[PATCH v7 1/2] serial: core: fix baud rate fallback in uart_get_baud_rate()

[Hui Peng <benquike@gmail.com>]

When uart_get_baud_rate() is called with a baud rate exceeding the port's
maximum supported speed (port->uartclk / 16), it clips baud to [min,
max - 1] and encodes it into termios via tty_termios_encode_baud_rate().

However, because the loop bound is for (try = 0; try < 2; try++), the
loop terminates immediately after try == 1 without re-evaluating
baud = tty_termios_baud_rate(termios) for the clipped rate, hitting
WARN_ON(1) and returning 0, which then triggers a fatal divide-by-zero
(Oops: divide error) in uart_get_divisor():

  WARNING: drivers/tty/serial/serial_core.c:548 at uart_get_baud_rate+0x136/0x260
  [ ... ]
  divide error: 0000 [#1] PREEMPT SMP KASAN

Increase the retry count in uart_get_baud_rate() from 2 to 3 iterations so
that clipped baud rates are re-evaluated in the third iteration.

Tested in QEMU against Linux 7.3.0-rc3 by setting B4000000 on /dev/ttyS0 via
tcsetattr(): on the unfixed kernel it triggers WARN_ON(1) and divide-by-zero
Oops, whereas with this fix applied uart_get_baud_rate() smoothly falls back
to 115200 without error.

Fixes: 091ea8e5d34e ("serial: core: prevent division by zero by always returning non-zero baud rate")
Cc: stable@vger.kernel.org
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Assisted-by: LLM
Signed-off-by: Hui Peng <benquike@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 f91bcfa30113..6ed0195e6912 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -470,7 +470,7 @@ unsigned int uart_get_baud_rate(struct uart_port *port, struct ktermios *termi
 	 * Ask the low level driver to verify the baud rate if it can't
 	 * then it will have encode_baud_rate set the Closet else .
 	 */
-	for (try = 0; try < 2; try++) {
+	for (try = 0; try < 3; try++) {
 		baud = tty_termios_baud_rate(termios);

 		/*
-- 
2.47.3