Re: [PATCH][next] rtw89: Fix potential dereference of the null pointer sta

[Kalle Valo <kvalo@codeaurora.org>]

Pkshih <pkshih@realtek.com> writes:

>> -----Original Message-----
>> From: Colin King <colin.king@canonical.com>
>> Sent: Friday, October 15, 2021 11:46 PM
>> To: Kalle Valo <kvalo@codeaurora.org>; David S . Miller <davem@davemloft.net>; Jakub Kicinski
>> <kuba@kernel.org>; Pkshih <pkshih@realtek.com>; linux-wireless@vger.kernel.org;
>> netdev@vger.kernel.org
>> Cc: kernel-janitors@vger.kernel.org; linux-kernel@vger.kernel.org
>> Subject: [PATCH][next] rtw89: Fix potential dereference of the null pointer sta
>> 
>> From: Colin Ian King <colin.king@canonical.com>
>> 
>> The pointer rtwsta is dereferencing pointer sta before sta is
>> being null checked, so there is a potential null pointer deference
>> issue that may occur. Fix this by only assigning rtwsta after sta
>> has been null checked. Add in a null pointer check on rtwsta before
>> dereferencing it too.
>> 
>> Fixes: e3ec7017f6a2 ("rtw89: add Realtek 802.11ax driver")
>> Addresses-Coverity: ("Dereference before null check")
>> Signed-off-by: Colin Ian King <colin.king@canonical.com>
>> ---
>>  drivers/net/wireless/realtek/rtw89/core.c | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/net/wireless/realtek/rtw89/core.c
>> b/drivers/net/wireless/realtek/rtw89/core.c
>> index 06fb6e5b1b37..26f52a25f545 100644
>> --- a/drivers/net/wireless/realtek/rtw89/core.c
>> +++ b/drivers/net/wireless/realtek/rtw89/core.c
>> @@ -1534,9 +1534,14 @@ static bool rtw89_core_txq_agg_wait(struct rtw89_dev *rtwdev,
>>  {
>>  	struct rtw89_txq *rtwtxq = (struct rtw89_txq *)txq->drv_priv;
>>  	struct ieee80211_sta *sta = txq->sta;
>> -	struct rtw89_sta *rtwsta = (struct rtw89_sta *)sta->drv_priv;
>
> 'sta->drv_priv' is only a pointer, we don't really dereference the
> data right here, so I think this is safe. More, compiler can optimize
> this instruction that reorder it to the place just right before using.
> So, it seems like a false alarm.
>
>> +	struct rtw89_sta *rtwsta;
>> 
>> -	if (!sta || rtwsta->max_agg_wait <= 0)
>> +	if (!sta)
>> +		return false;
>> +	rtwsta = (struct rtw89_sta *)sta->drv_priv;
>> +	if (!rtwsta)
>> +		return false;
>> +	if (rtwsta->max_agg_wait <= 0)
>>  		return false;
>> 
>>  	if (rtwdev->stats.tx_tfc_lv <= RTW89_TFC_MID)
>
> I check the size of object files before/after this patch, and
> the original one is smaller.
>
>    text    data     bss     dec     hex filename
>   16781    3392       1   20174    4ece core-0.o  // original
>   16819    3392       1   20212    4ef4 core-1.o  // after this patch
>
> Do you think it is worth to apply this patch?

I think that we should apply the patch. Even though the compiler _may_
reorder the code, it might choose not to do that.

Another question is that can txq->sta really be null? I didn't check the
code, but if it should be always set when the null check is not needed.

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