Re: tty crash in tty_ldisc_receive_buf()

[Wang YanQing]

On Thu, Apr 06, 2017 at 05:04:41PM +1000, Michael Neuling wrote:
> Hi all,
> 
> We are seeing the following crash (in linux-next but has been around since at
> least v4.10). 
> 
> [  417.514499] Unable to handle kernel paging request for data at address 0x00002260
> [  417.515361] Faulting instruction address: 0xc0000000006fad80
> cpu 0x15: Vector: 300 (Data Access) at [c00000799411f890]
>     pc: c0000000006fad80: n_tty_receive_buf_common+0xc0/0xbd0
>     lr: c0000000006fad5c: n_tty_receive_buf_common+0x9c/0xbd0
>     sp: c00000799411fb10
>    msr: 900000000280b033
>    dar: 2260
>  dsisr: 40000000
>   current = 0xc0000079675d1e00
>   paca    = 0xc00000000fb0d200	 softe: 0	 irq_happened: 0x01
>     pid   = 5, comm = kworker/u56:0
> Linux version 4.11.0-rc5-next-20170405 (mikey@bml86) (gcc version 5.4.0 20160609 (Ubuntu/IBM 5.4.0-6ubuntu1~16.04.4) ) #2 SMP Thu Apr 6 00:36:46 CDT 2017
> enter ? for help
> [c00000799411fbe0] c0000000006ff968 tty_ldisc_receive_buf+0x48/0xe0
> [c00000799411fc10] c0000000007009d8 tty_port_default_receive_buf+0x68/0xe0
> [c00000799411fc50] c0000000006ffce4 flush_to_ldisc+0x114/0x130
> [c00000799411fca0] c00000000010a0fc process_one_work+0x1ec/0x580
> [c00000799411fd30] c00000000010a528 worker_thread+0x98/0x5d0
> [c00000799411fdc0] c00000000011343c kthread+0x16c/0x1b0
> [c00000799411fe30] c00000000000b4e8 ret_from_kernel_thread+0x5c/0x74
> 
> It seems the null ptr deref is in n_tty_receive_buf_common() where we do:
> 
> 		size_t tail = smp_load_acquire(&ldata->read_tail);
> 
> ldata is NULL.
> 
> We see this usually on boot but can also see it if we kill a getty attached to
> tty (which is then respawned by systemd).  It seems like we are flushing data to
> a tty at the same time as it's being torn down and restarted.
> 
> I did try the below patch which avoids the crash but locks up one of the CPUs. I
> guess the data never gets flushed if we say nothing is processed.
> 
> This is on powerpc but has also been reported by parisc.
> 
> I'm not at all familiar with the tty layer and looking at the locks, mutexes,
> semaphores and reference counting in there scares the hell out of me. 
> 
> If anyone has an idea, I'm happy to try a patch.
> 
> Regards,
> Mikey
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index bdf0e6e899..99dd757aa4 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -1673,6 +1673,10 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
>  
>  	down_read(&tty->termios_rwsem);
>  
> +	/* This probably shouldn't happen, but return 0 data processed */
> +	if (!ldata)
> +		return 0;
> +
>  	while (1) {
>  		/*
>  		 * When PARMRK is set, each input char may take up to 3 chars

Maybe your patch should looks like:
+	/* This probably shouldn't happen, but return 0 data processed */
+	if (!ldata) {
+               up_read(&tty->termios_rwsem);
+		return 0;
+       }

or

Maybe below patch should work:
@@ -1668,11 +1668,12 @@ static int
 n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
                          char *fp, int count, int flow)
 {
-       struct n_tty_data *ldata = tty->disc_data;
+       struct n_tty_data *ldata;
			           int room, n, rcvd = 0, overflow;

        down_read(&tty->termios_rwsem);

+       ldata = tty->disc_data;