linux-kernel - Re: [PATCH] drivers/char/ipmi_si: prevent null deref during module exit

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Date:   Wed, 8 Nov 2017 14:00:44 -0600
From:   Corey Minyard <minyard@....org>
To:     Andrew Banman <abanman@....com>
Cc:     Dimitri Sivanich <sivanich@....com>,
        "Anderson, Russ" <russ.anderson@....com>,
        Mike Travis <mike.travis@....com>,
        openipmi-developer@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers/char/ipmi_si: prevent null deref during module
 exit

On 11/08/2017 11:11 AM, Andrew Banman wrote:
> On 11/8/17 11:06 AM, Andrew Banman wrote:
>> If there are uninitialized SMIs in the smi_infos list, i.e. with no
>> handlers set, then disable_si_irq() in cleanup_smi_one() will hit a null
>> pointer dereference when the former attempts to start the check enables
>> transaction. Thus, we panic during module exit.
>
> I think this points to a broader problem of holding uninitialized 
> smi_info
> structs in smi_infos list. There are many places where handlers and 
> other struct
> members are assumed. Maybe a better design would be to remove SMIs 
> from the list
> if we have no intention of initializing them?
>

This begs the question: How did you produce this?  From what I can tell,
there is no way you can get to this code if you don't have a working and
initialized smi_info structure, and that's not the only place this would
have to be fixed if it wasn't.  So it's not what you assume, I don't think
it's an uninitialized smi_info structure on the list.

As usual with these sorts of things, please send tracebacks and reproduction
procedures.

If you are removing the module and this happens, there may be a race
conditions, but this is the wrong fix.  More likely that the structure gets
cleaned up and this function is called afterwards.

-corey

> Andrew
>
>>
>> Avoid panicking when there are uninitialized SMIs by checking for a 
>> handler
>> pointer before starting the check enables transaction.
>>
>> Signed-off-by: Andrew Banman <abanman@....com>
>> ---
>>   drivers/char/ipmi/ipmi_si_intf.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/char/ipmi/ipmi_si_intf.c 
>> b/drivers/char/ipmi/ipmi_si_intf.c
>> index cb5719e..6c0b1b3 100644
>> --- a/drivers/char/ipmi/ipmi_si_intf.c
>> +++ b/drivers/char/ipmi/ipmi_si_intf.c
>> @@ -442,7 +442,7 @@ static void start_check_enables(struct smi_info 
>> *smi_info, bool start_timer)
>>
>>       if (start_timer)
>>           start_new_msg(smi_info, msg, 2);
>> -    else
>> +    else if (smi_info->handlers)
>> smi_info->handlers->start_transaction(smi_info->si_sm, msg, 2);
>>       smi_info->si_state = SI_CHECKING_ENABLES;
>>   }
>