[PATCH] serial: core: Fix atomicity violation in uart_tiocmget

[Gui-Dong Han <2045gemini@gmail.com>]

In uart_tiocmget():
    result = uport->mctrl;
    uart_port_lock_irq(uport);
    result |= uport->ops->get_mctrl(uport);
    uart_port_unlock_irq(uport);
    ...
    return result;

In uart_update_mctrl():
    uart_port_lock_irqsave(port, &flags);
    ...
    port->mctrl = (old & ~clear) | set;
    ...
    uart_port_unlock_irqrestore(port, flags);

An atomicity violation is identified due to the concurrent execution of
uart_tiocmget() and uart_update_mctrl(). After assigning
result = uport->mctrl, the mctrl value may change in uart_update_mctrl(),
leading to a mismatch between the value returned by
uport->ops->get_mctrl(uport) and the mctrl value previously read.
This can result in uart_tiocmget() returning an incorrect value.

This possible bug is found by an experimental static analysis tool
developed by our team, BassCheck[1]. This tool analyzes the locking APIs
to extract function pairs that can be concurrently executed, and then
analyzes the instructions in the paired functions to identify possible
concurrency bugs including data races and atomicity violations. The above
possible bug is reported when our tool analyzes the source code of
Linux 5.17.

To address this issue, it is suggested to move the line
result = uport->mctrl inside the uart_port_lock block to ensure atomicity
and prevent the mctrl value from being altered during the execution of
uart_tiocmget(). With this patch applied, our tool no longer reports the
bug, with the kernel configuration allyesconfig for x86_64. Due to the
absence of the requisite hardware, we are unable to conduct runtime
testing of the patch. Therefore, our verification is solely based on code
logic analysis.

[1] https://sites.google.com/view/basscheck/

Fixes: 559c7ff4e324 ("serial: core: Use port lock wrappers")
Cc: stable@vger.kernel.org
Signed-off-by: Gui-Dong Han <2045gemini@gmail.com>
---
 drivers/tty/serial/serial_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 80085b151b34..a9e39416d877 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1085,8 +1085,8 @@ static int uart_tiocmget(struct tty_struct *tty)
 		goto out;
 
 	if (!tty_io_error(tty)) {
-		result = uport->mctrl;
 		uart_port_lock_irq(uport);
+		result = uport->mctrl;
 		result |= uport->ops->get_mctrl(uport);
 		uart_port_unlock_irq(uport);
 	}
-- 
2.34.1