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serial: 8250: Fix RTS modem control while in rs485 mode

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Commit f45709df77 ("serial: 8250: Don't touch RTS modem control while
in rs485 mode") sought to prevent user space from interfering with rs485
communication by ignoring a TIOCMSET ioctl() which changes RTS polarity.

It did so in serial8250_do_set_mctrl(), which turns out to be too deep
in the call stack:  When a uart_port is opened, RTS polarity is set by
the rs485-aware function uart_port_dtr_rts().  It calls down to
serial8250_do_set_mctrl() and that particular RTS polarity change should
*not* be ignored.

The user-visible result is that on 8250_omap ports which use rs485 with
inverse polarity (RTS bit in MCR register is 1 to receive, 0 to send),
a newly opened port initially sets up RTS for sending instead of
receiving.  That's because omap_8250_startup() sets the cached value
up->mcr to 0 and omap_8250_restore_regs() subsequently writes it to the
MCR register.  Due to the commit, serial8250_do_set_mctrl() preserves
that incorrect register value:

do_sys_openat2
  do_filp_open
    path_openat
      vfs_open
        do_dentry_open
	  chrdev_open
	    tty_open
	      uart_open
	        tty_port_open
		  uart_port_activate
		    uart_startup
		      uart_port_startup
		        serial8250_startup
			  omap_8250_startup # up->mcr = 0
			uart_change_speed
			  serial8250_set_termios
			    omap_8250_set_termios
			      omap_8250_restore_regs
			        serial8250_out_MCR # up->mcr written
		  tty_port_block_til_ready
		    uart_dtr_rts
		      uart_port_dtr_rts
		        serial8250_set_mctrl
			  omap8250_set_mctrl
			    serial8250_do_set_mctrl # mcr[1] = 1 ignored

Fix by intercepting RTS changes from user space in uart_tiocmset()
instead.

Link: https://lore.kernel.org/linux-serial/20211027111644.1996921-1-baocheng.su@siemens.com/
Fixes: f45709df77 ("serial: 8250: Don't touch RTS modem control while in rs485 mode")
Cc: Chao Zeng <chao.zeng@siemens.com>
Cc: stable@vger.kernel.org # v5.7+
Reported-by: Su Bao Cheng <baocheng.su@siemens.com>
Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Tested-by: Su Bao Cheng <baocheng.su@siemens.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/21170e622a1aaf842a50b32146008b5374b3dd1d.1637596432.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Lukas Wunner 2021-11-22 16:58:24 +01:00 committed by Greg Kroah-Hartman
parent 4e9679738a
commit f85e04503f
2 changed files with 5 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
drivers/tty/serial/8250/8250_port.c
View File

@ -2024,13 +2024,6 @@ void serial8250_do_set_mctrl(struct uart_port *port, unsigned int mctrl)
struct uart_8250_port *up = up_to_u8250p(port);
unsigned char mcr;
if (port->rs485.flags & SER_RS485_ENABLED) {
if (serial8250_in_MCR(up) & UART_MCR_RTS)
mctrl |= TIOCM_RTS;
else
mctrl &= ~TIOCM_RTS;
}
mcr = serial8250_TIOCM_to_MCR(mctrl);
mcr = (mcr & up->mcr_mask) | up->mcr_force | up->mcr;

5
drivers/tty/serial/serial_core.c
View File

@ -1075,6 +1075,11 @@ uart_tiocmset(struct tty_struct *tty, unsigned int set, unsigned int clear)
goto out;
if (!tty_io_error(tty)) {
if (uport->rs485.flags & SER_RS485_ENABLED) {
set &= ~TIOCM_RTS;
clear &= ~TIOCM_RTS;
}
uart_update_mctrl(uport, set, clear);
ret = 0;
}