upstream: ssh-agent support for U2F/FIDO keys

feedback & ok markus@

OpenBSD-Commit-ID: bb544a44bc32e45d2ec8bf652db2046f38360acb

1 files changed, 143 insertions, 0 deletions

diff --git a/ssh-sk-helper.c b/ssh-sk-helper.c
new file mode 100644
index 000000000..0a0c92a44
--- /dev/null
+++ b/ssh-sk-helper.c
@@ -0,0 +1,143 @@
+/* $OpenBSD: ssh-sk-helper.c,v 1.1 2019/10/31 21:22:01 djm Exp $ */
+/*
+ * Copyright (c) 2019 Google LLC
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+/*
+ * This is a tiny program used to isolate the address space used for
+ * security key middleware signing operations from ssh-agent. It is similar
+ * to ssh-pkcs11-helper.c but considerably simpler as the signing operation
+ * for this case are stateless.
+ *
+ * It receives a signing request (key, provider, message, flags) from
+ * stdin, attempts to perform a signature using the security key provider
+ * and returns the resultant signature via stdout.
+ *
+ * In the future, this program might gain additional functions to support
+ * FIDO2 tokens such as enumerating resident keys. When this happens it will
+ * be necessary to crank SSH_SK_HELPER_VERSION below.
+ */
+
+#include "includes.h"
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <errno.h>
+
+#include "xmalloc.h"
+#include "log.h"
+#include "sshkey.h"
+#include "authfd.h"
+#include "misc.h"
+#include "sshbuf.h"
+#include "msg.h"
+#include "uidswap.h"
+#include "sshkey.h"
+#include "ssherr.h"
+#include "ssh-sk.h"
+
+extern char *__progname;
+
+int
+main(int argc, char **argv)
+{
+        SyslogFacility log_facility = SYSLOG_FACILITY_AUTH;
+        LogLevel log_level = SYSLOG_LEVEL_ERROR;
+        struct sshbuf *req, *resp, *kbuf;
+        struct sshkey *key;
+        uint32_t compat;
+        const u_char *message;
+        u_char version, *sig;
+        size_t msglen, siglen;
+        char *provider;
+        int in, out, ch, r, log_stderr = 0;
+
+        sanitise_stdfd();
+        log_init(__progname, log_level, log_facility, log_stderr);
+
+        while ((ch = getopt(argc, argv, "v")) != -1) {
+                switch (ch) {
+                case 'v':
+                        log_stderr = 1;
+                        if (log_level == SYSLOG_LEVEL_ERROR)
+                                log_level = SYSLOG_LEVEL_DEBUG1;
+                        else if (log_level < SYSLOG_LEVEL_DEBUG3)
+                                log_level++;
+                        break;
+                default:
+                        fprintf(stderr, "usage: %s [-v]\n", __progname);
+                        exit(1);
+                }
+        }
+        log_init(__progname, log_level, log_facility, log_stderr);
+
+        /*
+         * Rearrange our file descriptors a little; we don't trust the
+         * providers not to fiddle with stdin/out.
+         */
+        closefrom(STDERR_FILENO + 1);
+        if ((in = dup(STDIN_FILENO)) == -1 || (out = dup(STDOUT_FILENO)) == -1)
+                fatal("%s: dup: %s", __progname, strerror(errno));
+        close(STDIN_FILENO);
+        close(STDOUT_FILENO);
+        sanitise_stdfd(); /* resets to /dev/null */
+
+        if ((req = sshbuf_new()) == NULL || (resp = sshbuf_new()) == NULL)
+                fatal("%s: sshbuf_new failed", __progname);
+        if (ssh_msg_recv(in, req) < 0)
+                fatal("ssh_msg_recv failed");
+        close(in);
+        debug("%s: received message len %zu", __progname, sshbuf_len(req));
+
+        if ((r = sshbuf_get_u8(req, &version)) != 0)
+                fatal("%s: buffer error: %s", __progname, ssh_err(r));
+        if (version != SSH_SK_HELPER_VERSION) {
+                fatal("unsupported version: received %d, expected %d",
+                    version, SSH_SK_HELPER_VERSION);
+        }
+        if ((r = sshbuf_froms(req, &kbuf)) != 0 ||
+            (r = sshkey_private_deserialize(kbuf, &key)) != 0)
+                fatal("Unable to parse key: %s", ssh_err(r));
+        if (sshkey_type_plain(key->type) != KEY_ECDSA_SK)
+                fatal("Unsupported key type %s", sshkey_ssh_name(key));
+
+        if ((r = sshbuf_get_cstring(req, &provider, NULL)) != 0 ||
+            (r = sshbuf_get_string_direct(req, &message, &msglen)) != 0 ||
+            (r = sshbuf_get_u32(req, &compat)) != 0)
+                fatal("%s: buffer error: %s", __progname, ssh_err(r));
+        if (sshbuf_len(req) != 0)
+                fatal("%s: trailing data in request", __progname);
+
+        debug("%s: ready to sign with key %s, provider %s: "
+            "msg len %zu, compat 0x%lx", __progname, sshkey_type(key),
+            provider, msglen, (u_long)compat);
+
+        if ((r = sshsk_ecdsa_sign(provider, key, &sig, &siglen,
+            message, msglen, compat)) != 0)
+                fatal("Signing failed: %s", ssh_err(r));
+
+        /* send reply */
+        if ((r = sshbuf_put_string(resp, sig, siglen)) != 0)
+                fatal("%s: buffer error: %s", __progname, ssh_err(r));
+        debug("%s: reply len %zu", __progname, sshbuf_len(resp));
+        if (ssh_msg_send(out, SSH_SK_HELPER_VERSION, resp) == -1)
+                fatal("ssh_msg_send failed");
+        close(out);
+
+        return (0);
+}