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Date:   Tue, 28 Nov 2023 11:10:28 +0000
From:   Joey Gouly <joey.gouly@arm.com>
To:     "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Bill Wendling <morbo@google.com>,
        Kees Cook <keescook@chromium.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] neighbour: Fix __randomize_layout crash in struct
 neighbour
Message-ID: <20231128111028.GA2382233@e124191.cambridge.arm.com>
References: <ZWJoRsJGnCPdJ3+2@work>
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In-Reply-To: <ZWJoRsJGnCPdJ3+2@work>
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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@kernel.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@arm.com>