disallow remote addition of FIDO/PKCS11 keys

Depends on the local client performing the [email protected]
operation, so non-OpenSSH local client may circumvent this.

Author: djmdjm

ssh-agent.1

@@ -107,9 +107,27 @@ environment variable).
 .It Fl O Ar option
 Specify an option when starting
 .Nm .
-Currently only one option is supported:
+Currently two options are supported:
+.Cm allow-remote-pkcs11
+and
 .Cm no-restrict-websafe .
-This instructs
+.Pp
+The
+.Cm allow-remote-pkcs11
+option allows clients of a forwarded
+.Nm
+to load PKCS#11 or FIDO provider libraries.
+By default only local clients may perform this operation.
+Note that signalling that a
+.Nm
+client remote is performed by
+.Xr ssh 1 ,
+and use of other tools to forward access to the agent socket may circumvent
+this restriction.
+.Pp
+The
+.Cm no-restrict-websafe ,
+instructs
 .Nm
 to permit signatures using FIDO keys that might be web authentication
 requests.

ssh-agent.c

@@ -169,6 +169,12 @@ char socket_dir[PATH_MAX];
 /* Pattern-list of allowed PKCS#11/Security key paths */
 static char *allowed_providers;
 
+/*
+ * Allows PKCS11 providers or SK keys that use non-internal providers to
+ * be added over a remote connection (identified by [email protected]).
+ */
+static int remote_add_provider;
+
 /* locking */
 #define LOCK_SIZE	32
 #define LOCK_SALT_SIZE	16
@@ -1228,6 +1234,12 @@ process_add_identity(SocketEntry *e)
 		if (strcasecmp(sk_provider, "internal") == 0) {
 			debug_f("internal provider");
 		} else {
+			if (e->nsession_ids != 0 && !remote_add_provider) {
+				verbose("failed add of SK provider \"%.100s\": "
+				    "remote addition of providers is disabled",
+				    sk_provider);
+				goto out;
+			}
 			if (realpath(sk_provider, canonical_provider) == NULL) {
 				verbose("failed provider \"%.100s\": "
 				    "realpath: %s", sk_provider,
@@ -1391,6 +1403,11 @@ process_add_smartcard_key(SocketEntry *e)
 		error_f("failed to parse constraints");
 		goto send;
 	}
+	if (e->nsession_ids != 0 && !remote_add_provider) {
+		verbose("failed PKCS#11 add of \"%.100s\": remote addition of "
+		    "providers is disabled", provider);
+		goto send;
+	}
 	if (realpath(provider, canonical_provider) == NULL) {
 		verbose("failed PKCS#11 add of \"%.100s\": realpath: %s",
 		    provider, strerror(errno));
@@ -2050,7 +2067,9 @@ main(int ac, char **av)
 			break;
 		case 'O':
 			if (strcmp(optarg, "no-restrict-websafe") == 0)
-				restrict_websafe  = 0;
+				restrict_websafe = 0;
+			else if (strcmp(optarg, "allow-remote-pkcs11") == 0)
+				remote_add_provider = 1;
 			else
 				fatal("Unknown -O option");
 			break;