Subject: Re: [PATCH] rxrpc: Reuse SKCIPHER_REQUEST_ON_STACK buffer

From: Eric Biggers <[email-masked]>
Date: Mon, 16 Jul 2018 10:42:23 -0700

On Sun, Jul 15, 2018 at 08:49:47PM -0700, Kees Cook wrote:
> The use of SKCIPHER_REQUEST_ON_STACK() will trigger FRAME_WARN warnings
> (when less than 2048) once the VLA is no longer hidden from the check:
> 
> net/rxrpc/rxkad.c:398:1: warning: the frame size of 1152 bytes is larger than 1024 bytes [-Wframe-larger-than=]
> net/rxrpc/rxkad.c:242:1: warning: the frame size of 1152 bytes is larger than 1024 bytes [-Wframe-larger-than=]
> 
> This passes the initial SKCIPHER_REQUEST_ON_STACK allocation to the leaf
> functions for reuse. Two requests allocated on the stack are not needed
> when only one is used at a time.
> 
> Signed-off-by: Kees Cook <[email-masked]>
> ---
>  net/rxrpc/rxkad.c | 25 +++++++++++++------------
>  1 file changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/net/rxrpc/rxkad.c b/net/rxrpc/rxkad.c
> index 278ac0807a60..6393391fac86 100644
> --- a/net/rxrpc/rxkad.c
> +++ b/net/rxrpc/rxkad.c
> @@ -146,10 +146,10 @@ static int rxkad_prime_packet_security(struct rxrpc_connection *conn)
>  static int rxkad_secure_packet_auth(const struct rxrpc_call *call,
>  				    struct sk_buff *skb,
>  				    u32 data_size,
> -				    void *sechdr)
> +				    void *sechdr,
> +				    struct skcipher_request *req)
>  {
>  	struct rxrpc_skb_priv *sp = rxrpc_skb(skb);
> -	SKCIPHER_REQUEST_ON_STACK(req, call->conn->cipher);
>  	struct rxkad_level1_hdr hdr;
>  	struct rxrpc_crypt iv;
>  	struct scatterlist sg;
> @@ -183,12 +183,12 @@ static int rxkad_secure_packet_auth(const struct rxrpc_call *call,
>  static int rxkad_secure_packet_encrypt(const struct rxrpc_call *call,
>  				       struct sk_buff *skb,
>  				       u32 data_size,
> -				       void *sechdr)
> +				       void *sechdr,
> +				       struct skcipher_request *req)
>  {
>  	const struct rxrpc_key_token *token;
>  	struct rxkad_level2_hdr rxkhdr;
>  	struct rxrpc_skb_priv *sp;
> -	SKCIPHER_REQUEST_ON_STACK(req, call->conn->cipher);
>  	struct rxrpc_crypt iv;
>  	struct scatterlist sg[16];
>  	struct sk_buff *trailer;
> @@ -296,11 +296,12 @@ static int rxkad_secure_packet(struct rxrpc_call *call,
>  		ret = 0;
>  		break;
>  	case RXRPC_SECURITY_AUTH:
> -		ret = rxkad_secure_packet_auth(call, skb, data_size, sechdr);
> +		ret = rxkad_secure_packet_auth(call, skb, data_size, sechdr,
> +					       req);
>  		break;
>  	case RXRPC_SECURITY_ENCRYPT:
>  		ret = rxkad_secure_packet_encrypt(call, skb, data_size,
> -						  sechdr);
> +						  sechdr, req);
>  		break;
>  	default:
>  		ret = -EPERM;
> @@ -316,10 +317,10 @@ static int rxkad_secure_packet(struct rxrpc_call *call,
>   */
>  static int rxkad_verify_packet_1(struct rxrpc_call *call, struct sk_buff *skb,
>  				 unsigned int offset, unsigned int len,
> -				 rxrpc_seq_t seq)
> +				 rxrpc_seq_t seq,
> +				 struct skcipher_request *req)
>  {
>  	struct rxkad_level1_hdr sechdr;
> -	SKCIPHER_REQUEST_ON_STACK(req, call->conn->cipher);
>  	struct rxrpc_crypt iv;
>  	struct scatterlist sg[16];
>  	struct sk_buff *trailer;
> @@ -402,11 +403,11 @@ static int rxkad_verify_packet_1(struct rxrpc_call *call, struct sk_buff *skb,
>   */
>  static int rxkad_verify_packet_2(struct rxrpc_call *call, struct sk_buff *skb,
>  				 unsigned int offset, unsigned int len,
> -				 rxrpc_seq_t seq)
> +				 rxrpc_seq_t seq,
> +				 struct skcipher_request *req)
>  {
>  	const struct rxrpc_key_token *token;
>  	struct rxkad_level2_hdr sechdr;
> -	SKCIPHER_REQUEST_ON_STACK(req, call->conn->cipher);
>  	struct rxrpc_crypt iv;
>  	struct scatterlist _sg[4], *sg;
>  	struct sk_buff *trailer;
> @@ -549,9 +550,9 @@ static int rxkad_verify_packet(struct rxrpc_call *call, struct sk_buff *skb,
>  	case RXRPC_SECURITY_PLAIN:
>  		return 0;
>  	case RXRPC_SECURITY_AUTH:
> -		return rxkad_verify_packet_1(call, skb, offset, len, seq);
> +		return rxkad_verify_packet_1(call, skb, offset, len, seq, req);
>  	case RXRPC_SECURITY_ENCRYPT:
> -		return rxkad_verify_packet_2(call, skb, offset, len, seq);
> +		return rxkad_verify_packet_2(call, skb, offset, len, seq, req);
>  	default:
>  		return -ENOANO;
>  	}
> -- 
> 2.17.1

How about doing the 'skcipher_request_set_tfm(req, call->conn->cipher)' and
'skcipher_request_zero(req);' just once, in the top-level function
rxkad_verify_packet(), instead of before/after every time the request is used?

- Eric