答复: 答复: 答复: [PATCH][v4] tty: fix race between flush_to_ldisc and tty_open

["Li,Rongqing" <lirongqing@baidu.com>]

> -----邮件原件-----
> 发件人: Greg KH [mailto:gregkh@linuxfoundation.org]
> 发送时间: 2019年1月31日 14:52
> 收件人: Li,Rongqing <lirongqing@baidu.com>
> 抄送: jslaby@suse.com; linux-kernel@vger.kernel.org; gkohli@codeaurora.org;
> linux-serial@vger.kernel.org
> 主题: Re: 答复: 答复: [PATCH][v4] tty: fix race between flush_to_ldisc and
> tty_open
> 
> On Thu, Jan 31, 2019 at 02:15:35AM +0000, Li,Rongqing wrote:
> >
> >
> > > -----邮件原件-----
> > > 发件人: Greg KH [mailto:gregkh@linuxfoundation.org]
> > > 发送时间: 2019年1月30日 21:17
> > > 收件人: Li,Rongqing <lirongqing@baidu.com>
> > > 抄送: jslaby@suse.com; linux-kernel@vger.kernel.org;
> > > gkohli@codeaurora.org; linux-serial@vger.kernel.org
> > > 主题: Re: 答复: [PATCH][v4] tty: fix race between flush_to_ldisc and
> > > tty_open
> > >
> > > On Wed, Jan 30, 2019 at 12:48:42PM +0000, Li,Rongqing wrote:
> > > >
> > > >
> > > > > -----邮件原件-----
> > > > > 发件人: linux-kernel-owner@vger.kernel.org
> > > > > [mailto:linux-kernel-owner@vger.kernel.org] 代表 Greg KH
> > > > > 发送时间: 2019年1月30日 18:19
> > > > > 收件人: Li,Rongqing <lirongqing@baidu.com>
> > > > > 抄送: jslaby@suse.com; linux-kernel@vger.kernel.org;
> > > > > gkohli@codeaurora.org
> > > > > 主题: Re: [PATCH][v4] tty: fix race between flush_to_ldisc and
> > > > > tty_open
> > > > >
> > > > > On Fri, Jan 18, 2019 at 05:27:17PM +0800, Li RongQing wrote:
> > > > > > There still is a race window after the commit b027e2298bd588
> > > > > > ("tty: fix data race between tty_init_dev and flush of buf"),
> > > > > > and we encountered this crash issue if receive_buf call comes
> > > > > > before tty initialization completes in n_tty_open and
> > > > > > tty->driver_data may be NULL.
> > > > > >
> > > > > > CPU0                                    CPU1
> > > > > > ----                                    ----
> > > > > >                                  n_tty_open
> > > > > >                                    tty_init_dev
> > > > > >                                      tty_ldisc_unlock
> > > > > >                                        schedule
> flush_to_ldisc
> > > > > > receive_buf
> > > > > >   tty_port_default_receive_buf
> > > > > >    tty_ldisc_receive_buf
> > > > > >     n_tty_receive_buf_common
> > > > > >       __receive_buf
> > > > > >        uart_flush_chars
> > > > > >         uart_start
> > > > > >         /*tty->driver_data is NULL*/
> > > > > >                                    tty->ops->open
> > > > > >                                    /*init tty->driver_data*/
> > > > > >
> > > > > > it can be fixed by extending ldisc semaphore lock in
> > > > > > tty_init_dev to driver_data initialized completely after
> > > > > > tty->ops->open(), but this will lead to put lock on one
> > > > > > function and unlock in some other function, and hard to
> > > > > > maintain, so fix this race only by checking
> > > > > > tty->driver_data when receiving, and return if
> > > > > > tty->tty->driver_data
> > > > > > is NULL
> > > > > >
> > > > > > Signed-off-by: Wang Li <wangli39@baidu.com>
> > > > > > Signed-off-by: Zhang Yu <zhangyu31@baidu.com>
> > > > > > Signed-off-by: Li RongQing <lirongqing@baidu.com>
> > > > > > ---
> > > > > > V4: add version information
> > > > > > V3: not used ldisc semaphore lock, only checking
> > > > > > tty->driver_data with NULL
> > > > > > V2: fix building error by EXPORT_SYMBOL tty_ldisc_unlock
