tools/power/x86/intel-speed-select: Harden daemon pidfile open

Avoid symlink-based pidfile clobbering by opening the pidfile with
O_NOFOLLOW and validating it with fstat() before locking/writing.

The daemon currently uses a fixed pidfile path under /tmp. A local
unprivileged user can pre-create a symlink at that path and cause a
root-run daemon instance to write into an attacker-chosen file.

Fixes: 7fd786d ("tools/power/x86/intel-speed-select: OOB daemon mode")
Signed-off-by: Ali Ahmet MEMIS <dev@unknownbbqr.xyz>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: stable@kernel.org

Author: Ali Ahmet MEMIS

tools/power/x86/intel-speed-select/isst-daemon.c

@@ -148,6 +148,7 @@ static void daemonize(char *rundir, char *pidfile)
 {
 	int pid, sid, i;
 	char str[10];
+	struct stat st;
 	struct sigaction sig_actions;
 	sigset_t sig_set;
 	int ret;
@@ -200,11 +201,17 @@ static void daemonize(char *rundir, char *pidfile)
 	if (ret == -1)
 		exit(EXIT_FAILURE);
 
-	pid_file_handle = open(pidfile, O_RDWR | O_CREAT, 0600);
+	pid_file_handle = open(pidfile, O_RDWR | O_CREAT | O_NOFOLLOW, 0600);
 	if (pid_file_handle == -1) {
 		/* Couldn't open lock file */
 		exit(1);
 	}
+
+	if (fstat(pid_file_handle, &st) == -1)
+		exit(1);
+
+	if (!S_ISREG(st.st_mode))
+		exit(1);
 	/* Try to lock file */
 #ifdef LOCKF_SUPPORT
 	if (lockf(pid_file_handle, F_TLOCK, 0) == -1) {