Fix another crash at KEXINIT time, ahem.

This is the same code I previously fixed for failing to check NULL
pointers coming back from ssh_pkt_getstring if the server's KEXINIT
ended early, leading to an embarrassing segfault in place of a fatal
error message. But I've now also had it pointed out to me that the
fatal error message passes the string as %s, which is inappropriate
because (being read straight out of the middle of an SSH packet) it
isn't necessarily zero-terminated!

This is still just an embarrassing segfault in place of a fatal error
message, and not exploitable as far as I can see, because the string
is passed to a dupprintf, which will either read off the end of
allocated address space and segfault non-exploitably, or else it will
find a NUL after all and carefully allocate enough space to format an
error message containing all of the previous junk. But still, how
embarrassing to have messed up the same code _twice_.

[originally from svn r10211]

ssh.c

@@ -6226,6 +6226,10 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
 
 	pktin->savedpos += 16;	        /* skip garbage cookie */
 	ssh_pkt_getstring(pktin, &str, &len);    /* key exchange algorithms */
+        if (!str) {
+            bombout(("KEXINIT packet was incomplete"));
+            crStopV;
+        }
 
 	preferred = NULL;
 	for (i = 0; i < s->n_preferred_kex; i++) {
@@ -6245,8 +6249,8 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
 		break;
 	}
 	if (!ssh->kex) {
-	    bombout(("Couldn't agree a key exchange algorithm (available: %s)",
+            bombout(("Couldn't agree a key exchange algorithm"
-		     str ? str : "(null)"));
+                     " (available: %.*s)", len, str));
 	    crStopV;
 	}
 	/*
@@ -6256,6 +6260,10 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
 	 */
 	s->guessok = first_in_commasep_string(preferred, str, len);
 	ssh_pkt_getstring(pktin, &str, &len);    /* host key algorithms */
+        if (!str) {
+            bombout(("KEXINIT packet was incomplete"));
+            crStopV;
+        }
 	for (i = 0; i < lenof(hostkey_algs); i++) {
 	    if (in_commasep_string(hostkey_algs[i]->name, str, len)) {
 		ssh->hostkey = hostkey_algs[i];
@@ -6263,14 +6271,18 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
 	    }
 	}
 	if (!ssh->hostkey) {
-	    bombout(("Couldn't agree a host key algorithm (available: %s)",
+            bombout(("Couldn't agree a host key algorithm"
-		     str ? str : "(null)"));
+                     " (available: %.*s)", len, str));
 	    crStopV;
 	}
 
 	s->guessok = s->guessok &&
 	    first_in_commasep_string(hostkey_algs[0]->name, str, len);
 	ssh_pkt_getstring(pktin, &str, &len);    /* client->server cipher */
+        if (!str) {
+            bombout(("KEXINIT packet was incomplete"));
+            crStopV;
+        }
 	for (i = 0; i < s->n_preferred_ciphers; i++) {
 	    const struct ssh2_ciphers *c = s->preferred_ciphers[i];
 	    if (!c) {
@@ -6287,12 +6299,16 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
 		break;
 	}
 	if (!s->cscipher_tobe) {
-	    bombout(("Couldn't agree a client-to-server cipher (available: %s)",
+            bombout(("Couldn't agree a client-to-server cipher"
-		     str ? str : "(null)"));
+                     " (available: %.*s)", len, str));
 	    crStopV;
 	}
 
 	ssh_pkt_getstring(pktin, &str, &len);    /* server->client cipher */
+        if (!str) {
+            bombout(("KEXINIT packet was incomplete"));
+            crStopV;
+        }
 	for (i = 0; i < s->n_preferred_ciphers; i++) {
 	    const struct ssh2_ciphers *c = s->preferred_ciphers[i];
 	    if (!c) {
@@ -6309,12 +6325,16 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
 		break;
 	}
 	if (!s->sccipher_tobe) {
-	    bombout(("Couldn't agree a server-to-client cipher (available: %s)",
+            bombout(("Couldn't agree a server-to-client cipher"
-		     str ? str : "(null)"));
+                     " (available: %.*s)", len, str));
 	    crStopV;
 	}
 
 	ssh_pkt_getstring(pktin, &str, &len);    /* client->server mac */
+        if (!str) {
+            bombout(("KEXINIT packet was incomplete"));
+            crStopV;
+        }
 	for (i = 0; i < s->nmacs; i++) {
 	    if (in_commasep_string(s->maclist[i]->name, str, len)) {
 		s->csmac_tobe = s->maclist[i];
@@ -6322,6 +6342,10 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
 	    }
 	}
 	ssh_pkt_getstring(pktin, &str, &len);    /* server->client mac */
+        if (!str) {
+            bombout(("KEXINIT packet was incomplete"));
+            crStopV;
+        }
 	for (i = 0; i < s->nmacs; i++) {
 	    if (in_commasep_string(s->maclist[i]->name, str, len)) {
 		s->scmac_tobe = s->maclist[i];
@@ -6329,6 +6353,10 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
 	    }
 	}
 	ssh_pkt_getstring(pktin, &str, &len);  /* client->server compression */
+        if (!str) {
+            bombout(("KEXINIT packet was incomplete"));
+            crStopV;
+        }
 	for (i = 0; i < lenof(compressions) + 1; i++) {
 	    const struct ssh_compress *c =
 		i == 0 ? s->preferred_comp : compressions[i - 1];
@@ -6345,6 +6373,10 @@ static void do_ssh2_transport(Ssh ssh, void *vin, int inlen,
 	    }
 	}
 	ssh_pkt_getstring(pktin, &str, &len);  /* server->client compression */
+        if (!str) {
+            bombout(("KEXINIT packet was incomplete"));
+            crStopV;
+        }
 	for (i = 0; i < lenof(compressions) + 1; i++) {
 	    const struct ssh_compress *c =
 		i == 0 ? s->preferred_comp : compressions[i - 1];