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[45.78.65.84]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-3153dd4f0f0sm6470095eec.2.2026.07.31.07.22.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 31 Jul 2026 07:22:30 -0700 (PDT) From: Chengfeng Ye To: Greg Kroah-Hartman , Jiri Slaby , Richard Watts Cc: linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, Chengfeng Ye , stable@vger.kernel.org Subject: [PATCH] tty: serialize device registration and removal Date: Fri, 31 Jul 2026 22:22:20 +0800 Message-ID: <20260731142220.3000214-1-nicoyip.dev@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit tty_cdev_add() stores a newly allocated cdev in driver->cdevs[index] before initializing and registering it. tty_unregister_device() reads and deletes the same pointer without serialization. The race can proceed as follows: registration removal ------------ ------- driver->cdevs[index] = cdev cdev_del(driver->cdevs[index]) kobject_put() frees cdev driver->cdevs[index]->ops = &tty_fops The final write accesses freed memory. Concurrent registration can also replace the slot while another caller is still using its cdev. KASAN reported: BUG: KASAN: slab-use-after-free in tty_cdev_add+0x5c7/0x670 Write of size 8 at addr ffff88810b298c48 Call Trace: tty_cdev_add+0x5c7/0x670 tty_register_device_attr+0x458/0x810 gsm_activate_mux+0x105/0x2e0 gsmld_ioctl+0x92f/0x14d0 Allocated by task 111: cdev_alloc+0x99/0x130 tty_cdev_add+0x93/0x670 Freed by task 110: cdev_dynamic_release+0x38/0x50 kobject_put+0x14d/0x280 tty_unregister_device+0x1b7/0x3c0 Add a per-driver mutex and hold it across complete device registration and removal. This prevents either transition from replacing or freeing a cdev until the other has finished. Fixes: a3a10ce3429e ("Avoid usb reset crashes by making tty_io cdevs truly dynamic") Cc: stable@vger.kernel.org Signed-off-by: Chengfeng Ye --- drivers/tty/tty_io.c | 13 ++++++++++--- include/linux/tty_driver.h | 3 +++ 2 files changed, 13 insertions(+), 3 deletions(-) diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index 6b283fd03ff8..926a9b586c7d 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -3185,7 +3185,8 @@ static int tty_cdev_add(struct tty_driver *driver, dev_t dev, * that bit is not set, this function should not be called by a tty * driver. * - * Locking: ?? + * Locking: device registration and removal are serialized by + * @driver->device_mutex. * * Return: A pointer to the struct device for this tty device (or * ERR_PTR(-EFOO) on error). @@ -3217,7 +3218,8 @@ static void tty_device_create_release(struct device *dev) * tty driver's flags have the %TTY_DRIVER_DYNAMIC_DEV bit set. If that bit is * not set, this function should not be called by a tty driver. * - * Locking: ?? + * Locking: device registration and removal are serialized by + * @driver->device_mutex. * * Return: A pointer to the struct device for this tty device (or * ERR_PTR(-EFOO) on error). @@ -3257,6 +3259,7 @@ struct device *tty_register_device_attr(struct tty_driver *driver, dev_set_drvdata(dev, drvdata); dev_set_uevent_suppress(dev, 1); + guard(mutex)(&driver->device_mutex); retval = device_register(dev); if (retval) @@ -3300,10 +3303,13 @@ EXPORT_SYMBOL_GPL(tty_register_device_attr); * If a tty device is registered with a call to tty_register_device() then * this function must be called when the tty device is gone. * - * Locking: ?? + * Locking: device registration and removal are serialized by + * @driver->device_mutex. */ void tty_unregister_device(struct tty_driver *driver, unsigned index) { + guard(mutex)(&driver->device_mutex); + device_destroy(&tty_class, MKDEV(driver->major, driver->minor_start) + index); if (!(driver->flags & TTY_DRIVER_DYNAMIC_ALLOC)) { cdev_del(driver->cdevs[index]); @@ -3338,6 +3344,7 @@ struct tty_driver *__tty_alloc_driver(unsigned int lines, struct module *owner, return ERR_PTR(-ENOMEM); kref_init(&driver->kref); + mutex_init(&driver->device_mutex); driver->num = lines; driver->owner = owner; driver->flags = flags; diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h index 1f2896e56e77..bf8c9d8ec698 100644 --- a/include/linux/tty_driver.h +++ b/include/linux/tty_driver.h @@ -7,6 +7,7 @@ #include #include #include +#include #include #include #include @@ -497,6 +498,7 @@ struct tty_operations { * @kref: reference counting. Reaching zero frees all the internals and the * driver. * @cdevs: allocated/registered character /dev devices + * @device_mutex: serializes device registration and removal * @owner: modules owning this driver. Used drivers cannot be rmmod'ed. * Automatically set by tty_alloc_driver(). * @driver_name: name of the driver used in /proc/tty @@ -532,6 +534,7 @@ struct tty_operations { struct tty_driver { struct kref kref; struct cdev **cdevs; + struct mutex device_mutex; struct module *owner; const char *driver_name; const char *name; -- 2.43.0