LDAP: Use randomized ccname for storing credentials

https://fedorahosted.org/sssd/ticket/2410

If two ldap_child processes attempt to prime the ccache at the same time
for the same domain, the ldap_child might fail with:
    [ldap_child_get_tgt_sync] (0x0040): Failed to init ccache: Internal credentials cache error
    [main] (0x0020): ldap_child_get_tgt_sync failed.

To avoid the race-condition, the ldap_child process now creates the
ccache randomized and before returning to the caller, renames the
randomized ccache to a permanent one.

Reviewed-by: Sumit Bose <sbose@redhat.com>

1 files changed, 39 insertions, 5 deletions

diff --git a/src/providers/ldap/ldap_child.c b/src/providers/ldap/ldap_child.c
index 7d793bbe4..fbdf89a26 100644
--- a/src/providers/ldap/ldap_child.c
+++ b/src/providers/ldap/ldap_child.c
@@ -168,7 +168,9 @@ static krb5_error_code ldap_child_get_tgt_sync(TALLOC_CTX *memctx,
                                                const char **ccname_out,
                                                time_t *expire_time_out)
 {
+    int fd;
     char *ccname;
+    char *ccname_dummy;
     char *realm_name = NULL;
     char *full_princ = NULL;
     char *default_realm = NULL;
@@ -185,6 +187,8 @@ static krb5_error_code ldap_child_get_tgt_sync(TALLOC_CTX *memctx,
     int kdc_time_offset_usec;
     int ret;
     TALLOC_CTX *tmp_ctx;
+    char *ccname_file_dummy;
+    char *ccname_file;
 
     krberr = krb5_init_context(&context);
     if (krberr) {
@@ -290,14 +294,34 @@ static krb5_error_code ldap_child_get_tgt_sync(TALLOC_CTX *memctx,
         goto done;
     }
 
-    ccname = talloc_asprintf(tmp_ctx, "FILE:%s/ccache_%s", DB_PATH, realm_name);
-    if (!ccname) {
-        krberr = KRB5KRB_ERR_GENERIC;
+    ccname_file_dummy = talloc_asprintf(tmp_ctx, "%s/ccache_%s_XXXXXX",
+                                        DB_PATH, realm_name);
+    ccname_file = talloc_asprintf(tmp_ctx, "%s/ccache_%s",
+                                  DB_PATH, realm_name);
+    if (ccname_file_dummy == NULL || ccname_file == NULL) {
+        ret = ENOMEM;
         goto done;
     }
-    DEBUG(SSSDBG_TRACE_INTERNAL, "keytab ccname: [%s]\n", ccname);
 
-    krberr = krb5_cc_resolve(context, ccname, &ccache);
+    fd = mkstemp(ccname_file_dummy);
+    if (fd == -1) {
+        ret = errno;
+        goto done;
+    }
+    /* We only care about creating a unique file name here, we don't
+     * need the fd
+     */
+    close(fd);
+
+    ccname_dummy = talloc_asprintf(tmp_ctx, "FILE:%s", ccname_file_dummy);
+    ccname = talloc_asprintf(tmp_ctx, "FILE:%s", ccname_file);
+    if (ccname_dummy == NULL || ccname == NULL) {
+        krberr = ENOMEM;
+        goto done;
+    }
+    DEBUG(SSSDBG_TRACE_INTERNAL, "keytab ccname: [%s]\n", ccname_dummy);
+
+    krberr = krb5_cc_resolve(context, ccname_dummy, &ccache);
     if (krberr) {
         DEBUG(SSSDBG_OP_FAILURE, "Failed to set cache name: %s\n",
                   sss_krb5_get_error_message(context, krberr));
@@ -368,6 +392,16 @@ static krb5_error_code ldap_child_get_tgt_sync(TALLOC_CTX *memctx,
     kdc_time_offset = 0;
 #endif
 
+    DEBUG(SSSDBG_TRACE_INTERNAL,
+          "Renaming [%s] to [%s]\n", ccname_file_dummy, ccname_file);
+    ret = rename(ccname_file_dummy, ccname_file);
+    if (ret == -1) {
+        ret = errno;
+        DEBUG(SSSDBG_CRIT_FAILURE,
+              "rename failed [%d][%s].\n", ret, strerror(ret));
+        goto done;
+    }
+
     krberr = 0;
     *ccname_out = talloc_steal(memctx, ccname);
     *expire_time_out = my_creds.times.endtime - kdc_time_offset;