*
* - OUR_V2_BIGWIN is the window size we advertise for the only
* channel in a simple connection. It must be <= INT_MAX.
+ *
+ * - OUR_V2_MAXPKT is the official "maximum packet size" we send
+ * to the remote side. This actually has nothing to do with the
+ * size of the _packet_, but is instead a limit on the amount
+ * of data we're willing to receive in a single SSH2 channel
+ * data message.
+ *
+ * - OUR_V2_PACKETLIMIT is actually the maximum size of SSH
+ * _packet_ we're prepared to cope with. It must be a multiple
+ * of the cipher block size, and must be at least 35000.
*/
#define SSH1_BUFFER_LIMIT 32768
#define OUR_V2_WINSIZE 16384
#define OUR_V2_BIGWIN 0x7fffffff
#define OUR_V2_MAXPKT 0x4000UL
+#define OUR_V2_PACKETLIMIT 0x9000UL
/* Maximum length of passwords/passphrases (arbitrary) */
#define SSH_MAX_PASSWORD_LEN 100
st->cipherblk = 8;
if (st->cipherblk < 8)
st->cipherblk = 8;
+ st->maclen = ssh->scmac ? ssh->scmac->len : 0;
- st->pktin->data = snewn(st->cipherblk + APIEXTRA, unsigned char);
+ if (ssh->sccipher && (ssh->sccipher->flags & SSH_CIPHER_IS_CBC) &&
+ ssh->scmac) {
+ /*
+ * When dealing with a CBC-mode cipher, we want to avoid the
+ * possibility of an attacker's tweaking the ciphertext stream
+ * so as to cause us to feed the same block to the block
+ * cipher more than once and thus leak information
+ * (VU#958563). The way we do this is not to take any
+ * decisions on the basis of anything we've decrypted until
+ * we've verified it with a MAC. That includes the packet
+ * length, so we just read data and check the MAC repeatedly,
+ * and when the MAC passes, see if the length we've got is
+ * plausible.
+ */
- /*
- * Acquire and decrypt the first block of the packet. This will
- * contain the length and padding details.
- */
- for (st->i = st->len = 0; st->i < st->cipherblk; st->i++) {
- while ((*datalen) == 0)
- crReturn(NULL);
- st->pktin->data[st->i] = *(*data)++;
- (*datalen)--;
- }
+ /* May as well allocate the whole lot now. */
+ st->pktin->data = snewn(OUR_V2_PACKETLIMIT + st->maclen + APIEXTRA,
+ unsigned char);
- if (ssh->sccipher)
- ssh->sccipher->decrypt(ssh->sc_cipher_ctx,
- st->pktin->data, st->cipherblk);
+ /* Read an amount corresponding to the MAC. */
+ for (st->i = 0; st->i < st->maclen; st->i++) {
+ while ((*datalen) == 0)
+ crReturn(NULL);
+ st->pktin->data[st->i] = *(*data)++;
+ (*datalen)--;
+ }
- /*
- * Now get the length and padding figures.
- */
- st->len = GET_32BIT(st->pktin->data);
- st->pad = st->pktin->data[4];
+ st->packetlen = 0;
+ {
+ unsigned char seq[4];
+ ssh->scmac->start(ssh->sc_mac_ctx);
+ PUT_32BIT(seq, st->incoming_sequence);
+ ssh->scmac->bytes(ssh->sc_mac_ctx, seq, 4);
+ }
- /*
- * _Completely_ silly lengths should be stomped on before they
- * do us any more damage.
- */
- if (st->len < 0 || st->len > 35000 || st->pad < 4 ||
- st->len - st->pad < 1 || (st->len + 4) % st->cipherblk != 0) {
- bombout(("Incoming packet was garbled on decryption"));
- ssh_free_packet(st->pktin);
- crStop(NULL);
- }
+ for (;;) { /* Once around this loop per cipher block. */
+ /* Read another cipher-block's worth, and tack it onto the end. */
+ for (st->i = 0; st->i < st->cipherblk; st->i++) {
+ while ((*datalen) == 0)
+ crReturn(NULL);
+ st->pktin->data[st->packetlen+st->maclen+st->i] = *(*data)++;
+ (*datalen)--;
+ }
+ /* Decrypt one more block (a little further back in the stream). */
+ ssh->sccipher->decrypt(ssh->sc_cipher_ctx,
+ st->pktin->data + st->packetlen,
+ st->cipherblk);
+ /* Feed that block to the MAC. */
+ ssh->scmac->bytes(ssh->sc_mac_ctx,
+ st->pktin->data + st->packetlen, st->cipherblk);
+ st->packetlen += st->cipherblk;
+ /* See if that gives us a valid packet. */
+ if (ssh->scmac->verresult(ssh->sc_mac_ctx,
+ st->pktin->data + st->packetlen) &&
+ (st->len = GET_32BIT(st->pktin->data)) + 4 == st->packetlen)
+ break;
+ if (st->packetlen >= OUR_V2_PACKETLIMIT) {
+ bombout(("No valid incoming packet found"));
+ ssh_free_packet(st->pktin);
+ crStop(NULL);
+ }
+ }
+ st->pktin->maxlen = st->packetlen + st->maclen;
+ st->pktin->data = sresize(st->pktin->data,
+ st->pktin->maxlen + APIEXTRA,
+ unsigned char);
+ } else {
+ st->pktin->data = snewn(st->cipherblk + APIEXTRA, unsigned char);
- /*
- * This enables us to deduce the payload length.
