fix: improve file checks for procfs knobs

CodeQL checks flagged this pattern as a TOCTOU race.

Signed-off-by: Robin H. Johnson <[email protected]>

privsep-linux.c

@@ -183,25 +183,32 @@ static int set_interface_var(const char *iface, const char *var, const char *nam
 
 	/* No path traversal */
 	if (!iface[0] || !strcmp(iface, ".") || !strcmp(iface, "..") || strchr(iface, '/'))
-		goto cleanup;
+		goto errmsg;
 
+	/* If the file does NOT exist, do NOT create it; this is not an error, as we have to support old & new names */
 	if (access(spath, F_OK) != 0)
 		goto cleanup;
 
-	fp = fopen(spath, "w");
-	if (!fp) {
-		if (name)
-			flog(LOG_ERR, "failed to set %s (%u) for %s: %s", name, val, iface, strerror(errno));
-		goto cleanup;
-	}
+	// recheck to avoid TOCTOU anyway
+	int fd = open(spath, O_WRONLY, S_IRWXU);
+	if (fd == -1)
+		goto errmsg;
+
+	fp = fdopen(fd, "w");
+	if (!fp)
+		goto errmsg;
 
 	if (0 > fprintf(fp, "%u", val)) {
-		goto cleanup;
+		goto errmsg;
 	}
 
 	retval = 0;
 
+errmsg:
+	if (name && retval != 0)
+		flog(LOG_ERR, "failed to set %s (%u) for %s: %s", name, val, iface, strerror(errno));
 cleanup:
+
 	if (fp)
 		fclose(fp);