linux-hardening - Re: [PATCH 02/64] mac80211: Use flex-array for radiotap header bitmap

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Message-ID: <202107281454.F96505E15@keescook>
Date:   Wed, 28 Jul 2021 14:54:52 -0700
From:   Kees Cook <keescook@...omium.org>
To:     dsterba@...e.cz, Dan Carpenter <dan.carpenter@...cle.com>,
        linux-hardening@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Keith Packard <keithpac@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-staging@...ts.linux.dev, linux-block@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH 02/64] mac80211: Use flex-array for radiotap header bitmap

On Wed, Jul 28, 2021 at 11:23:23AM +0200, David Sterba wrote:
> On Wed, Jul 28, 2021 at 10:35:56AM +0300, Dan Carpenter wrote:
> > @@ -372,7 +372,7 @@ ieee80211_add_rx_radiotap_header(struct ieee80211_local *local,
> >  			ieee80211_calculate_rx_timestamp(local, status,
> >  							 mpdulen, 0),
> >  			pos);
> > -		rthdr->it_present |= cpu_to_le32(1 << IEEE80211_RADIOTAP_TSFT);
> > +		rthdr->data.it_present |= cpu_to_le32(1 << IEEE80211_RADIOTAP_TSFT);
> 
> A drive-by comment, not related to the patchset, but rather the
> ieee80211 driver itself.
> 
> Shift expressions with (1 << NUMBER) can be subtly broken once the
> NUMBER is 31 and the value gets silently cast to a 64bit type. It will
> become 0xfffffffff80000000.
> 
> I've checked the IEEE80211_RADIOTAP_* defintions if this is even remotely
> possible and yes, IEEE80211_RADIOTAP_EXT == 31. Fortunatelly it seems to
> be used with used with a 32bit types (eg. _bitmap_shifter) so there are
> no surprises.
> 
> The recommended practice is to always use unsigned types for shifts, so
> "1U << ..." at least.

Ah, good catch! I think just using BIT() is the right replacement here,
yes? I suppose that should be a separate patch.

-- 
Kees Cook