ssh: extend 'dh_gex_limits' to server side

OTP-13066

4 files changed, 57 insertions, 7 deletions

diff --git a/lib/ssh/doc/src/ssh.xml b/lib/ssh/doc/src/ssh.xml
index 1e4dd91eb6..7c1b9ea0dc 100644
--- a/lib/ssh/doc/src/ssh.xml
+++ b/lib/ssh/doc/src/ssh.xml
@@ -475,6 +475,17 @@ kex is implicit but public_key is set explicitly.</p>
 	    </p>
 	  </item>
 
+	  <tag><c><![CDATA[{dh_gex_limits,{Min=integer(),Max=integer()}}]]></c></tag>
+	  <item>
+	    <p>Limits what a client can ask for in diffie-hellman-group-exchange. The effective value will be
+	    <c>MaxUsed = min(MaxClient,Max), MinUsed = max(MinClient,Min)</c>.
+	    </p>
+	    <p>If <c>MaxUsed &lt; MinUses</c> in a key exchange, it will fail with a disconnect.
+	    </p>
+	    <p>See RFC 4419 for the function of the max an min values.
+	    </p>
+	  </item>
+
 	  <tag><c><![CDATA[{pwdfun, fun(User::string(), password::string()) -> boolean()}]]></c></tag>
 	  <item>
 	    <p>Provides a function for password validation. This function is called
diff --git a/lib/ssh/src/ssh.erl b/lib/ssh/src/ssh.erl
index 9befceb51b..39cf441090 100644
--- a/lib/ssh/src/ssh.erl
+++ b/lib/ssh/src/ssh.erl
@@ -474,9 +474,14 @@ handle_ssh_option({dh_gex_groups,{Tag,File=[C|_]}}=Opt) when is_integer(C), C>0,
     end;	    
     
 
+handle_ssh_option({dh_gex_limits,{Min,Max}} = Opt) when is_integer(Min), Min>0, 
+							is_integer(Max), Max>=Min ->
+    %% Server
+    Opt;
 handle_ssh_option({dh_gex_limits,{Min,I,Max}} = Opt) when is_integer(Min), Min>0, 
 							  is_integer(I),   I>=Min,
 							  is_integer(Max), Max>=I ->
+    %% Client
     Opt;
 handle_ssh_option({connect_timeout, Value} = Opt) when is_integer(Value); Value == infinity ->
     Opt;
diff --git a/lib/ssh/src/ssh_transport.erl b/lib/ssh/src/ssh_transport.erl
index 35bfadb1df..d61fc76c0a 100644
--- a/lib/ssh/src/ssh_transport.erl
+++ b/lib/ssh/src/ssh_transport.erl
@@ -441,13 +441,14 @@ handle_kexdh_reply(#ssh_msg_kexdh_reply{public_host_key = PeerPubHostKey,
 %%%
 %%% diffie-hellman-group-exchange-sha1
 %%% 
-handle_kex_dh_gex_request(#ssh_msg_kex_dh_gex_request{min = Min,
+handle_kex_dh_gex_request(#ssh_msg_kex_dh_gex_request{min = Min0,
 						      n   = NBits,
-						      max = Max}, 
-			  Ssh0=#ssh{opts=Opts}) when Min=<NBits, NBits=<Max ->
+						      max = Max0}, 
+			  Ssh0=#ssh{opts=Opts}) when Min0=<NBits, NBits=<Max0 ->
     %% server
+    {Min, Max} = adjust_gex_min_max(Min0, Max0, Opts),
     case public_key:dh_gex_group(Min, NBits, Max,
-			     proplists:get_value(dh_gex_groups,Opts)) of
+				 proplists:get_value(dh_gex_groups,Opts)) of
 	{ok, {_Sz, {G,P}}} ->
 	    {Public, Private} = generate_key(dh, [P,G]),
 	    {SshPacket, Ssh} = 
@@ -471,6 +472,26 @@ handle_kex_dh_gex_request(_, _) ->
 	    language = ""}
 	}).
 
