Re: [PATCH] mac80211: fix radiotap header generation

[Kees Cook <keescook@chromium.org>]

On Tue, Nov 09, 2021 at 10:02:04AM +0100, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@intel.com>
> 
> In commit 8c89f7b3d3f2 ("mac80211: Use flex-array for radiotap header
> bitmap") we accidentally pointed the position to the wrong place, so
> we overwrite a present bitmap, and thus cause all kinds of trouble.
> 
> To see the issue, note that the previous code read:
> 
>   pos = (void *)(it_present + 1);
> 
> The requirement now is that we need to calculate pos via it_optional,
> to not trigger the compiler hardening checks, as:
> 
>   pos = (void *)&rthdr->it_optional[...];
> 
> Rewriting the original expression, we get (obviously, since that just
> adds "+ x - x" terms):
> 
>   pos = (void *)(it_present + 1 + rthdr->it_optional - rthdr->it_optional)
> 
> and moving the "+ rthdr->it_optional" outside to be used as an array:
> 
>   pos = (void *)&rthdr->it_optional[it_present + 1 - rthdr->it_optional];
> 
> The original is off by one, fix it.
> 
> Cc: stable@vger.kernel.org
> Fixes: 8c89f7b3d3f2 ("mac80211: Use flex-array for radiotap header bitmap")
> Reported-by: Sid Hayn <sidhayn@gmail.com>
> Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Argh! Thank you for the fix and sorry for the trouble the earlier patch
created!

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  net/mac80211/rx.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/mac80211/rx.c b/net/mac80211/rx.c
> index fc5c608d02e2..3562730ea0f8 100644
> --- a/net/mac80211/rx.c
> +++ b/net/mac80211/rx.c
> @@ -364,7 +364,7 @@ ieee80211_add_rx_radiotap_header(struct ieee80211_local *local,
>  	 * the compiler to think we have walked past the end of the
>  	 * struct member.
>  	 */
> -	pos = (void *)&rthdr->it_optional[it_present - rthdr->it_optional];
> +	pos = (void *)&rthdr->it_optional[it_present + 1 - rthdr->it_optional];
>  
>  	/* the order of the following fields is important */
>  
> -- 
> 2.31.1
> 

-- 
Kees Cook