(rc->kctx, rc->vctx, ai, secret, &srv->serv);
     if (retval != 0) {
@@ -173,28 +166,12 @@ request_new(krad_client *rc, krad_code code, const krad_attrset *attrs,
     return 0;
 }
 
-/* Close remotes that haven't been used in a while. */
-static void
-age(struct server_head *head, time_t currtime)
-{
-    server *srv, *tmp;
-
-    K5_LIST_FOREACH_SAFE(srv, head, list, tmp) {
-        if (currtime == (time_t)-1 || currtime - srv->last > 60 * 60) {
-            K5_LIST_REMOVE(srv, list);
-            kr_remote_free(srv->serv);
-            free(srv);
-        }
-    }
-}
-
 /* Handle a response from a server (or related errors). */
 static void
 on_response(krb5_error_code retval, const krad_packet *reqp,
             const krad_packet *rspp, void *data)
 {
     request *req = data;
-    time_t currtime;
     size_t i;
 
     /* Do nothing if we are already completed. */
@@ -221,10 +198,6 @@ on_response(krb5_error_code retval, const krad_packet *reqp,
     for (i = 0; req->remotes[i].remote != NULL; i++)
         kr_remote_cancel(req->remotes[i].remote, req->remotes[i].packet);
 
-    /* Age out servers that haven't been used in a while. */
-    if (time(&currtime) != (time_t)-1)
-        age(&req->rc->servers, currtime);
-
     request_free(req);
 }
 
@@ -247,10 +220,23 @@ krad_client_new(krb5_context kctx, verto_ctx *vctx, krad_client **out)
 void
 krad_client_free(krad_client *rc)
 {
+    server *srv;
+
     if (rc == NULL)
         return;
 
-    age(&rc->servers, -1);
+    /* Cancel all requests before freeing any remotes, since each request's
+     * callback data may contain references to multiple remotes. */
+    K5_LIST_FOREACH(srv, &rc->servers, list)
+        kr_remote_cancel_all(srv->serv);
+
+    while (!K5_LIST_EMPTY(&rc->servers)) {
+        srv = K5_LIST_FIRST(&rc->servers);
+        K5_LIST_REMOVE(srv, list);
+        kr_remote_free(srv->serv);
+        free(srv);
+    }
+
     free(rc);
 }
 
diff --git a/src/lib/krad/internal.h b/src/lib/krad/internal.h
index 312dc8258..b086598fb 100644
--- a/src/lib/krad/internal.h
+++ b/src/lib/krad/internal.h
@@ -120,6 +120,10 @@ kr_remote_send(krad_remote *rr, krad_code code, krad_attrset *attrs,
 void
 kr_remote_cancel(krad_remote *rr, const krad_packet *pkt);
 
+/* Cancel all requests awaiting responses. */
+void
+kr_remote_cancel_all(krad_remote *rr);
+
 /* Determine if this remote object refers to the remote resource identified
  * by the addrinfo struct and the secret. */
 krb5_boolean
diff --git a/src/lib/krad/remote.c b/src/lib/krad/remote.c
index 0f90443ce..b5dd8cd19 100644
--- a/src/lib/krad/remote.c
+++ b/src/lib/krad/remote.c
@@ -421,15 +421,20 @@ error:
     return retval;
 }
 
+void
+kr_remote_cancel_all(krad_remote *rr)
+{
+    while (!K5_TAILQ_EMPTY(&rr->list))
+        request_finish(K5_TAILQ_FIRST(&rr->list), ECANCELED, NULL);
+}
+
 void
 kr_remote_free(krad_remote *rr)
 {
     if (rr == NULL)
         return;
 
-    while (!K5_TAILQ_EMPTY(&rr->list))
-        request_finish(K5_TAILQ_FIRST(&rr->list), ECANCELED, NULL);
-
+    kr_remote_cancel_all(rr);
     free(rr->secret);
     if (rr->info != NULL)
         free(rr->info->ai_addr);
-- 
2.35.3