netdev - Re: [PATCH net-next] net: Silence false field-spanning write warning in ip_tunnel_info_opts

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Message-ID: <7dae76b8-4b79-40dd-9aa5-050c44e060f0@nvidia.com>
Date: Wed, 8 Jan 2025 08:56:00 +0200
From: Gal Pressman <gal@...dia.com>
To: Kees Cook <kees@...nel.org>, "David S. Miller" <davem@...emloft.net>,
 Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, David Ahern <dsahern@...nel.org>,
 Simon Horman <horms@...nel.org>, linux-hardening@...r.kernel.org,
 Cosmin Ratiu <cratiu@...dia.com>
Subject: Re: [PATCH net-next] net: Silence false field-spanning write warning
 in ip_tunnel_info_opts_set() memcpy

On 08/01/2025 1:28, Kees Cook wrote:
> 
> 
> On January 7, 2025 8:55:09 AM PST, Gal Pressman <gal@...dia.com> wrote:
>> When metadata_dst struct is allocated (using metadata_dst_alloc()), it
>> reserves room for options at the end of the struct.
>>
>> Similar to [1], change the memcpy() to unsafe_memcpy() as it is
>> guaranteed that enough room (md_size bytes) was allocated and the
>> field-spanning write is intentional.
> 
> Why not just add an "options" flex array to struct ip_tunnel_info?
> 
> E.g.:
> 
> struct ip_tunnel_info {
> 	struct ip_tunnel_key	key;
> 	struct ip_tunnel_encap	encap;
> #ifdef CONFIG_DST_CACHE
> 	struct dst_cache	dst_cache;
> #endif
> 	u8			options_len;
> 	u8			mode;
>   u8   options[] __counted_by(options_len);
> };
> 
>>
>> This resolves the following warning:
>>  memcpy: detected field-spanning write (size 8) of single field "_Generic(info, const struct ip_tunnel_info * : ((const void *)((info) + 1)), struct ip_tunnel_info * : ((void *)((info) + 1)) )" at include/net/ip_tunnels.h:662 (size 0)
> 
> Then you can drop this macro and just use: info->options
> 
> Looks like you'd need to do it for all the types in struct metadata_dst, but at least you could stop hiding it from the compiler. :)
> 
> -Kees
> 
> 

Thanks for the review Kees, I'll take a look into that.