Re: [PATCH v2] batman-adv: Add flex array to struct batadv_tvlv_tt_data

[Kees Cook]

On Fri, Apr 26, 2024 at 07:22:47PM +0200, Erick Archer wrote:
> The "struct batadv_tvlv_tt_data" uses a dynamically sized set of
> trailing elements. Specifically, it uses an array of structures of type
> "batadv_tvlv_tt_vlan_data". So, use the preferred way in the kernel
> declaring a flexible array [1].
> 
> At the same time, prepare for the coming implementation by GCC and Clang
> of the __counted_by attribute. Flexible array members annotated with
> __counted_by can have their accesses bounds-checked at run-time via
> CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
> strcpy/memcpy-family functions). In this case, it is important to note
> that the attribute used is specifically __counted_by_be since variable
> "num_vlan" is of type __be16.
> 
> The order in which the structure batadv_tvlv_tt_data and the structure
> batadv_tvlv_tt_vlan_data are defined must be swap to avoid an incomplete
> type error.
> 
> Also, avoid the open-coded arithmetic in memory allocator functions [2]
> using the "struct_size" macro and use the "flex_array_size" helper to
> clarify some calculations, when possible.
> 
> Moreover, the new structure member also allow us to avoid the open-coded
> arithmetic on pointers in some situations. Take advantage of this.
> 
> This code was detected with the help of Coccinelle, and audited and
> modified manually.
> 
> Link: https://www.kernel.org/doc/html/next/process/deprecated.html#zero-length-and-one-element-arrays [1]
> Link: https://www.kernel.org/doc/html/next/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [2]
> Signed-off-by: Erick Archer <erick.archer@xxxxxxxxxxx>

Thanks for this! I think the readability is significantly improved.

> [...]
> @@ -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);


-Kees

-- 
Kees Cook