linux-hardening - Re: [PATCH] NFC: hci: Split memcpy() of struct hcp

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Message-ID: <Yy6HVVMGdSCyEHbZ@work>
Date:   Fri, 23 Sep 2022 23:28:05 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] NFC: hci: Split memcpy() of struct hcp_message flexible
 array

On Fri, Sep 23, 2022 at 09:08:35PM -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
> split already existed for the "firstfrag" case, so just generalize the
> logic further.
> 
> [1] https://lore.kernel.org/linux-hardening/20220901065914.1417829-2-keescook@chromium.org/
> 
> Cc: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Paolo Abeni <pabeni@...hat.com>
> Cc: netdev@...r.kernel.org
> Reported-by: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Signed-off-by: Kees Cook <keescook@...omium.org>

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

Thanks!
--
Gustavo

> ---
>  net/nfc/hci/hcp.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/net/nfc/hci/hcp.c b/net/nfc/hci/hcp.c
> index 05c60988f59a..4902f5064098 100644
> --- a/net/nfc/hci/hcp.c
> +++ b/net/nfc/hci/hcp.c
> @@ -73,14 +73,12 @@ int nfc_hci_hcp_message_tx(struct nfc_hci_dev *hdev, u8 pipe,
>  		if (firstfrag) {
>  			firstfrag = false;
>  			packet->message.header = HCP_HEADER(type, instruction);
> -			if (ptr) {
> -				memcpy(packet->message.data, ptr,
> -				       data_link_len - 1);
> -				ptr += data_link_len - 1;
> -			}
>  		} else {
> -			memcpy(&packet->message, ptr, data_link_len);
> -			ptr += data_link_len;
> +			packet->message.header = *ptr++;
> +		}
> +		if (ptr) {
> +			memcpy(packet->message.data, ptr, data_link_len - 1);
> +			ptr += data_link_len - 1;
>  		}
>  
>  		/* This is the last fragment, set the cb bit */
> -- 
> 2.34.1
>