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

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Date: Tue, 02 Apr 2024 21:06:35 +0200
From: Sven Eckelmann <sven@...fation.org>
To: Marek Lindner <mareklindner@...mailbox.ch>,
 Simon Wunderlich <sw@...onwunderlich.de>, Antonio Quartulli <a@...table.cc>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Kees Cook <keescook@...omium.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Erick Archer <erick.archer@...look.com>
Cc: Erick Archer <erick.archer@...look.com>, b.a.t.m.a.n@...ts.open-mesh.org,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH] batman-adv: Add flex array to struct batadv_tvlv_tt_data

On Tuesday, 2 April 2024 19:23:01 CEST 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].
> 
> 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@...look.com>

> ---
> Hi,
> 
> I would like to add the "__counted_by(num_vlan)" tag to the new flex member
> but I don't know if this line can affect it.
> 
> ntohs(tt_data->num_vlan)


Yes, num_vlan is a __be16. I could only identify the kernel-doc related 
scripts as consumer. But maybe they are more - so I would defer this question 
to kernel-hardening.


And with this change, I get a lot of additional warnings (-Wsparse-all)


cfg: BLA=n DAT=y DEBUG=y TRACING=n NC=y MCAST=n BATMAN_V=n
    net/batman-adv/translation-table.c:574:21: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:859:25: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:859:25: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:938:25: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:938:25: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:2932:16: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:2932:16: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:3378:21: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:3378:21: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:3982:30: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:3986:27: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:4026:30: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:4030:27: warning: using sizeof on a flexible structure
    net/batman-adv/translation-table.c:4032:23: warning: cast from restricted __be16
    net/batman-adv/translation-table.c:4032:23: warning: restricted __be16 degrades to integer
    net/batman-adv/translation-table.c:4032:23: warning: incorrect type in argument 1 (different base types)
    net/batman-adv/translation-table.c:4032:23:    expected unsigned long [usertype] factor1
    net/batman-adv/translation-table.c:4032:23:    got restricted __be16 [usertype] num_vlan

[...]
>  	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;

This helper would need an #include of <linux/overflow.h> in 
net/batman-adv/translation-table.c

[....]
>  /**
> @@ -4039,8 +4029,7 @@ static int batadv_tt_tvlv_unicast_handler_v1(struct batadv_priv *bat_priv,
>  	tt_data = tvlv_value;
>  	tvlv_value_len -= sizeof(*tt_data);
>  
> -	tt_vlan_len = sizeof(struct batadv_tvlv_tt_vlan_data);
> -	tt_vlan_len *= ntohs(tt_data->num_vlan);
> +	tt_vlan_len = flex_array_size(tt_data, vlan_data, tt_data->num_vlan);

This is definitely wrong on little endian systems. You first need to convert 
num_vlan from network (big endian) to host order.

Kind regards,
	Sven

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