linux-kernel - Re: [PATCH] iwlwifi: mvm: Fix potential NULL dereference for sta

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Date:   Fri, 21 Jan 2022 12:30:12 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Takashi Iwai <tiwai@...e.de>,
        Luca Coelho <luciano.coelho@...el.com>,
        Oliver Neukum <oneukum@...e.de>,
        linux-wireless@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iwlwifi: mvm: Fix potential NULL dereference for sta

On Fri, 21 Jan 2022 12:22:05 +0100,
Johannes Berg wrote:
> 
> On Fri, 2022-01-21 at 12:14 +0100, Takashi Iwai wrote:
> > The recent fix for NULL sta in iwl_mvm_get_tx_rate() may still hit a
> > potential NULL dereference, as iwl_mvm_sta_from_mac80211() is called
> > unconditionally (although this doesn't seem happening, practically
> > seen, thanks to the compiler optimization).
> > 
> 
> No objection to the patch, but I think the description isn't quite
> right?
> 
> static inline struct iwl_mvm_sta *
> iwl_mvm_sta_from_mac80211(struct ieee80211_sta *sta)
> {
>         return (void *)sta->drv_priv;
> }
> 
> looks like a dereference, but I _think_
> 
> struct ieee80211_sta {
> 	[...]
> 
>         /* must be last */
>         u8 drv_priv[] __aligned(sizeof(void *));
> };
> 
> 
> means it's just an address calculation, i.e. the same as if we had
> 
> 	return (void *)((u8 *)sta + offsetof(typeof(*sta), drv_priv));
> 
> no?

Yeah, indeed, that won't access the member.

> I guess technically it's still UB doing calculations on a NULL pointer,
> but practically that's going to work.
> 
> Anyway, no objections :)

OK, I'll submit v2 with rephrasing for avoid confusion.


Takashi