Re: [PATCH v2] mac80211: minstrel_ht: fix NULL pointer dereference during auth/assoc

["Arend van Spriel" <arend@broadcom.com>]

On 02/08/2012 05:08 PM, Felix Fietkau wrote:
> On 2012-02-08 1:44 PM, Arend van Spriel wrote:
>> This patch fixes the following NULL pointer dereference:
>>
>>> I rerun the test on a kernel with some more lock checking and got lucky.
>>>
>>> Feb  8 08:40:17 lb-bun-10 kernel: [  514.512283] wlan0: authenticate
>>> with 98:fc:11:8e:94:57
>>> Feb  8 08:40:17 lb-bun-10 kernel: [  514.512515] wlan0: send auth to
>>> 98:fc:11:8e:94:57 (try 1/3)
>>> Feb  8 08:40:17 lb-bun-10 kernel: [  514.514184] BUG: unable to handle
>>> kernel NULL pointer dereference at 00000004
>>> Feb  8 08:40:17 lb-bun-10 kernel: [  514.514233] IP: [<f8648f08>]
>>> minstrel_tx_status+0x48/0xe0 [mac80211]
>>
>> static inline int
>> rix_to_ndx(struct minstrel_sta_info *mi, int rix)
>> {
>>         int i = rix;
>>         for (i = rix; i >= 0; i--)
>>                 if (mi->r[i].rix == rix)
>>   48:   3b 58 04                cmp    0x4(%eax),%ebx
>>   4b:   74 4b                   je     98 <minstrel_tx_status+0x98>
>>   4d:   83 c2 01                add    $0x1,%edx
>>
>> It fails because of mi->r[i] being a NULL pointer. It is allocated in
>> minstel_alloc_sta, but minstrel_ht_alloc_sta does not call that function.
>>
>> struct minstrel_ht_sta_priv holds a union with struct minstrel_ht_sta and
>> struct minstrel_sta_info. During authenticate the bool is_ht is false so
>> minstrel_ht_tx_status calls minstrel_tx_status, but minstrel_sta_info::r
>> is not set until rate_init which is after assoc.
> I'm not sure this is the right fix. If I understand the issue correctly,
> mi->r is only NULL because .rate_init has not been called yet, which is
> a bug that other rate control modules might trip over as well, maybe in
> more subtle ways. If this is the case, then I believe this bug should be
> fixed in mac80211 instead.
> 
> Maybe it would even be a good idea to set a flag after calling
> rate_control_rate_init, and calling WARN_ON or even BUG_ON when doing tx
> before that.
> 
> - Felix
> 

Hi Felix,

I am not sure I can agree. minstrel_ht allocates the struct
minstrel_ht_sta_priv initializing is_ht to false thereby choosing a
default and zero'ing the legacy field. When setting is_ht to false
consequently the contents of the legacy field should be properly set as
is done in minstrel_ht_update_caps (at use_legacy label).

Gr. AvS