Re: uclinux 2.4.32 panic

Steve Clark
2006-05-25T14:11:29Z

Hi Willy,

Thanks for your analysis - see my comments below;

Steve

Willy TARREAU wrote:
> On Wed, May 24, 2006 at 05:24:33PM -0400, Steve Clark wrote:
> (...)
>
>>>>>>LR; 008727cc <rs_write+148/294>
>>>>
>>>>>>PC; 0090e5fc <memmove+25c/460> <=====
>>>>
>>>>Trace; 0090e3a0 <memcpy+0/0>
>>>>Trace; 008727cc <rs_write+148/294>
>>>>
>>>>>>r8; 00956228 <tmp_buf+0/1000>
>>>>
>>>>Trace; 00872684 <rs_write+0/294>
>
> (...)
>
>
>>The hardware is the ActionTec DualPC Modem it has a conexant cx82100
>>arm processor.
>>I can reproduce it at will by connecting to the internet and running
>>nttcp thru it at the same time
>>I am scping file both ways from and to, and then finally starting a
>>getty on /dev/ttyS0, the modem is
>>at ttyS1 and also ttyS0 is where all the kernel printk messages come out.
>>
>>When I start the getty if I have all the other traffic going it
>>usually will panic in under a minute. IF I don't
>>have the getty running it will run for hours and not panic.
>>
>>
>>
>>2.4.32-uc0 with patches from
>>http://www.bettina-attack.de/jonny/view.php/projects/uclinux_on_cx82100/
>
>
> Well, at least the cnxtserial.c file looks suspicious to me :
>
> static int rs_write(struct tty_struct * tty, int from_user,
> const unsigned char *buf, int count)
> {
> int c, total = 0;
> unsigned long flags;
> struct cnxt_serial *info = (struct cnxt_serial *)tty->driver_data;
> ^^^^^
>
> if (serial_paranoia_check(info,tty->device, "rs_write"))
> ^^^^^
> return 0;
>
> if (!tty || !info->xmit_buf)
> return 0;
>
> => tty already referenced twice before the check. Either the check is
> useless, or the person who wrote it had a good reason for it which
> was not considered when writing the two lines above. I would suggest
> to start from something like this :
>
> static int rs_write(struct tty_struct * tty, int from_user,
> const unsigned char *buf, int count)
> {
> int c, total = 0;
> unsigned long flags;
> struct cnxt_serial *info;
>
> if (!tty)
> return 0;
>
> info = (struct cnxt_serial *)tty->driver_data;
> if (serial_paranoia_check(info, tty->device, "rs_write"))
> return 0;
>
> if (!info->xmit_buf)
> return 0;

I did this - actually I matched to serial.c
>
>
> Further :
>
> c = MIN(count, MIN(SERIAL_XMIT_SIZE - info->xmit_cnt - 1,
> SERIAL_XMIT_SIZE - info->xmit_head));
> if (c <= 0)
> break;
>
> => info->xmit_cnt and info->xmit_head are signed ints. If you encounter
> memory corruption (eg: during your ethernet transfers) and those get
> negative, nothing prevents the buffer from being overwritten past the
> end.
>
> Further :
>
> if (from_user) {
> down(&tmp_buf_sem);
> copy_from_user(tmp_buf, buf, c);
> c = MIN(c, MIN(SERIAL_XMIT_SIZE - info->xmit_cnt - 1,
> SERIAL_XMIT_SIZE - info->xmit_head));
> ^^^^^^^^^^^^^^^^^^
> memcpy(info->xmit_buf + info->xmit_head, tmp_buf, c);
> up(&tmp_buf_sem);
> } else
>
> => What the hell is this ? c was assigned the same value above, so
> we get :
> c = MIN(MIN(count, MIN(SERIAL_XMIT_SIZE - info->xmit_cnt - 1,
> SERIAL_XMIT_SIZE - info->xmit_head)),
> MIN(SERIAL_XMIT_SIZE - info->xmit_cnt - 1,
> SERIAL_XMIT_SIZE - info->xmit_head));
>
> I'm not sure this was what the developper originally intented to do,
> but although useless, it does not seem incorrect. However, I don't
> know if he wanted to further reduce the buffer for any reason.
>
> Also, it appears that nothing prevents any code running outside the
> loop from changing info->xmit_buf between the restore_flags() and
> the cli(). I don't know if this is functionnaly possible, but at
> least it is possible by memory corruption (eg: padding too large
> for a packet and writing zeroes past the end of one buffer).
>
I moved the cli() outside of the while loop - similar to serial.c

> You should definitely add printks or at least double checks
> everywhere within this loop I think.
>
I did and info->xmit_head is getting clobbered!
see below:
> That's all I can tell, I don't know this platform at all.
>
> Regards,
> Willy
>
>
>
xmit_buf+head=a2279c,tmp_buf=956228,c=250
rs_write:xmit_buf=a22000,tmp_buf=956228
xmit_buf+head=a22896,tmp_buf=956228,c=250
rs_write:xmit_buf=a22000,tmp_buf=956228
xmit_buf+head=a22990,tmp_buf=956228,c=19
rs_write:xmit_buf=170f1200,tmp_buf=956228
^^^^^^
xmit_buf+head=170f1200,tmp_buf=956228,c=13
Unable to handle kernel NULL pointer dereference at virtual address
0000003f
fault-common.c(97): start_code=0xdc4040, start_stack=0xe11f38)
Internal error: Oops: ffffffff
CPU: 0
pc : [<0090e6a0>] lr : [<00872808>] Not tainted
sp : 00a1fe94 ip : 00000001 fp : 00a1feb8
r10: 00000fff r9 : 00956228 r8 : 0093d26c
r7 : 0000000d r6 : 009922a0 r5 : 00955f00 r4 : 0000000d
r3 : 0000007e r2 : 00000009 r1 : 009922ac r0 : 170f120d
Flags: Nzcv IRQs off FIQs on Mode SVC_32 Segment user
Control: C000107D
Process pppd (pid: 64, stackpage=00a1f000)