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Date:   Wed, 28 Jul 2021 11:23:23 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Dan Carpenter <dan.carpenter@oracle.com>
Cc:     Kees Cook <keescook@chromium.org>, linux-hardening@vger.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Keith Packard <keithpac@amazon.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org,
        netdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-staging@lists.linux.dev, linux-block@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: Re: [PATCH 02/64] mac80211: Use flex-array for radiotap header bitmap
Message-ID: <20210728092323.GW5047@twin.jikos.cz>
Reply-To: dsterba@suse.cz
Mail-Followup-To: dsterba@suse.cz, Dan Carpenter <dan.carpenter@oracle.com>,
        Kees Cook <keescook@chromium.org>, linux-hardening@vger.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Keith Packard <keithpac@amazon.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org,
        netdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-staging@lists.linux.dev, linux-block@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com
References: <20210727205855.411487-1-keescook@chromium.org>
 <20210727205855.411487-3-keescook@chromium.org>
 <20210728073556.GP1931@kadam>
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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.