Disallow remote addition of FIDO/PKCS11 provider libraries to

ssh-agent by default.

The old behaviour of allowing remote clients from loading providers
can be restored using `ssh-agent -O allow-remote-pkcs11`.

Detection of local/remote clients requires a ssh(1) that supports
the `session-bind@openssh.com` extension. Forwarding access to a
ssh-agent socket using non-OpenSSH tools may circumvent this control.

ok markus@

diff --git a/usr.bin/ssh/ssh-agent.1 b/usr.bin/ssh/ssh-agent.1
index b0bf65d..beb8186 100644 (file)
--- a/usr.bin/ssh/ssh-agent.1
+++ b/usr.bin/ssh/ssh-agent.1
@@ -1,4 +1,4 @@
-.\" $OpenBSD: ssh-agent.1,v 1.75 2022/10/07 06:00:58 jmc Exp $
+.\" $OpenBSD: ssh-agent.1,v 1.76 2023/07/19 13:56:33 djm Exp $
 .\"
 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -34,7 +34,7 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd $Mdocdate: October 7 2022 $
+.Dd $Mdocdate: July 19 2023 $
 .Dt SSH-AGENT 1
 .Os
 .Sh NAME
@@ -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.

diff --git a/usr.bin/ssh/ssh-agent.c b/usr.bin/ssh/ssh-agent.c
index 79b1161..0b2ee97 100644 (file)
--- a/usr.bin/ssh/ssh-agent.c
+++ b/usr.bin/ssh/ssh-agent.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-agent.c,v 1.299 2023/07/10 04:51:26 djm Exp $ */
+/* $OpenBSD: ssh-agent.c,v 1.300 2023/07/19 13:56:33 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -156,6 +156,12 @@ 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 session-bind@openssh.com).
+ */
+static int remote_add_provider;
+
 /* locking */
 #define LOCK_SIZE      32
 #define LOCK_SALT_SIZE 16
@@ -1215,6 +1221,12 @@ 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,
@@ -1378,6 +1390,11 @@ 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));
@@ -2032,7 +2049,9 @@ 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;