[PATCH v4 3/3] serial: core: reject baud_base values that overflow port->uartclk in uart_set_info()

[Hui Peng <benquike@gmail.com>]

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 before multiplying by 16 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@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@gmail.com>
---
Changes in v4:
- Check new_info->baud_base > UINT_MAX / 16 before multiplying by 16,
  as suggested by Ilpo Järvinen.

Changes in v3:
- Add missing #include <linux/limits.h> for UINT_MAX and move the
  new_info->baud_base > UINT_MAX / 16 check to the uartclk == 0 check, as
  suggested by Ilpo Järvinen.

 drivers/tty/serial/serial_core.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 95774b0f1484..75bb1eaa27e7 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -16,6 +16,7 @@
 #include <linux/console.h>
 #include <linux/gpio/consumer.h>
 #include <linux/kernel.h>
+#include <linux/limits.h>
 #include <linux/of.h>
 #include <linux/pm_runtime.h>
 #include <linux/proc_fs.h>
@@ -931,8 +932,13 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
 	old_custom_divisor = uport->custom_divisor;
 
 	if (!(uport->flags & UPF_FIXED_PORT)) {
-		unsigned int uartclk = new_info->baud_base * 16;
+		unsigned int uartclk;
+
 		/* check needs to be done here before other settings made */
+		if (new_info->baud_base > UINT_MAX / 16)
+			return -EINVAL;
+
+		uartclk = new_info->baud_base * 16;
 		if (uartclk == 0)
 			return -EINVAL;
 	}
-- 
2.55.0.1082.g2b9226bbc0-goog