util/shm: clear mode permission bits in allocate_shm_file_pair

This ensures the file cannot be re-opened with write permissions.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3429

util/shm.c

@@ -3,6 +3,7 @@
 #include <fcntl.h>
 #include <string.h>
 #include <sys/mman.h>
+#include <sys/stat.h>
 #include <time.h>
 #include <unistd.h>
 #include <wlr/config.h>
@@ -73,6 +74,14 @@ bool allocate_shm_file_pair(size_t size, int *rw_fd_ptr, int *ro_fd_ptr) {
 
 	shm_unlink(name);
 
+	// Make sure the file cannot be re-opened in read-write mode (e.g. via
+	// "/proc/self/fd/" on Linux)
+	if (fchmod(rw_fd, 0) != 0) {
+		close(rw_fd);
+		close(ro_fd);
+		return false;
+	}
+
 	int ret;
 	do {
 		ret = ftruncate(rw_fd, size);