[PATCH] tty: Serialize saved termios access with device registration

[Chengfeng Ye]

tty_register_device_attr() frees saved termios data when reusing a minor
number without serializing with tty_init_termios() or tty_save_termios().
The tty_mutex held by the open and close paths does not protect against
this registration-side free.

For example, an n_gsm mux reconfiguration can register a device while its
previous tty is being released. tty_save_termios() loads the saved pointer,
registration clears the array entry and frees the object, and
tty_save_termios() then writes through its stale pointer. The saved-termios
copy in tty_init_termios() is vulnerable to the same lifetime race.

KASAN reported:

  BUG: KASAN: slab-use-after-free in tty_save_termios+0x39a/0x3d0
  Write of size 44 at addr ffff8881012e8180 by task poc/114

  Call Trace:
   tty_save_termios+0x39a/0x3d0
   release_tty+0xb3/0x7a0
   tty_release_struct+0xa0/0xd0
   tty_release+0xc1b/0x11b0
   __fput+0x2f8/0x9e0
   fput_close_sync+0xe2/0x190
   __x64_sys_close+0x78/0xd0

  Allocated by task 110:
   tty_save_termios+0x2c1/0x3d0
   release_tty+0xb3/0x7a0
   tty_release_struct+0xa0/0xd0
   tty_release+0xc1b/0x11b0

  Freed by task 113:
   kfree+0x131/0x3c0
   tty_register_device_attr+0x498/0x8b0
   gsm_activate_mux+0xfb/0x210
   gsmld_ioctl+0x92f/0x14d0
   tty_ioctl+0x915/0x1240
   __x64_sys_ioctl+0x134/0x1c0

Serialize the saved-termios copies and reset with a private mutex. Taking
tty_mutex during registration would invert existing driver lock ordering:
UART registration holds port->mutex, while tty_find_polling_driver() holds
tty_mutex when calling uart_poll_init(), which takes port->mutex. Keep the
new lock confined to the saved-termios operations, with no driver callbacks
inside the critical sections.

Fixes: 93857edd9829 ("tty: reset termios state on device registration")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: GPT-6-Astra
Signed-off-by: Chengfeng Ye <nicoyip.dev@xxxxxxxxx>
---
 drivers/tty/tty_io.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 48569035da56..26bdc4560a4c 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -143,6 +143,7 @@ LIST_HEAD(tty_drivers);			/* linked list of tty drivers */
 
 /* Mutex to protect creating and releasing a tty */
 DEFINE_MUTEX(tty_mutex);
+static DEFINE_MUTEX(tty_termios_mutex);
 
 static ssize_t tty_read(struct kiocb *, struct iov_iter *);
 static ssize_t tty_write(struct kiocb *, struct iov_iter *);
@@ -1219,6 +1220,8 @@ void tty_init_termios(struct tty_struct *tty)
 	if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
 		tty->termios = tty->driver->init_termios;
 	else {
+		guard(mutex)(&tty_termios_mutex);
+
 		/* Check for lazy saved data */
 		tp = tty->driver->termios[idx];
 		if (tp != NULL) {
@@ -1441,6 +1444,8 @@ void tty_save_termios(struct tty_struct *tty)
 	if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
 		return;
 
+	guard(mutex)(&tty_termios_mutex);
+
 	/* Stash the termios data */
 	tp = tty->driver->termios[idx];
 	if (tp == NULL) {
@@ -3242,10 +3247,12 @@ struct device *tty_register_device_attr(struct tty_driver *driver,
 		 * Free any saved termios data so that the termios state is
 		 * reset when reusing a minor number.
 		 */
-		tp = driver->termios[index];
-		if (tp) {
-			driver->termios[index] = NULL;
-			kfree(tp);
+		scoped_guard(mutex, &tty_termios_mutex) {
+			tp = driver->termios[index];
+			if (tp) {
+				driver->termios[index] = NULL;
+				kfree(tp);
+			}
 		}
 
 		retval = tty_cdev_add(driver, devt, index, 1);
-- 
2.43.0