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

[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@vger.kernel.org
Assisted-by: GPT-6-Astra
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 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

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

[Johan Hovold]

On Sun, Sep 27, 2026 at 02:41:54AM +0800, Chengfeng Ye wrote:
> 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.

Thanks for reporting this. The intention here was that the TTY drivers
would not be reusing a minor number while it is still in use, but that
is clearly not adhered to everywhere.

> The saved-termios
> copy in tty_init_termios() is vulnerable to the same lifetime race.

How could this happen? TTY drivers should hang up their ports during
deregistration so this shouldn't be an issue unless we have a driver bug
(which we do in n_gsm apparently).

> 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.

I'm a bit torn about this, but I think we should avoid adding another
mutex in favouring of making the saved termios reset opt-in.

Clearing the saved termios state on device registration doesn't make
much sense if a minor number may still be in use as an open TTY would
prevent the termios from being reset.

I've just posted a patch for this here:

	https://lore.kernel.org/r/20260930091938.1715754-1-johan@kernel.org

Johan

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

[Chengfeng Ye]

Hi Johan,

On Wed, Sep 30, 2026 at 5:20 PM Johan Hovold <johan@kernel.org> wrote:
>
> On Sun, Sep 27, 2026 at 02:41:54AM +0800, Chengfeng Ye wrote:
> > 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.
>
> Thanks for reporting this. The intention here was that the TTY drivers
> would not be reusing a minor number while it is still in use, but that
> is clearly not adhered to everywhere.
>
> > The saved-termios
> > copy in tty_init_termios() is vulnerable to the same lifetime race.
>
> How could this happen? TTY drivers should hang up their ports during
> deregistration so this shouldn't be an issue unless we have a driver bug
> (which we do in n_gsm apparently).

Right, I did not reproduce or otherwise establish a valid interleaving
involving tty_init_termios(), and the KASAN report only demonstrates
the use-after-free in tty_save_termios(). That statement came from an
LLM-generated draft, sorry for the confusion.

> > 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.
>
> I'm a bit torn about this, but I think we should avoid adding another
> mutex in favouring of making the saved termios reset opt-in.
>
> Clearing the saved termios state on device registration doesn't make
> much sense if a minor number may still be in use as an open TTY would
> prevent the termios from being reset.
>
> I've just posted a patch for this here:
>
>         https://lore.kernel.org/r/20260930091938.1715754-1-johan@kernel.org
>
> Johan

Thank you for preparing the fix. Making the saved-termios reset opt-in
is a much cleaner way to address the demonstrated issue.

Best regards,
Chengfeng