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

[Dan Carpenter]

On Mon, Oct 18, 2021 at 03:35:28AM +0000, Pkshih wrote:
> 
> > -----Original Message-----
> > From: Colin King <colin.king@xxxxxxxxxxxxx>
> > Sent: Friday, October 15, 2021 11:46 PM
> > To: Kalle Valo <kvalo@xxxxxxxxxxxxxx>; David S . Miller <davem@xxxxxxxxxxxxx>; Jakub Kicinski
> > <kuba@xxxxxxxxxx>; Pkshih <pkshih@xxxxxxxxxxx>; linux-wireless@xxxxxxxxxxxxxxx;
> > netdev@xxxxxxxxxxxxxxx
> > Cc: kernel-janitors@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Subject: [PATCH][next] rtw89: Fix potential dereference of the null pointer sta
> > 
> > From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxx>
> > ---
> >  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.

The warning is about "sta" not "sta->priv".  It's not a false positive.

I have heard discussions about compilers trying to work around these
bugs by re-ordering the code.  Is that an option in GCC?  It's not
something we should rely on, but I'm just curious if it exists in
released versions.

regards,
dan carpenter