From 455ce986fa356ff43a43c0d363ba95fa152f21d5 Mon Sep 17 00:00:00 2001 From: Jiayuan Chen Date: Wed, 4 Feb 2026 15:43:20 +0800 Subject: [PATCH] serial: core: fix infinite loop in handle_tx() for PORT_UNKNOWN uart_write_room() and uart_write() behave inconsistently when xmit_buf is NULL (which happens for PORT_UNKNOWN ports that were never properly initialized): - uart_write_room() returns kfifo_avail() which can be > 0 - uart_write() checks xmit_buf and returns 0 if NULL This inconsistency causes an infinite loop in drivers that rely on tty_write_room() to determine if they can write: while (tty_write_room(tty) > 0) { written = tty->ops->write(...); // written is always 0, loop never exits } For example, caif_serial's handle_tx() enters an infinite loop when used with PORT_UNKNOWN serial ports, causing system hangs. Fix by making uart_write_room() also check xmit_buf and return 0 if it's NULL, consistent with uart_write(). Reproducer: https://gist.github.com/mrpre/d9a694cc0e19828ee3bc3b37983fde13 Signed-off-by: Jiayuan Chen Cc: stable Link: https://patch.msgid.link/20260204074327.226165-1-jiayuan.chen@linux.dev Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/serial_core.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 487756947a96..89cebdd27841 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -643,7 +643,10 @@ static unsigned int uart_write_room(struct tty_struct *tty) unsigned int ret; port = uart_port_ref_lock(state, &flags); - ret = kfifo_avail(&state->port.xmit_fifo); + if (!state->port.xmit_buf) + ret = 0; + else + ret = kfifo_avail(&state->port.xmit_fifo); uart_port_unlock_deref(port, flags); return ret; }