[PATCH v2 3/3] serial: core: reject baud_base values that overflow port->uartclk in uart_set_info()

[Hui Peng]

In uart_set_info(), new_info->baud_base is multiplied by 16 and stored in
uport->uartclk (an unsigned int):

  uport->uartclk = new_info->baud_base * 16;

While uart_set_info() checks if (uartclk == 0) and
if (new_info->baud_base < 9600), when new_info->baud_base exceeds
UINT_MAX / 16 with low bits set (for example, 0x10000001), multiplying
by 16 wraps around in 32-bit unsigned arithmetic to a small non-zero value
(16), bypassing both uartclk == 0 and new_info->baud_base < 9600 and
setting uport->uartclk = 16 (baud_base = 1, well below the required
minimum of 9600 * 16).

Reject new_info->baud_base > UINT_MAX / 16 with -EINVAL in
uart_set_info().

Tested in QEMU against Linux 7.3.0-rc3 by calling ioctl(fd, TIOCSSERIAL,
&ss) with ss.baud_base = 0x10000001 on /dev/ttyS1: on the unfixed kernel
TIOCSSERIAL succeeds (ret = 0) and wraps uport->uartclk to 16
(TIOCGSERIAL reports baud_base = 1), whereas with the fix applied
TIOCSSERIAL returns -EINVAL and preserves the existing uport->uartclk.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: 6eabce6608d6 ("serial: core: check uartclk for zero to avoid divide by zero")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@xxxxxxxxx>
---
Changes in v2:
- Split out into patch 3/3 of the series, add Cc: stable@xxxxxxxxxxxxxxx,
  and document the QEMU test method, as requested by Greg Kroah-Hartman.

 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 128fc056f5c9..4cd4fb0f7ee3 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -965,7 +965,7 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
 	}
 
 	if ((new_info->irq >= irq_get_nr_irqs()) || (new_info->irq < 0) ||
-	    (new_info->baud_base < 9600))
+	    (new_info->baud_base < 9600) || (new_info->baud_base > UINT_MAX / 16))
 		return -EINVAL;
 
 	if (change_port || change_irq) {
-- 
2.49.0