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

Kalle Valo
2021-10-20T08:36:16Z

Pkshih <pkshih@realtek.com> writes:

>> -----Original Message-----
>> From: kvalo=codeaurora.org@mg.codeaurora.org
>> <kvalo=codeaurora.org@mg.codeaurora.org> On
>> Behalf Of Kalle Valo
>> Sent: Monday, October 18, 2021 8:12 PM
>> To: Pkshih <pkshih@realtek.com>
>> Cc: Colin King <colin.king@canonical.com>; David S . Miller
>> <davem@davemloft.net>; Jakub
>> Kicinski <kuba@kernel.org>; linux-wireless@vger.kernel.org; netdev@vger.kernel.org;
>> kernel-janitors@vger.kernel.org; linux-kernel@vger.kernel.org
>> Subject: Re: [PATCH][next] rtw89: Fix potential dereference of the null pointer sta
>>
>> 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.
>
> Understand.
>
> I have another way to fix this coverity warning, like:
>
> @@ -1617,7 +1617,7 @@ 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;
> + struct rtw89_sta *rtwsta = sta ? (struct rtw89_sta *)sta->drv_priv : NULL;
>
> if (!sta || rtwsta->max_agg_wait <= 0)
> return false;
>
> Is this acceptable?
> It has a little redundant checking of 'sta', but the code looks clean.

I feel that Colin's fix is more readable, but this is just matter of
taste. You can choose.

>> 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.
>>
>
> It says
>
> * struct ieee80211_txq - Software intermediate tx queue
> * @sta: station table entry, %NULL for per-vif queue
>
> So, we need to check if 'sta' is NULL.

Ok, thanks for checking (no pun intended) :)

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