macsec: restore uAPI after addition of GCM-AES-256

Commit ccfdec908922 ("macsec: Add support for GCM-AES-256 cipher suite")
changed a few values in the uapi headers for MACsec.

Because of existing userspace implementations, we need to preserve the
value of MACSEC_DEFAULT_CIPHER_ID. Not doing that resulted in
wpa_supplicant segfaults when a secure channel was created using the
default cipher. Thus, swap MACSEC_DEFAULT_CIPHER_{ID,ALT} back to their
original values.

Changing the maximum length of the MACSEC_SA_ATTR_KEY attribute is
unnecessary, as the previous value (MACSEC_MAX_KEY_LEN, which was 128B)
is large enough to carry 32-bytes keys. This patch reverts
MACSEC_MAX_KEY_LEN to 128B and restores the old length check on
MACSEC_SA_ATTR_KEY.

Fixes: ccfdec908922 ("macsec: Add support for GCM-AES-256 cipher suite")
Signed-off-by: Davide Caratti <[e-mail]>
Signed-off-by: Sabrina Dubroca <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/macsec.c

···
 #define MACSEC_GCM_AES_128_SAK_LEN 16
 #define MACSEC_GCM_AES_256_SAK_LEN 32
 
-#define MAX_SAK_LEN MACSEC_GCM_AES_256_SAK_LEN
-
 #define DEFAULT_SAK_LEN MACSEC_GCM_AES_128_SAK_LEN
 #define DEFAULT_SEND_SCI true
 #define DEFAULT_ENCRYPT false
···
 	[MACSEC_SA_ATTR_KEYID] = { .type = NLA_BINARY,
 				   .len = MACSEC_KEYID_LEN, },
 	[MACSEC_SA_ATTR_KEY] = { .type = NLA_BINARY,
-				 .len = MAX_SAK_LEN, },
+				 .len = MACSEC_MAX_KEY_LEN, },
 };
 
 static int parse_sa_config(struct nlattr **attrs, struct nlattr **tb_sa)
···
 
 	switch (secy->key_len) {
 	case MACSEC_GCM_AES_128_SAK_LEN:
-		csid = MACSEC_CIPHER_ID_GCM_AES_128;
+		csid = MACSEC_DEFAULT_CIPHER_ID;
 		break;
 	case MACSEC_GCM_AES_256_SAK_LEN:
 		csid = MACSEC_CIPHER_ID_GCM_AES_256;
···
 	if (data[IFLA_MACSEC_CIPHER_SUITE]) {
 		switch (nla_get_u64(data[IFLA_MACSEC_CIPHER_SUITE])) {
 		case MACSEC_CIPHER_ID_GCM_AES_128:
-		case MACSEC_DEFAULT_CIPHER_ALT:
+		case MACSEC_DEFAULT_CIPHER_ID:
 			secy->key_len = MACSEC_GCM_AES_128_SAK_LEN;
 			break;
 		case MACSEC_CIPHER_ID_GCM_AES_256:
···
 	switch (csid) {
 	case MACSEC_CIPHER_ID_GCM_AES_128:
 	case MACSEC_CIPHER_ID_GCM_AES_256:
-	case MACSEC_DEFAULT_CIPHER_ALT:
+	case MACSEC_DEFAULT_CIPHER_ID:
 		if (icv_len < MACSEC_MIN_ICV_LEN ||
 		    icv_len > MACSEC_STD_ICV_LEN)
 			return -EINVAL;
···
 
 	switch (secy->key_len) {
 	case MACSEC_GCM_AES_128_SAK_LEN:
-		csid = MACSEC_CIPHER_ID_GCM_AES_128;
+		csid = MACSEC_DEFAULT_CIPHER_ID;
 		break;
 	case MACSEC_GCM_AES_256_SAK_LEN:
 		csid = MACSEC_CIPHER_ID_GCM_AES_256;

include/uapi/linux/if_macsec.h

···
 #define MACSEC_GENL_NAME "macsec"
 #define MACSEC_GENL_VERSION 1
 
-#define MACSEC_MAX_KEY_LEN 256
+#define MACSEC_MAX_KEY_LEN 128
 
 #define MACSEC_KEYID_LEN 16
 
···
 #define MACSEC_CIPHER_ID_GCM_AES_128 0x0080C20001000001ULL
 #define MACSEC_CIPHER_ID_GCM_AES_256 0x0080C20001000002ULL
 
-#define MACSEC_DEFAULT_CIPHER_ID     MACSEC_CIPHER_ID_GCM_AES_128
 /* deprecated cipher ID for GCM-AES-128 */
-#define MACSEC_DEFAULT_CIPHER_ALT    0x0080020001000001ULL
+#define MACSEC_DEFAULT_CIPHER_ID     0x0080020001000001ULL
+#define MACSEC_DEFAULT_CIPHER_ALT    MACSEC_CIPHER_ID_GCM_AES_128
 
 #define MACSEC_MIN_ICV_LEN 8
 #define MACSEC_MAX_ICV_LEN 32