- */
- st->payload = st->len - st->pad - 1;
+ /*
+ * Acquire and decrypt the first block of the packet. This will
+ * contain the length and padding details.
+ */
+ for (st->i = st->len = 0; st->i < st->cipherblk; st->i++) {
+ while ((*datalen) == 0)
+ crReturn(NULL);
+ st->pktin->data[st->i] = *(*data)++;
+ (*datalen)--;
+ }
- st->pktin->length = st->payload + 5;
+ if (ssh->sccipher)
+ ssh->sccipher->decrypt(ssh->sc_cipher_ctx,
+ st->pktin->data, st->cipherblk);
- /*
- * So now we can work out the total packet length.
- */
- st->packetlen = st->len + 4;
- st->maclen = ssh->scmac ? ssh->scmac->len : 0;
+ /*
+ * Now get the length figure.
+ */
+ st->len = GET_32BIT(st->pktin->data);
- /*
- * Allocate memory for the rest of the packet.
- */
- st->pktin->maxlen = st->packetlen + st->maclen;
- st->pktin->data = sresize(st->pktin->data,
- st->pktin->maxlen + APIEXTRA,
- unsigned char);
+ /*
+ * _Completely_ silly lengths should be stomped on before they
+ * do us any more damage.
+ */
+ if (st->len < 0 || st->len > OUR_V2_PACKETLIMIT ||
+ (st->len + 4) % st->cipherblk != 0) {
+ bombout(("Incoming packet was garbled on decryption"));
+ ssh_free_packet(st->pktin);
+ crStop(NULL);
+ }
- /*
- * Read and decrypt the remainder of the packet.
- */
- for (st->i = st->cipherblk; st->i < st->packetlen + st->maclen;
- st->i++) {
- while ((*datalen) == 0)
- crReturn(NULL);
- st->pktin->data[st->i] = *(*data)++;
- (*datalen)--;
- }
- /* Decrypt everything _except_ the MAC. */
- if (ssh->sccipher)
- ssh->sccipher->decrypt(ssh->sc_cipher_ctx,
- st->pktin->data + st->cipherblk,
- st->packetlen - st->cipherblk);
+ /*
+ * So now we can work out the total packet length.
+ */
+ st->packetlen = st->len + 4;
- st->pktin->encrypted_len = st->packetlen;
+ /*
+ * Allocate memory for the rest of the packet.
+ */
+ st->pktin->maxlen = st->packetlen + st->maclen;
+ st->pktin->data = sresize(st->pktin->data,
+ st->pktin->maxlen + APIEXTRA,
+ unsigned char);
- /*
- * Check the MAC.
- */
- if (ssh->scmac
- && !ssh->scmac->verify(ssh->sc_mac_ctx, st->pktin->data, st->len + 4,
- st->incoming_sequence)) {
- bombout(("Incorrect MAC received on packet"));
+ /*
+ * Read and decrypt the remainder of the packet.
+ */
+ for (st->i = st->cipherblk; st->i < st->packetlen + st->maclen;
+ st->i++) {
+ while ((*datalen) == 0)
+ crReturn(NULL);
+ st->pktin->data[st->i] = *(*data)++;
+ (*datalen)--;
+ }
+ /* Decrypt everything _except_ the MAC. */
+ if (ssh->sccipher)
+ ssh->sccipher->decrypt(ssh->sc_cipher_ctx,
+ st->pktin->data + st->cipherblk,
+ st->packetlen - st->cipherblk);
+
+ /*
+ * Check the MAC.
+ */
+ if (ssh->scmac
+ && !ssh->scmac->verify(ssh->sc_mac_ctx, st->pktin->data,
+ st->len + 4, st->incoming_sequence)) {
+ bombout(("Incorrect MAC received on packet"));
+ ssh_free_packet(st->pktin);
+ crStop(NULL);
+ }
+ }
+ /* Get and sanity-check the amount of random padding. */
+ st->pad = st->pktin->data[4];
+ if (st->pad < 4 || st->len - st->pad < 1) {
+ bombout(("Invalid padding length on received packet"));
ssh_free_packet(st->pktin);
crStop(NULL);
}
+ /*
+ * This enables us to deduce the payload length.
+ */
+ st->payload = st->len - st->pad - 1;
+
+ st->pktin->length = st->payload + 5;
+ st->pktin->encrypted_len = st->packetlen;
st->pktin->sequence = st->incoming_sequence++;
projects / u / mdw / putty / blobdiff