Re: gigaset: freeing an active object

From: Peter Hurley
Date: Sun Nov 29 2015 - 13:22:30 EST


Hi Tilman,

On 11/29/2015 10:30 AM, Tilman Schmidt wrote:
> Hi Sasha,
>
> thanks for the report. As the original author of the code in question, I
> am somewhat at a loss what to make of it.
>
> Am 27.11.2015 um 16:19 schrieb Sasha Levin:
>> Fuzzing with syzkaller on the latest -next kernel produced this error:
>
> Is there a way to know the actual sequence of events that triggered this
> warning?
>
>> [ 413.536749] WARNING: CPU: 6 PID: 25400 at lib/debugobjects.c:263 debug_print_object+0x1c4/0x1e0()
>> [ 413.538111] ODEBUG: free active (active state 0) object type: timer_list hint: delayed_work_timer_fn+0x0/0x90
>
> This message seems to indicate that an object of type timer_list was
> freed which was still active. However the driver in question
> (ser_gigaset) does not use any timers.
>
> What are the exact conditions for producing this message? IOW how does
> the ODEBUG code determine that an object of type timer_list is being
> freed, and that it is still in use?
>
> Are there any messages from ser_gigaset or another one of the gigaset
> drivers before that warning?
>
>> [ 413.539598] Modules linked in:3470693efef57268844f02f5de3ab392d8cf5e209671ddd87163cb964c510659
>
> This message does not tell me anything. What does that hex string after
> the colon mean?
>
>> [ 413.540448] CPU: 6 PID: 25400 Comm: syzkaller_execu Not tainted 4.4.0-rc2-next-20151126-sasha-00005-g00d303e-dirty #2653
>> [ 413.547614] Call Trace:
>> [ 413.548077] [<ffffffffa8e6b5bb>] dump_stack+0x72/0xb7
>> [ 413.548765] [<ffffffffa73531d3>] warn_slowpath_common+0x113/0x140
>> [ 413.551151] [<ffffffffa73532cb>] warn_slowpath_fmt+0xcb/0x100
>> [ 413.554295] [<ffffffffa8ed0194>] debug_print_object+0x1c4/0x1e0
>> [ 413.556592] [<ffffffffa8ed1035>] __debug_check_no_obj_freed+0x215/0x7a0
>> [ 413.560526] [<ffffffffa8ed2b6c>] debug_check_no_obj_freed+0x2c/0x40
>> [ 413.561328] [<ffffffffa77aac4c>] kfree+0x1fc/0x2f0
>
> Judging from the backtrace below this must be the call
>
> kfree(cs->hw.ser);
>
> in drivers/isdn/gigaset/ser-gigaset.c line 375.
> cs->hw.ser is of type struct ser_cardstate *.
> struct ser_cardstate consists of a struct platform_device, a struct
> completion, an atomic_t and a pointer. No timer_list.
>
>> [ 413.561970] [<ffffffffae74b021>] gigaset_freecshw+0xe1/0x120
>
> There are functions by this name in all three Gigaset hardware dependent
> modules (bas_gigaset, ser_gigaset and usb_gigaset), but ...
>
>> [ 413.562723] [<ffffffffae70669d>] gigaset_freecs+0x2ad/0x600
>> [ 413.564240] [<ffffffffae74ba60>] gigaset_tty_close+0x210/0x280
>
> this function only exists in ser_gigaset.

The platform_device embedded in struct ser_cardstate hasn't been released when
you kfree() the memory it's in.

Regards,
Peter Hurley

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