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Date: Tue, 28 Nov 2023 11:10:28 +0000
From: Joey Gouly <joey.gouly@....com>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Bill Wendling <morbo@...gle.com>, Kees Cook <keescook@...omium.org>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] neighbour: Fix __randomize_layout crash in struct
 neighbour

Hi,

On Sat, Nov 25, 2023 at 03:33:58PM -0600, Gustavo A. R. Silva wrote:
> Previously, one-element and zero-length arrays were treated as true
> flexible arrays, even though they are actually "fake" flex arrays.
> The __randomize_layout would leave them untouched at the end of the
> struct, similarly to proper C99 flex-array members.
> 
> However, this approach changed with commit 1ee60356c2dc ("gcc-plugins:
> randstruct: Only warn about true flexible arrays"). Now, only C99
> flexible-array members will remain untouched at the end of the struct,
> while one-element and zero-length arrays will be subject to randomization.
> 
> Fix a `__randomize_layout` crash in `struct neighbour` by transforming
> zero-length array `primary_key` into a proper C99 flexible-array member.
> 
> Fixes: 1ee60356c2dc ("gcc-plugins: randstruct: Only warn about true flexible arrays")
> Closes: https://lore.kernel.org/linux-hardening/20231124102458.GB1503258@e124191.cambridge.arm.com/
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  include/net/neighbour.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/net/neighbour.h b/include/net/neighbour.h
> index 07022bb0d44d..0d28172193fa 100644
> --- a/include/net/neighbour.h
> +++ b/include/net/neighbour.h
> @@ -162,7 +162,7 @@ struct neighbour {
>  	struct rcu_head		rcu;
>  	struct net_device	*dev;
>  	netdevice_tracker	dev_tracker;
> -	u8			primary_key[0];
> +	u8			primary_key[];
>  } __randomize_layout;
>  
>  struct neigh_ops {

Fixes the crash for me!

Tested-by: Joey Gouly <joey.gouly@....com>

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