linux-kernel - Re: [PATCH] ath9k: fix null-ptr-deref in ath_chanctx

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Message-ID: <87il8uta8f.fsf@toke.dk>
Date:   Fri, 01 Sep 2023 13:16:00 +0200
From:   Toke Høiland-Jørgensen <toke@...e.dk>
To:     Dongliang Mu <dzm91@...t.edu.cn>, Kalle Valo <kvalo@...nel.org>,
        Sujith Manoharan <c_manoha@....qualcomm.com>,
        "John W. Linville" <linville@...driver.com>
Cc:     hust-os-kernel-patches@...glegroups.com,
        linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ath9k: fix null-ptr-deref in ath_chanctx_event

Dongliang Mu <dzm91@...t.edu.cn> writes:

> On 2023/9/1 18:41, 'Toke Høiland-Jørgensen' via HUST OS Kernel 
> Contribution wrote:
>> Dongliang Mu <dzm91@...t.edu.cn> writes:
>>
>>> Smatch reports:
>>>
>>> ath_chanctx_event() error: we previously assumed 'vif' could be null
>>>
>>> The function ath_chanctx_event can be called with vif argument as NULL.
>>> If vif is NULL, ath_dbg can trigger a null pointer dereference.
>>>
>>> Fix this by adding a null pointer check.
>>>
>>> Fixes: 878066e745b5 ("ath9k: Add more debug statements for channel context")
>>> Signed-off-by: Dongliang Mu <dzm91@...t.edu.cn>
>>> ---
>>>   drivers/net/wireless/ath/ath9k/channel.c | 4 +++-
>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/net/wireless/ath/ath9k/channel.c b/drivers/net/wireless/ath/ath9k/channel.c
>>> index 571062f2e82a..e343c8962d14 100644
>>> --- a/drivers/net/wireless/ath/ath9k/channel.c
>>> +++ b/drivers/net/wireless/ath/ath9k/channel.c
>>> @@ -576,7 +576,9 @@ void ath_chanctx_event(struct ath_softc *sc, struct ieee80211_vif *vif,
>>>   		if (sc->sched.state != ATH_CHANCTX_STATE_WAIT_FOR_BEACON)
>>>   			break;
>>>   
>>> -		ath_dbg(common, CHAN_CTX, "Preparing beacon for vif: %pM\n", vif->addr);
>>> +		if (vif)
>>> +			ath_dbg(common, CHAN_CTX,
>>> +				"Preparing beacon for vif: %pM\n", vif->addr);
>> Please don't send patches for static checker errors without actually
>> checking if there is a valid bug. Which there isn't in this case.
>
> Before sending this patch, I searched in the code, there are many call 
> sites of ath_chanctx_event with argument vif as NULL.
>
> Functions calling this function: ath_chanctx_event
>
>    File      Function                   Line
> 0 beacon.c  ath9k_beacon_tasklet        459 ath_chanctx_event(sc, vif, 
> ATH_CHANCTX_EVENT_BEACON_PREPARE);

But only this one has ATH_CHANCTX_EVENT_BEACON_PREPARE as an argument,
which is required to hit the code path you are changing.

> 1 channel.c ath_chanctx_check_active    321 ath_chanctx_event(sc, NULL,
> 2 channel.c ath_chanctx_beacon_sent_ev  781 ath_chanctx_event(sc, NULL, ev);
> 3 channel.c ath_chanctx_beacon_recv_ev  787 ath_chanctx_event(sc, NULL, ev);
> 4 channel.c ath_chanctx_timer          1054 ath_chanctx_event(sc, NULL, 
> ATH_CHANCTX_EVENT_TSF_TIMER);
> 5 channel.c ath_chanctx_set_next       1321 ath_chanctx_event(sc, NULL, 
> ATH_CHANCTX_EVENT_SWITCH);
> 6 channel.c ath9k_p2p_ps_timer         1566 ath_chanctx_event(sc, NULL, 
> ATH_CHANCTX_EVENT_TSF_TIMER);
> 7 main.c    ath9k_sta_state            1671 ath_chanctx_event(sc, vif,
> 8 main.c    ath9k_remove_chanctx       2577 ath_chanctx_event(sc, NULL, 
> ATH_CHANCTX_EVENT_UNASSIGN);
> 9 xmit.c    ath_tx_edma_tasklet        2749 ath_chanctx_event(sc, NULL,
>
> This NULL parameters would cause some abnormal behaviors.
>
>> Specifically, that branch of the switch statement dereferences the avp
>> pointer, which will be NULL if 'vif' is. Meaning we will have crashed
>> way before reaching this statement if vif is indeed NULL.
> Yeah, you are right. However, no matter where or which variable causing 
> the null-ptr-def crash, the crash is there.

There is no crash, see above.

-Toke