linux-hardening - Re: [PATCH v2] batman-adv: Add flex array to struct batadv_tvlv_tt

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Date: Wed, 1 May 2024 15:35:57 +0200
From: Erick Archer <erick.archer@...look.com>
To: Kees Cook <keescook@...omium.org>
Cc: Erick Archer <erick.archer@...look.com>,
	Marek Lindner <mareklindner@...mailbox.ch>,
	Simon Wunderlich <sw@...onwunderlich.de>,
	Antonio Quartulli <a@...table.cc>,
	Sven Eckelmann <sven@...fation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	b.a.t.m.a.n@...ts.open-mesh.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, linux-hardening@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: Re: [PATCH v2] batman-adv: Add flex array to struct
 batadv_tvlv_tt_data

Hi Kees,

On Mon, Apr 29, 2024 at 10:36:23AM -0700, Kees Cook wrote:
> On Fri, Apr 26, 2024 at 07:22:47PM +0200, Erick Archer wrote:
> >  
> > @@ -3957,17 +3947,18 @@ static void batadv_tt_tvlv_ogm_handler_v1(struct batadv_priv *bat_priv,
> >  
> >  	num_vlan = ntohs(tt_data->num_vlan);
> >  
> > -	if (tvlv_value_len < sizeof(*tt_vlan) * num_vlan)
> > +	flex_size = flex_array_size(tt_data, vlan_data, num_vlan);
> > +	if (tvlv_value_len < flex_size)
> >  		return;
> >  
> > -	tt_vlan = (struct batadv_tvlv_tt_vlan_data *)(tt_data + 1);
> > -	tt_change = (struct batadv_tvlv_tt_change *)(tt_vlan + num_vlan);
> > -	tvlv_value_len -= sizeof(*tt_vlan) * num_vlan;
> > +	tt_change = (struct batadv_tvlv_tt_change *)(tt_data->vlan_data +
> > +						     num_vlan);
> 
> This is the only part I'm a little worried about. The math all checks
> out, but the compiler may get bothered about performing a pointer
> calculation using the vlan_data flexible array memory. As in, this may
> be calculated as an array offset, since it is the same as:
> 
> 	&tt_data->vlan_data[num_vlan]
> 
> Which, for big endian where __counted_by is in effect, the bounds
> checker may throw a fit since we're going past the end of the array. In
> other "multiple trailing flexible array" situations, we've done the
> addressing from the base pointer, since the compiler either knows the
> full allocation size or it knows nothing about it (this case, since it
> came from a "void *" function argument). I would suggest:
> 
> tt_change = (struct batadv_tvlv_tt_change *)((void *)tt_data + flex_size);
> 
First of all thanks for the review and the great explanation. The change
looks reasonable to me. I will send a new version with this suggestion.

Regards,
Erick