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Date:	Fri, 19 Sep 2014 08:56:08 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Robert Święcki <robert@...ecki.net>,
	linux-kernel@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>
CC:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Subject: Re: Div-by-zero in the 8250 serial driver (3.17-rc5)

[ +cc Greg Kroah-Hartman, AlanC ]

On 09/18/2014 10:57 AM, Robert Święcki wrote:
> Hi,
> 
> # setserial /dev/ttyS0 spd_hi baud_base 38400
> 
> Entering kdb (current=0xffff8805ee033200, pid 1798) on processor 9 Oops: (null)
> due to oops @ 0xffffffff8149c01e
> .CPU: 9 PID: 1798 Comm: setserial Tainted: G        W I
> 3.17.0-031700rc5-generic-201409151105
> task: ffff8805ee033200 ti: ffff8800e39e0000 task.ti: ffff8800e39e0000
> RIP: 0010:[<ffffffff8149c01e>]  [<ffffffff8149c01e>] uart_get_divisor+0x1e/0x40
> RSP: 0018:ffff8800e39e3c30  EFLAGS: 00010206
> RAX: 0000000000096000 RBX: ffffffff81fab060 RCX: 0000000000000006
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81fab060
> RBP: ffff8800e39e3c78 R08: 000000000000000a R09: 00000000000003d2
> R10: 0000000000000000 R11: 00000000000003d1 R12: 0000000000000013
> R13: ffff8805f0748d2c R14: 0000000000000000 R15: 0000000000000010
> FS:  00007f0fa653d740(0000) GS:ffff880613920000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f0fa6003330 CR3: 00000000365f8000 CR4: 00000000000007e0
> Stack:
>  ffffffff814a2978 ffffffff81fab060 0000000000000010 ffff8800e39e3ca8
>  ffff8805eca08000 ffff8805f0748c00 ffff8800e39e3dc0 ffffffff81fab060
>  0000000000000010 ffff8800e39e3c88 ffffffff814a2d55 ffff8800e39e3ca8
> Call Trace:
>  [<ffffffff814a2978>] ? serial8250_do_set_termios+0xd8/0x490
>  [<ffffffff814a2d55>] serial8250_set_termios+0x25/0x30
>  [<ffffffff8149dce2>] uart_change_speed+0x52/0xb0
>  [<ffffffff8149ec9c>] uart_set_info+0x20c/0x5f0
>  [<ffffffff8149c75a>] ? do_uart_get_info+0xfa/0x1a0
>  [<ffffffff8149fbdf>] uart_ioctl+0x14f/0x220
>  [<ffffffff814804d8>] tty_ioctl+0x298/0x8f0
>  [<ffffffff811fc025>] do_vfs_ioctl+0x75/0x2c0
>  [<ffffffff811fc301>] SyS_ioctl+0x91/0xb0
>  [<ffffffff817a436d>] system_call_fastpath+0x1a/0x1f
> Code: c0 00 00 00 c3 0f 1f 84 00 00 00 00 00 66 66 66 66 90 55 81 fe
> 00 96 00 00 48 89 e5 74 17 c1 e6 04 31 d2 89 f0 d1 e8 03 47 58 5d <f7>
> f6 c3 0f 1f 80 00 00 00 00 8b 87 b8 00 00 00 25 30 10 00 00

Hi Robert,

Thanks for the report.

I was able to reproduce this crash on all recent kernels I tested. The
patch below fixed the problem for me. If possible, please test the patch
on your system.

Regards,
Peter Hurley

--- >% ---
Subject: [PATCH] serial: Fix divide-by-zero fault in uart_get_divisor()

uart_get_baud_rate() will return baud == 0 if the max rate is set
to the "magic" 38400 rate and the SPD_* flags are also specified.
On the first iteration, if the current baud rate is higher than the
max, the baud rate is clamped at the max (which in the degenerate
case is 38400). On the second iteration, the now-"magic" 38400 baud
rate selects the possibly higher alternate baud rate indicated by
the SPD_* flag. Since only two loop iterations are performed, the
loop is exited, a kernel WARNING is generated and a baud rate of
0 is returned.

Reproducible with:
 setserial /dev/ttyS0 spd_hi base_baud 38400

Only perform the "magic" 38400 -> SPD_* baud transform on the first
loop iteration, which prevents the degenerate case from recognizing
the clamped baud rate as the "magic" 38400 value.

Reported-by: Robert Święcki <robert@...ecki.net>
Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
 drivers/tty/serial/serial_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 66e2de6..d0a5fc9 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -359,7 +359,7 @@ uart_get_baud_rate(struct uart_port *port, struct ktermios *termios,
 		 * The spd_hi, spd_vhi, spd_shi, spd_warp kludge...
 		 * Die! Die! Die!
 		 */
-		if (baud == 38400)
+		if (try == 0 && baud == 38400)
 			baud = altbaud;
 
 		/*
-- 
2.1.0




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