From: Geert Uytterhoeven <geert+renesas@xxxxxxxxxxxxxx>
When unbinding a serial driver that's being used as a serial console,
the kernel may crash with a NULL pointer dereference in a uart_*() function
called from uart_close () (e.g. uart_flush_buffer() or
uart_chars_in_buffer()).
To fix this, let uart_close() check for port->count == 0. If this is the
case, bail out early. Else tty_port_close_start() will make the port
counts inconsistent, printing out warnings like
tty_port_close_start: tty->count = 1 port count = 0.
and
tty_port_close_start: count = -1
and once uport == NULL, it will also crash.
Also fix the related crash in pr_debug() by checking for a non-NULL uport
first.
Detailed description:
On driver unbind, uart_remove_one_port() is called. Basically it;
- marks the port dead,
- calls tty_vhangup(),
- sets state->uart_port = NULL.
What will happen depends on whether the port is just in use by e.g. getty,
or was also opened as a console.
A. If the tty was not opened as a console:
- tty_vhangup() will (in __tty_hangup()):
- mark all file descriptors for this tty hung up by pointing them to
hung_up_tty_fops,
- call uart_hangup(), which sets port->count to 0.
- A subsequent uart_open() (this may be through /dev/ttyS*, or through
/dev/console if this is a serial console) will fail with -ENXIO as the
port was marked dead,
- uart_close() after the failed uart_open() will return early, as
tty_hung_up_p() (called from tty_port_close_start()) will notice it was
hung up.
B. If the tty was also opened as a console:
- tty_vhangup() will (in __tty_hangup()):
- mark non-console file descriptors for this tty hung up by pointing
them to hung_up_tty_fops,
- NOT call uart_hangup(), but instead call uart_close() for every
non-console file descriptor, so port->count will still have a
non-zero value afterwards.
- A subsequent uart_open() will fail with -ENXIO as the port was
marked dead,
- uart_close() after the failed uart_open() starts to misbehave:
- tty_hung_up_p() will not notice it was hung up,
- As port->count is non-zero, tty_port_close_start() will decrease
port->count, making the tty and port counts inconsistent. Later,
warnings like these will be printed:
tty_port_close_start: tty->count = 1 port count = 0.
and
tty_port_close_start: count = -1
- If all of this happens after state->uart_port was set to zero, a
NULL pointer dereference will happen.
Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxxxxxxx>
---
This still doesn't fix everything, but avoids the NULL pointer dereference.
In case B, port->count is still non-zero after tty_vhangup(). Hence when
uart_close() is called after a failed uart_open(), it will decrement
port->count, and, on reaching zero, it will close the port for real (incl.
calling uart_shutdown()). This doesn't seem to cause any harm, though.
Besides, if uart_close() wouldn't do that, who else would shut down the
port?
As tty_open() always calls tty_release() on failure of ->open(),
->close() is always called.
How can uart_close() after a failed uart_open() know it should not do
anything? Set a (new) flag in its port structure?
---
drivers/tty/serial/serial_core.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 21084f0b8ea4..56dda84f82a5 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1319,9 +1319,9 @@ static void uart_close(struct tty_struct *tty, struct file *filp)
uport = state->uart_port;
port = &state->port;
- pr_debug("uart_close(%d) called\n", uport->line);
+ pr_debug("uart_close(%d) called\n", uport ? uport->line : -1);
- if (tty_port_close_start(port, tty, filp) == 0)
+ if (!port->count || tty_port_close_start(port, tty, filp) == 0)
return;
/*
--
1.7.9.5
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