+
+adjust_gex_min_max(Min0, Max0, Opts) ->
+    case proplists:get_value(dh_gex_limits, Opts) of
+	undefined ->
+	    {Min0, Max0};
+	{Min1, Max1} ->
+	    Min2 = max(Min0, Min1),
+	    Max2 = min(Max0, Max1),
+	    if
+		Min2 =< Max2 ->
+		    {Min2, Max2};
+		Max2 < Min2 ->
+		    throw(#ssh_msg_disconnect{
+			     code = ?SSH_DISCONNECT_PROTOCOL_ERROR,
+			     description = "No possible diffie-hellman-group-exchange group possible", 
+			     language = ""})
+	    end
+    end.
+		    
+
 handle_kex_dh_gex_group(#ssh_msg_kex_dh_gex_group{p = P, g = G}, Ssh0) ->
     %% client
     {Public, Private} = generate_key(dh, [P,G]),
diff --git a/lib/ssh/test/ssh_protocol_SUITE.erl b/lib/ssh/test/ssh_protocol_SUITE.erl
index 0292c8d149..3a7f47c2dd 100644
--- a/lib/ssh/test/ssh_protocol_SUITE.erl
+++ b/lib/ssh/test/ssh_protocol_SUITE.erl
@@ -67,6 +67,7 @@ groups() ->
      {kex, [], [no_common_alg_server_disconnects,
 		no_common_alg_client_disconnects,
 		gex_client_init_option_groups,
+		gex_server_gex_limit,
 		gex_client_init_option_groups_moduli_file,
 		gex_client_init_option_groups_file
 		]},
@@ -92,7 +93,8 @@ init_per_testcase(no_common_alg_server_disconnects, Config) ->
 
 init_per_testcase(TC, Config) when TC == gex_client_init_option_groups ;
 				   TC == gex_client_init_option_groups_moduli_file ;
-				   TC == gex_client_init_option_groups_file ->
+				   TC == gex_client_init_option_groups_file ;
+				   TC == gex_server_gex_limit ->
     Opts = case TC of
 	       gex_client_init_option_groups ->
 		   [{dh_gex_groups, [{2345, 3, 41}]}];
@@ -104,6 +106,12 @@ init_per_testcase(TC, Config) when TC == gex_client_init_option_groups ;
 		   DataDir = ?config(data_dir, Config),
 		   F = filename:join(DataDir, "dh_group_test.moduli"),
 		   [{dh_gex_groups, {ssh_moduli_file,F}}];
+	       gex_server_gex_limit ->
+		    [{dh_gex_groups, [{ 500, 3, 18},
+				      {1000, 7, 91},
+				      {3000, 5, 61}]},
+		     {dh_gex_limits,{500,1500}}
+		    ];
 	       _ ->
 		   []
 	   end,
@@ -117,7 +125,8 @@ end_per_testcase(no_common_alg_server_disconnects, Config) ->
     stop_std_daemon(Config);
 end_per_testcase(TC, Config) when TC == gex_client_init_option_groups ;
 				  TC == gex_client_init_option_groups_moduli_file ;
-				  TC == gex_client_init_option_groups_file ->
+				  TC == gex_client_init_option_groups_file ;
+				  TC == gex_server_gex_limit ->
     stop_std_daemon(Config);
 end_per_testcase(_TestCase, Config) ->
     check_std_daemon_works(Config, ?LINE).
@@ -337,7 +346,6 @@ gex_client_init_option_groups(Config) ->
     do_gex_client_init(Config, {2000, 2048, 4000}, 
 		       {3,41}).
 
-
 gex_client_init_option_groups_file(Config) ->
     do_gex_client_init(Config, {2000, 2048, 4000},
 		       {5,61}).
@@ -346,6 +354,11 @@ gex_client_init_option_groups_moduli_file(Config) ->
     do_gex_client_init(Config, {2000, 2048, 4000},
 		       {5,16#B7}).
 
+gex_server_gex_limit(Config) ->
+    do_gex_client_init(Config, {1000, 3000, 4000},
+		       {7,91}).
+
+
 do_gex_client_init(Config, {Min,N,Max}, {G,P}) ->
     {ok,_} =
 	ssh_trpt_test_lib:exec(