Re: [PATCH v3] batman-adv: Add flex array to struct batadv_tvlv_tt_data

Kees Cook
2024-05-01T20:17:16Z

On Wed, May 01, 2024 at 05:02:42PM +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 following change to the "batadv_tt_tvlv_ogm_handler_v1" function:
>
> - tt_vlan = (struct batadv_tvlv_tt_vlan_data *)(tt_data + 1);
> - tt_change = (struct batadv_tvlv_tt_change *)(tt_vlan + num_vlan);
>
> + tt_change = (struct batadv_tvlv_tt_change *)((void *)tt_data
> + + flex_size);
>
> is intended to prevent the compiler from generating an "out-of-bounds"
> notification due to the __counted_by attribute. The compiler can do a
> pointer calculation using the vlan_data flexible array memory, or in
> other words, this may be calculated as an array offset, since it is the
> same as:
>
> &tt_data->vlan_data[num_vlan]
>
> Therefore, we go past the end of the array. In other "multiple trailing
> flexible array" situations, this has been solved by 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).
>
> 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@outlook.com>

Thanks for the tweak!

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

--
Kees Cook