serial: sh-sci: Fix off-by-one error in FIFO threshold register setting

commit 2ea2e019c190ee3973ef7bcaf829d8762e56e635 upstream.

The Receive FIFO Data Count Trigger field (RTRG[6:0]) in the Receive
FIFO Data Count Trigger Register (HSRTRGR) of HSCIF can only hold values
ranging from 0-127.  As the FIFO size is equal to 128 on HSCIF, the user
can write an out-of-range value, touching reserved bits.

Fix this by limiting the trigger value to the FIFO size minus one.
Reverse the order of the checks, to avoid rx_trig becoming zero if the
FIFO size is one.

Note that this change has no impact on other SCIF variants, as their
maximum supported trigger value is lower than the FIFO size anyway, and
the code below takes care of enforcing these limits.

Fixes: a380ed461f66d1b8 ("serial: sh-sci: implement FIFO threshold register setting")
Reported-by: Linh Phung <linh.phung.jy@renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/5eff320aef92ffb33d00e57979fd3603bbb4a70f.1620648218.git.geert+renesas@glider.be
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index 06cf474072d6dfe73c9c3c0e0386a09cd184836e..3a2e312039a42558916c3c152fd123700310a1ab 100644 (file)
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -994,10 +994,10 @@ static int scif_set_rtrg(struct uart_port *port, int rx_trig)
 {
        unsigned int bits;
 
+       if (rx_trig >= port->fifosize)
+               rx_trig = port->fifosize - 1;
        if (rx_trig < 1)
                rx_trig = 1;
-       if (rx_trig >= port->fifosize)
-               rx_trig = port->fifosize;
 
        /* HSCIF can be set to an arbitrary level. */
        if (sci_getreg(port, HSRTRGR)->size) {