-----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 <redacted>; David S . Miller <davem@davemloft.net>; Jakub
Kicinski [off-list ref]; 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 [off-list ref] writes:

quoted

quoted

-----Original Message-----
From: Colin King <redacted>
Sent: Friday, October 15, 2021 11:46 PM
To: Kalle Valo <redacted>; David S . Miller <davem@davemloft.net>; Jakub Kicinski
[off-list ref]; Pkshih [off-list ref]; 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 <redacted>

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 <redacted>
---
 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.

quoted

+	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.

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.

--
Ping-Ke