[PATCH] rxrpc: Reuse SKCIPHER_REQUEST_ON_STACK buffer

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 <redacted>
---
 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


-- 
Kees Cook
Pixel Security

On Mon, Jul 16, 2018 at 5:49 AM, Kees Cook [off-list ref] 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 <redacted>

This looks like a very nice solution to the problem.

Acked-by: Arnd Bergmann <arnd@arndb.de>

Since the large stack usage could already cause problems in older kernels,
should this be backported to stable kernels as well?

      Arnd

On Sun, Jul 15, 2018 at 08:49:47PM -0700, Kees Cook wrote:

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

On Mon, Jul 16, 2018 at 3:17 AM, Arnd Bergmann [off-list ref] wrote:

On Mon, Jul 16, 2018 at 5:49 AM, Kees Cook [off-list ref] wrote:

quoted

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 <redacted>

This looks like a very nice solution to the problem.

Acked-by: Arnd Bergmann <arnd@arndb.de>

Since the large stack usage could already cause problems in older kernels,
should this be backported to stable kernels as well?

I don't think this hits the bar for doing a -stable patch, but if you
think I should, I can do it.

-Kees

-- 
Kees Cook
Pixel Security

On Mon, Jul 16, 2018 at 10:42 AM, Eric Biggers [off-list ref] wrote:

On Sun, Jul 15, 2018 at 08:49:47PM -0700, Kees Cook wrote:

quoted

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 <redacted>
---
 net/rxrpc/rxkad.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)
[...]

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?

Hm. While that does sound reasonable, I like having it be
operationally unchanged. And changing this makes the caller function a
bit more weird. Right now it's doing direct returns, and we'd need to
cover error paths, etc. Having the "zero" _right_ after the
encrypt/decrypt seems more robust to me.

-Kees

-- 
Kees Cook
Pixel Security