[PATCH] tty: serial: uartlite: ensure uart driver is registered

[Elodie Decerle <elodie.decerle@nokia.com>]

When two instances of uart devices are probing, a concurrency race can
occur. If one thread calls uart_register_driver function, which first
allocates and assigns memory to 'uart_state' member of uart_driver
structure, the other instance can bypass uart driver registration and
call ulite_assign. This calls uart_add_one_port, which expects the uart
driver to be fully initialized. This leads to a kernel panic due to a
null pointer dereference:

[    8.143581] BUG: kernel NULL pointer dereference, address: 00000000000002b8
[    8.156982] #PF: supervisor write access in kernel mode
[    8.156984] #PF: error_code(0x0002) - not-present page
[    8.156986] PGD 0 P4D 0
...
[    8.180668] RIP: 0010:mutex_lock+0x19/0x30
[    8.188624] Call Trace:
[    8.188629]  ? __die_body.cold+0x1a/0x1f
[    8.195260]  ? page_fault_oops+0x15c/0x290
[    8.209183]  ? __irq_resolve_mapping+0x47/0x80
[    8.209187]  ? exc_page_fault+0x64/0x140
[    8.209190]  ? asm_exc_page_fault+0x22/0x30
[    8.209196]  ? mutex_lock+0x19/0x30
[    8.223116]  uart_add_one_port+0x60/0x440
[    8.223122]  ? proc_tty_register_driver+0x43/0x50
[    8.223126]  ? tty_register_driver+0x1ca/0x1e0
[    8.246250]  ulite_probe+0x357/0x4b0 [uartlite]

Adding a mutex lock around the uart_register_driver call in the probe
function prevents this race condition and ensures that the uart driver
structure is fully initialized and registered before it is used.

Signed-off-by: Elodie Decerle <elodie.decerle@nokia.com>
Reviewed-by: Jakub Lewalski <jakub.lewalski@nokia.com>
---
 drivers/tty/serial/uartlite.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/tty/serial/uartlite.c b/drivers/tty/serial/uartlite.c
index a41e7fc373b7..460eb2032efa 100644
--- a/drivers/tty/serial/uartlite.c
+++ b/drivers/tty/serial/uartlite.c
@@ -23,6 +23,8 @@
 #include <linux/clk.h>
 #include <linux/pm_runtime.h>
 
+static DEFINE_MUTEX(uart_driver_register_lock);
+
 #define ULITE_NAME		"ttyUL"
 #if CONFIG_SERIAL_UARTLITE_NR_UARTS > 4
 #define ULITE_MAJOR             0       /* use dynamic node allocation */
@@ -880,6 +882,8 @@ static int ulite_probe(struct platform_device *pdev)
 	pm_runtime_set_active(&pdev->dev);
 	pm_runtime_enable(&pdev->dev);
 
+	mutex_lock(&uart_driver_register_lock);
+
 	if (!ulite_uart_driver.state) {
 		dev_dbg(&pdev->dev, "uartlite: calling uart_register_driver()\n");
 		ret = uart_register_driver(&ulite_uart_driver);
@@ -890,6 +894,8 @@ static int ulite_probe(struct platform_device *pdev)
 		}
 	}
 
+	mutex_unlock(&uart_driver_register_lock);
+
 	ret = ulite_assign(&pdev->dev, id, res->start, irq, pdata);
 
 	pm_runtime_mark_last_busy(&pdev->dev);
-- 
2.43.0