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Date:   Tue, 27 Sep 2022 16:26:32 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        linux-hyperv@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] Drivers: hv: vmbus: Split memcpy of flex-array

On Tue, Sep 27, 2022 at 02:17:36PM -0700, Kees Cook wrote:
> To work around a misbehavior of the compiler's ability to see into
> composite flexible array structs (as detailed in the coming memcpy()
> hardening series[1]), split the memcpy() of the header and the payload
> so no false positive run-time overflow warning will be generated. This
> results in the already inlined memcpy getting unrolled a little more,
> which very slightly increases text size:
> 
> $ size drivers/hv/vmbus_drv.o.before drivers/hv/vmbus_drv.o
>    text    data     bss     dec     hex filename
>   22968    5239     232   28439    6f17 drivers/hv/vmbus_drv.o.before
>   23032    5239     232   28503    6f57 drivers/hv/vmbus_drv.o
> 
> Avoids the run-time false-positive warning:
> 
>   memcpy: detected field-spanning write (size 212) of single field "&ctx->msg" at drivers/hv/vmbus_drv.c:1133 (size 16)
> 
> [1] https://lore.kernel.org/linux-hardening/20220901065914.1417829-2-keescook@chromium.org/
> 
> Cc: "K. Y. Srinivasan" <kys@...rosoft.com>
> Cc: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: Stephen Hemminger <sthemmin@...rosoft.com>
> Cc: Wei Liu <wei.liu@...nel.org>
> Cc: Dexuan Cui <decui@...rosoft.com>
> Cc: linux-hyperv@...r.kernel.org
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Reported-by: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Tested-by: Nathan Chancellor <nathan@...nel.org>
> Signed-off-by: Kees Cook <keescook@...omium.org>

Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Thanks!
--
Gustavo

> ---
> v2: - fix commit log, add tags and exact warning test from Nathan
> v1: https://lore.kernel.org/lkml/20220924030741.3345349-1-keescook@chromium.org
> ---
>  drivers/hv/vmbus_drv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 23c680d1a0f5..9b111a8262e3 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1131,7 +1131,8 @@ void vmbus_on_msg_dpc(unsigned long data)
>  			return;
>  
>  		INIT_WORK(&ctx->work, vmbus_onmessage_work);
> -		memcpy(&ctx->msg, &msg_copy, sizeof(msg->header) + payload_size);
> +		ctx->msg.header = msg_copy.header;
> +		memcpy(&ctx->msg.payload, msg_copy.u.payload, payload_size);
>  
>  		/*
>  		 * The host can generate a rescind message while we
> -- 
> 2.34.1
> 

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