> > > > > > V1: extend ldisc lock to protect that tty->driver_data is
> > > > > > inited
> > > > > >
> > > > > > drivers/tty/tty_port.c | 3 +++
> > > > > >  1 file changed, 3 insertions(+)
> > > > > >
> > > > > > diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
> > > > > > index
> > > > > > 044c3cbdcfa4..86d0bec38322 100644
> > > > > > --- a/drivers/tty/tty_port.c
> > > > > > +++ b/drivers/tty/tty_port.c
> > > > > > @@ -31,6 +31,9 @@ static int
> > > > > > tty_port_default_receive_buf(struct
> > > > > > tty_port
> > > > > *port,
> > > > > >  	if (!tty)
> > > > > >  		return 0;
> > > > > >
> > > > > > +	if (!tty->driver_data)
> > > > > > +		return 0;
> > > > > > +
> > > > >
> > > > > How is this working?  What is setting driver_data to NULL to
> > > > > "stop" this
> > > race?
> > > > >
> > > >
> > > >
> > > > if tty->driver_data is NULL and return,
> > > > tty_port_default_receive_buf will not step to uart_start which
> > > > access tty->driver_data and trigger panic before tty_open, so it
> > > > can fix the system panic
> > > >
> > > > > There's no requirement that a tty driver set this field to NULL
> > > > > when it is
> > > "done"
> > > > > with the tty device, so I think you are just getting lucky in
> > > > > that your specific driver happens to be doing this.
> > > > >
> > > >
> > > > when tty_open is running, tty is allocated by kzalloc in
> > > > tty_init_dev which called by tty_open_by_driver, tty is inited to
> > > > 0
> > > >
> > > > > What driver are you testing this against?
> > > > >
> > > >
> > > > 8250
> > >
> > > Ok, as this is specific to the uart core, how about this patch instead:
> > >
> > > diff --git a/drivers/tty/serial/serial_core.c
> > > b/drivers/tty/serial/serial_core.c
> > > index 5c01bb6d1c24..b56a6250df3f 100644
> > > --- a/drivers/tty/serial/serial_core.c
> > > +++ b/drivers/tty/serial/serial_core.c
> > > @@ -130,6 +130,9 @@ static void uart_start(struct tty_struct *tty)
> > >  	struct uart_port *port;
> > >  	unsigned long flags;
> > >
> > > +	if (!state)
> > > +		return;
> > > +
> > >  	port = uart_port_lock(state, flags);
> > >  	__uart_start(tty);
> > >  	uart_port_unlock(port, flags);
> >
> >
> > If move the check into uart_start, i am afraid that it maybe not fully
> > fix this issue, Since n_tty_receive_buf_common maybe call
> > n_tty_check_throttle/ tty_unthrottle_safe which maybe use the
> > tty->driver_data
> >
> > if tty is not fully opened, I think no gain to step into more function
> 
> But as I said, the tty core has no knowledge of the "driver_data", field.  It
> does not know if a driver really is even using that field, so it means nothing to
> the tty core, so it can not check it.  Your specific tty driver does happen to use
> it, so it can check it.
> 
> If you also need to check this in unthrottle, how about this patch too?
> Does the combination of these two patches solve the problem for your
> systems?
> 
> thanks,
> 
> greg k-h
> 

Thanks for you explanation, I see now
Your suggestion should work, I will send V5 based on your suggestion

-RongQing

> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 5c01bb6d1c24..e33d4c181123 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -727,6 +727,9 @@ static void uart_unthrottle(struct tty_struct *tty)
>  	upstat_t mask = UPSTAT_SYNC_FIFO;
>  	struct uart_port *port;
> 
> +	if (!state)
> +		return;
> +
>  	port = uart_port_ref(state);
>  	if (!port)
>  		return;