[PATCH 1/5] serial: core: fix division by zero if old termios is present but has invalid rate

[Hugo Villeneuve <hugo@hugovil.com>]

From: Hugo Villeneuve <hvilleneuve@dimonoff.com>

If the old baud rate termios is present, it is implicitely assumed to be
valid. However, if base_baud is lowered via TIOCSSERIAL, and the new
max value is lower than the old baud rate, the later will become invalid,
and uart_get_baud_rate() loop will exit, and return a baud rate of zero,
causing this:

  WARNING: drivers/tty/serial/serial_core.c:547 at
    uart_get_baud_rate+0x104/0x180, CPU#0: uart_baud_test/782
  ...
  Division by zero in kernel.
  CPU: 0 UID: 0 PID: 782 Comm: uart_baud_test Tainted... 7.3.0-rc6-...
  Tainted: [W]=WARN
  Hardware name: Freescale i.MX6 Ultralite (Device Tree)
  Call trace:
   unwind_backtrace from show_stack+0x10/0x14
   show_stack from dump_stack_lvl+0x50/0x64
   dump_stack_lvl from Ldiv0_64+0x8/0x18
   Ldiv0_64 from div64_u64+0xb0/0xd4
   div64_u64 from uart_update_timeout+0x3c/0x44
   uart_update_timeout from sc16is7xx_set_termios+0x308/0x428 [sc16is7xx]
   sc16is7xx_set_termios [sc16is7xx] from uart_change_line_settings+0x34/0x1a8
   uart_change_line_settings from uart_set_termios+0x98/0x13c
   uart_set_termios from tty_set_termios+0x154/0x208
   ...

Reproducing scenario:
  set serial port speed to B115200
  set baud_base to 9600 via TIOCSSERIAL:
    this will lower maximum allowed baud rate to 9600
  call tcsetattr() with B57600 (old = B115200)
    uart_get_baud_rate() will return 0

Fix by running the loop an additional iteration, so to always make sure to
return a valid (clipped) baud rate if old is present but invalid.

Update confusing comments about @termios structure update: it will be
updated only if the desired baud rate is invalid, and also if @old
contained B0.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reported-by: Hui Peng <benquike@gmail.com>
Link: https://lore.kernel.org/all/20260930125901.778868-1-benquike@gmail.com/
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
---
 drivers/tty/serial/serial_core.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index c0deec0e6af41..c3c1a1d76405d 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -465,12 +465,9 @@ EXPORT_SYMBOL(uart_update_timeout);
  * magic 38400 baud rate (with spd_* flags), and mapping the %B0 rate to 9600
  * baud or min argument, whichever is greater.
  *
- * If the new baud rate is invalid, try the @old termios setting. If it's still
- * invalid, clip to the nearest chip supported rate.
- *
- * The @termios structure is updated to reflect the baud rate we're actually
- * going to be using. Don't do this for the case where B0 is requested ("hang
- * up").
+ * If the desired baud rate is invalid, try the @old termios setting. If @old
+ * termios is absent or if it's still invalid, clip to the nearest chip
+ * supported rate. For these cases, @termios is updated with the fallback rate.
  *
  * Locking: caller dependent
  */
@@ -501,7 +498,7 @@ uart_get_baud_rate(struct uart_port *port, struct ktermios *termios,
 		break;
 	}
 
-	for (try = 0; try < 2; try++) {
+	for (try = 0; try < 3; try++) {
 		baud = tty_termios_baud_rate(termios);
 
 		/*
-- 
2.47.3