From 445e57387e36ee73ebf43e79fe8dcf291d242945 Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Wed, 10 Jul 2024 17:18:55 +0200
Subject: [PATCH] terminal-util: when querying bg color, ensure input fd and
 output fd refer to same tty

Let's add an extra safety check: before issuing the ansi sequence to
query the bg color, let's make sure input and output fd actually
reference the same tty. because otherwise it's unlikely we'll be able to
read back the response from the tty driver.

This is mostly just paranoia.
---
 src/basic/terminal-util.c | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/src/basic/terminal-util.c b/src/basic/terminal-util.c
index 2ea502c2bcd..fb6c9177d49 100644
--- a/src/basic/terminal-util.c
+++ b/src/basic/terminal-util.c
@@ -1576,6 +1576,37 @@ void termios_disable_echo(struct termios *termios) {
         termios->c_cc[VTIME] = 0;
 }
 
+static int terminal_verify_same(int input_fd, int output_fd) {
+        assert(input_fd >= 0);
+        assert(output_fd >= 0);
+
+        /* Validates that the specified fds reference the same TTY */
+
+        if (input_fd != output_fd) {
+                struct stat sti;
+                if (fstat(input_fd, &sti) < 0)
+                        return -errno;
+
+                if (!S_ISCHR(sti.st_mode)) /* TTYs are character devices */
+                        return -ENOTTY;
+
+                struct stat sto;
+                if (fstat(output_fd, &sto) < 0)
+                        return -errno;
+
+                if (!S_ISCHR(sto.st_mode))
+                        return -ENOTTY;
+
+                if (sti.st_rdev != sto.st_rdev)
+                        return -ENOLINK;
+        }
+
+        if (!isatty_safe(input_fd)) /* The check above was just for char device, but now let's ensure it's actually a tty */
+                return -ENOTTY;
+
+        return 0;
+}
+
 typedef enum BackgroundColorState {
         BACKGROUND_TEXT,
         BACKGROUND_ESCAPE,
@@ -1736,8 +1767,9 @@ int get_default_background_color(double *ret_red, double *ret_green, double *ret
         if (!colors_enabled())
                 return -EOPNOTSUPP;
 
-        if (!isatty(STDIN_FILENO) || !isatty(STDOUT_FILENO))
-                return -EOPNOTSUPP;
+        r = terminal_verify_same(STDIN_FILENO, STDOUT_FILENO);
+        if (r < 0)
+                return r;
 
         if (streq_ptr(getenv("TERM"), "linux")) {
                 /* Linux console is black */
-- 
2.47.3