Subject: Fix race condition when creating/rotating keys (#123)
Origin: upstream, v13-3-g8dbbed1 <https://github.com/latchset/tang/commit/v13-3-g8dbbed1>
Upstream-Author: Sergio Correia <scorreia@redhat.com>
Date: Wed Jun 14 10:53:20 2023 -0300

    When we create/rotate keys using either the tangd-keygen and
    tangd-rotate-keys helpers, there is a small window between the
    keys being created and then the proper ownership permissions being
    set. This also happens when there are no keys and tang creates a
    pair of keys itself.

    In certain situations, such as the keys directory having wide open
    permissions, a user with local access could exploit this race
    condition and read the keys before they are set to more restrictive
    permissions.

    To prevent this issue, we now set the default umask to 0337 before
    creating the files, so that they are already created with restrictive
    permissions; afterwards, we set the proper ownership as usual.

    Issue reported by Brian McDermott of CENSUS labs.

    Fixes CVE-2023-1672


    Reviewed-by: Sergio Arroutbi <sarroutb@redhat.com>
    Signed-off-by: Sergio Correia <scorreia@redhat.com>

--- a/src/keys.c
+++ b/src/keys.c
@@ -17,6 +17,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <sys/stat.h>
 #include <stdlib.h>
 #include <string.h>
 #include <dirent.h>
@@ -304,6 +305,9 @@
     const char** hashes = supported_hashes();
     const char* alg[] = {"ES512", "ECMR", NULL};
     char path[PATH_MAX];
+
+    /* Set default umask for file creation. */
+    umask(0337);
     for (int i = 0; alg[i] != NULL; i++) {
         json_auto_t* jwk = jwk_generate(alg[i]);
         if (!jwk) {
--- a/src/tangd-keygen
+++ b/src/tangd-keygen
@@ -27,6 +27,9 @@
 
 [ $# -eq 3 ] && sig=$2 && exc=$3
 
+# Set default umask for file creation.
+umask 0337
+
 jwe=`jose jwk gen -i '{"alg":"ES512"}'`
 [ -z "$sig" ] && sig=`echo "$jwe" | jose jwk thp -i-`
 echo "$jwe" > $1/$sig.jwk
--- a/src/tangd-rotate-keys
+++ b/src/tangd-rotate-keys
@@ -72,6 +72,10 @@
 
     # Create a new set of keys.
     DEFAULT_THP_HASH="S256"
+
+    # Set default umask for file creation.
+    umask 0337
+
     for alg in "ES512" "ECMR"; do
         json="$(printf '{"alg": "%s"}' "${alg}")"
         jwe="$(jose jwk gen --input "${json}")"