- markus@cvs.openbsd.org 2009/01/26 09:58:15
[cipher.c cipher.h packet.c]
Work around the CPNI-957037 Plaintext Recovery Attack by always
reading 256K of data on packet size or HMAC errors (in CBC mode only).
Help, feedback and ok djm@
Feedback from Martin Albrecht and Paterson Kenny
diff --git a/packet.c b/packet.c
index 4ded17f..33559cd 100644
--- a/packet.c
+++ b/packet.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: packet.c,v 1.158 2008/11/21 15:47:38 markus Exp $ */
+/* $OpenBSD: packet.c,v 1.159 2009/01/26 09:58:15 markus Exp $ */
/*
* Author: Tatu Ylonen <ylo@cs.hut.fi>
* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -84,6 +84,8 @@
#define DBG(x)
#endif
+#define PACKET_MAX_SIZE (256 * 1024)
+
/*
* This variable contains the file descriptors used for communicating with
* the other side. connection_in is used for reading; connection_out for
@@ -160,6 +162,10 @@
/* roundup current message to extra_pad bytes */
static u_char extra_pad = 0;
+/* XXX discard incoming data after MAC error */
+static u_int packet_discard = 0;
+static Mac *packet_discard_mac = NULL;
+
struct packet {
TAILQ_ENTRY(packet) next;
u_char type;
@@ -209,6 +215,36 @@
packet_timeout_ms = timeout * count * 1000;
}
+static void
+packet_stop_discard(void)
+{
+ if (packet_discard_mac) {
+ char buf[1024];
+
+ memset(buf, 'a', sizeof(buf));
+ while (buffer_len(&incoming_packet) < PACKET_MAX_SIZE)
+ buffer_append(&incoming_packet, buf, sizeof(buf));
+ (void) mac_compute(packet_discard_mac,
+ p_read.seqnr,
+ buffer_ptr(&incoming_packet),
+ PACKET_MAX_SIZE);
+ }
+ logit("Finished discarding for %.200s", get_remote_ipaddr());
+ cleanup_exit(255);
+}
+
+static void
+packet_start_discard(Enc *enc, Mac *mac, u_int packet_length, u_int discard)
+{
+ if (!cipher_is_cbc(enc->cipher))
+ packet_disconnect("Packet corrupt");
+ if (packet_length != PACKET_MAX_SIZE && mac && mac->enabled)
+ packet_discard_mac = mac;
+ if (buffer_len(&input) >= discard)
+ packet_stop_discard();
+ packet_discard = discard - buffer_len(&input);
+}
+
/* Returns 1 if remote host is connected via socket, 0 if not. */
int
@@ -1127,6 +1163,9 @@
Mac *mac = NULL;
Comp *comp = NULL;
+ if (packet_discard)
+ return SSH_MSG_NONE;
+
if (newkeys[MODE_IN] != NULL) {
enc = &newkeys[MODE_IN]->enc;
mac = &newkeys[MODE_IN]->mac;
@@ -1148,12 +1187,14 @@
block_size);
cp = buffer_ptr(&incoming_packet);
packet_length = get_u32(cp);
- if (packet_length < 1 + 4 || packet_length > 256 * 1024) {
+ if (packet_length < 1 + 4 || packet_length > PACKET_MAX_SIZE) {
#ifdef PACKET_DEBUG
buffer_dump(&incoming_packet);
#endif
- packet_disconnect("Bad packet length %-10u",
- packet_length);
+ logit("Bad packet length %u.", packet_length);
+ packet_start_discard(enc, mac, packet_length,
+ PACKET_MAX_SIZE);
+ return SSH_MSG_NONE;
}
DBG(debug("input: packet len %u", packet_length+4));
buffer_consume(&input, block_size);
@@ -1165,7 +1206,9 @@
if (need % block_size != 0) {
logit("padding error: need %d block %d mod %d",
need, block_size, need % block_size);
- packet_disconnect("Bad packet length %-10u", packet_length);
+ packet_start_discard(enc, mac, packet_length,
+ PACKET_MAX_SIZE - block_size);
+ return SSH_MSG_NONE;
}
/*
* check if the entire packet has been received and
@@ -1188,11 +1231,19 @@
macbuf = mac_compute(mac, p_read.seqnr,
buffer_ptr(&incoming_packet),
buffer_len(&incoming_packet));
- if (memcmp(macbuf, buffer_ptr(&input), mac->mac_len) != 0)
- packet_disconnect("Corrupted MAC on input.");
+ if (memcmp(macbuf, buffer_ptr(&input), mac->mac_len) != 0) {
+ logit("Corrupted MAC on input.");
+ if (need > PACKET_MAX_SIZE)
+ fatal("internal error need %d", need);
+ packet_start_discard(enc, mac, packet_length,
+ PACKET_MAX_SIZE - need);
+ return SSH_MSG_NONE;
+ }
+
DBG(debug("MAC #%d ok", p_read.seqnr));
buffer_consume(&input, mac->mac_len);
}
+ /* XXX now it's safe to use fatal/packet_disconnect */
if (seqnr_p != NULL)
*seqnr_p = p_read.seqnr;
if (++p_read.seqnr == 0)
@@ -1325,6 +1376,13 @@
void
packet_process_incoming(const char *buf, u_int len)
{
+ if (packet_discard) {
+ keep_alive_timeouts = 0; /* ?? */
+ if (len >= packet_discard)
+ packet_stop_discard();
+ packet_discard -= len;
+ return;
+ }
buffer_append(&input, buf, len);
}