wolfsshd: wipe host key copies left behind

The shell child frees its session and the server context before it
drops to the user, so the host private key and the session keys are
wiped from its image ahead of the exec. It keeps copies of the user
name and command it still needs. A short read of a key file wipes
the buffer before freeing it.

Issue: F-8841, F-14227

Author: ejohnstown

apps/wolfsshd/wolfsshd.c

@@ -438,6 +438,8 @@ static byte* getBufferFromFile(const char* fileName, word32* bufSz, void* heap,
         readSz = (word32)WFREAD(NULL, buf, 1, fileSz, file);
         if (readSz < (size_t)fileSz) {
             WFCLOSE(NULL, file);
+            /* whatever part of a key was read */
+            WS_FORCEZERO(buf, (word32)fileSz + 1);
             WFREE(buf, heap, DYNTYPE_SSHD);
             return NULL;
         }
@@ -2962,11 +2964,33 @@ static int SHELL_Subsystem(WOLFSSHD_CONNECTION* conn, WOLFSSH* ssh,
         const char *args[] = {"-sh", NULL, NULL, NULL};
         char cmd[MAX_COMMAND_SZ];
         char shell[MAX_COMMAND_SZ];
+        char* userName;
         int ret;
 
         signal(SIGINT,  SIG_DFL);
         signal(SIGCHLD, SIG_DFL);
 
+        /* The shell needs none of the session, so drop the host key and the
+         * session keys this image inherited before giving up root. The
+         * command can live in the session, so keep a copy of it. */
+        userName = WSTRDUP(wolfSSH_GetUsername(ssh), NULL, DYNTYPE_SSHD);
+        if (forcedCmd != NULL) {
+            forcedCmd = WSTRDUP(forcedCmd, NULL, DYNTYPE_SSHD);
+            if (forcedCmd == NULL) {
+                wolfSSH_Log(WS_LOG_ERROR,
+                    "[SSHD] Error copying forced command");
+                exit(1);
+            }
+        }
+        if (userName == NULL) {
+            wolfSSH_Log(WS_LOG_ERROR, "[SSHD] Error copying user name");
+            exit(1);
+        }
+        wolfSSH_free(ssh);
+        ssh = NULL;
+        wolfSSH_CTX_free(conn->ctx);
+        conn->ctx = NULL;
+
         if (!ptyReq || forcedCmd) {
             close(stdoutPipe[0]);
             close(stderrPipe[0]);
@@ -2993,7 +3017,7 @@ static int SHELL_Subsystem(WOLFSSHD_CONNECTION* conn, WOLFSSH* ssh,
         }
 
         /* set additional groups if needed */
-        if (wolfSSHD_AuthSetGroups(conn->auth, wolfSSH_GetUsername(ssh),
+        if (wolfSSHD_AuthSetGroups(conn->auth, userName,
                 pPasswd->pw_gid) != WS_SUCCESS) {
             wolfSSH_Log(WS_LOG_ERROR, "[SSHD] Error setting groups");
             exit(1);