upstream: Extends the SK API to accept a set of key/value options

for all operations. These are intended to future-proof the API a little by
making it easier to specify additional fields for without having to change
the API version for each.

At present, only two options are defined: one to explicitly specify
the device for an operation (rather than accepting the middleware's
autoselection) and another to specify the FIDO2 username that may
be used when generating a resident key. These new options may be
invoked at key generation time via ssh-keygen -O

This also implements a suggestion from Markus to avoid "int" in favour
of uint32_t for the algorithm argument in the API, to make implementation
of ssh-sk-client/helper a little easier.

feedback, fixes and ok markus@

OpenBSD-Commit-ID: 973ce11704609022ab36abbdeb6bc23c8001eabc

1 files changed, 104 insertions, 17 deletions

diff --git a/ssh-sk.c b/ssh-sk.c
index b1d0d6c58..0ef52e299 100644
--- a/ssh-sk.c
+++ b/ssh-sk.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh-sk.c,v 1.23 2019/12/30 09:24:45 djm Exp $ */
+/* $OpenBSD: ssh-sk.c,v 1.24 2020/01/06 02:00:47 djm Exp $ */
 /*
  * Copyright (c) 2019 Google LLC
  *
@@ -53,29 +53,32 @@ struct sshsk_provider {
         /* Enroll a U2F key (private key generation) */
         int (*sk_enroll)(int alg, const uint8_t *challenge,
             size_t challenge_len, const char *application, uint8_t flags,
-            const char *pin, struct sk_enroll_response **enroll_response);
+            const char *pin, struct sk_option **opts,
+            struct sk_enroll_response **enroll_response);
 
         /* Sign a challenge */
         int (*sk_sign)(int alg, const uint8_t *message, size_t message_len,
             const char *application,
             const uint8_t *key_handle, size_t key_handle_len,
-            uint8_t flags, const char *pin,
+            uint8_t flags, const char *pin, struct sk_option **opts,
             struct sk_sign_response **sign_response);
 
         /* Enumerate resident keys */
-        int (*sk_load_resident_keys)(const char *pin,
+        int (*sk_load_resident_keys)(const char *pin, struct sk_option **opts,
             struct sk_resident_key ***rks, size_t *nrks);
 };
 
 /* Built-in version */
 int ssh_sk_enroll(int alg, const uint8_t *challenge,
     size_t challenge_len, const char *application, uint8_t flags,
-    const char *pin, struct sk_enroll_response **enroll_response);
+    const char *pin, struct sk_option **opts,
+    struct sk_enroll_response **enroll_response);
 int ssh_sk_sign(int alg, const uint8_t *message, size_t message_len,
     const char *application,
     const uint8_t *key_handle, size_t key_handle_len,
-    uint8_t flags, const char *pin, struct sk_sign_response **sign_response);
-int ssh_sk_load_resident_keys(const char *pin,
+    uint8_t flags, const char *pin, struct sk_option **opts,
+    struct sk_sign_response **sign_response);
+int ssh_sk_load_resident_keys(const char *pin, struct sk_option **opts,
     struct sk_resident_key ***rks, size_t *nrks);
 
 static void
@@ -339,9 +342,80 @@ skerr_to_ssherr(int skerr)
         }
 }
 
+static void
+sshsk_free_options(struct sk_option **opts)
+{
+        size_t i;
+
+        if (opts == NULL)
+                return;
+        for (i = 0; opts[i] != NULL; i++) {
+                free(opts[i]->name);
+                free(opts[i]->value);
+                free(opts[i]);
+        }
+        free(opts);
+}
+
+static int
+sshsk_add_option(struct sk_option ***optsp, size_t *noptsp,
+    const char *name, const char *value, uint8_t required)
+{
+        struct sk_option **opts = *optsp;
+        size_t nopts = *noptsp;
+
+        if ((opts = recallocarray(opts, nopts, nopts + 2, /* extra for NULL */
+            sizeof(*opts))) == NULL) {
+                error("%s: array alloc failed", __func__);
+                return SSH_ERR_ALLOC_FAIL;
+        }
+        *optsp = opts;
+        *noptsp = nopts + 1;
+        if ((opts[nopts] = calloc(1, sizeof(**opts))) == NULL) {
+                error("%s: alloc failed", __func__);
+                return SSH_ERR_ALLOC_FAIL;
+        }
+        if ((opts[nopts]->name = strdup(name)) == NULL ||
+            (opts[nopts]->value = strdup(value)) == NULL) {
+                error("%s: alloc failed", __func__);
+                return SSH_ERR_ALLOC_FAIL;
+        }
+        opts[nopts]->required = required;
+        return 0;
+}
+
+static int
+make_options(const char *device, const char *user_id,
+    struct sk_option ***optsp)
+{
+        struct sk_option **opts = NULL;
+        size_t nopts = 0;
+        int r, ret = SSH_ERR_INTERNAL_ERROR;
+
+        if (device != NULL &&
+            (r = sshsk_add_option(&opts, &nopts, "device", device, 0)) != 0) {
+                ret = r;
+                goto out;
+        }
+        if (user_id != NULL &&
+            (r = sshsk_add_option(&opts, &nopts, "user", user_id, 0)) != 0) {
+                ret = r;
+                goto out;
+        }
+        /* success */
+        *optsp = opts;
+        opts = NULL;
+        nopts = 0;
+        ret = 0;
+ out:
+        sshsk_free_options(opts);
+        return ret;
+}
+
 int
-sshsk_enroll(int type, const char *provider_path, const char *application,
-    uint8_t flags, const char *pin, struct sshbuf *challenge_buf,
+sshsk_enroll(int type, const char *provider_path, const char *device,
+    const char *application, const char *userid, uint8_t flags,
+    const char *pin, struct sshbuf *challenge_buf,
     struct sshkey **keyp, struct sshbuf *attest)
 {
         struct sshsk_provider *skp = NULL;
@@ -350,17 +424,23 @@ sshsk_enroll(int type, const char *provider_path, const char *application,
         const u_char *challenge;
         size_t challenge_len;
         struct sk_enroll_response *resp = NULL;
+        struct sk_option **opts = NULL;
         int r = SSH_ERR_INTERNAL_ERROR;
         int alg;
 
-        debug("%s: provider \"%s\", application \"%s\", flags 0x%02x, "
-            "challenge len %zu%s", __func__, provider_path, application,
-            flags, challenge_buf == NULL ? 0 : sshbuf_len(challenge_buf),
+        debug("%s: provider \"%s\", device \"%s\", application \"%s\", "
+            "userid \"%s\", flags 0x%02x, challenge len %zu%s", __func__,
+            provider_path, device, application, userid, flags,
+            challenge_buf == NULL ? 0 : sshbuf_len(challenge_buf),
             (pin != NULL && *pin != '\0') ? " with-pin" : "");
 
         *keyp = NULL;
         if (attest)
                 sshbuf_reset(attest);
+
+        if ((r = make_options(device, userid, &opts)) != 0)
+                goto out;
+
         switch (type) {
 #ifdef WITH_OPENSSL
         case KEY_ECDSA_SK:
@@ -407,7 +487,7 @@ sshsk_enroll(int type, const char *provider_path, const char *application,
         /* XXX validate flags? */
         /* enroll key */
         if ((r = skp->sk_enroll(alg, challenge, challenge_len, application,
-            flags, pin, &resp)) != 0) {
+            flags, pin, opts, &resp)) != 0) {
                 error("Security key provider \"%s\" returned failure %d",
                     provider_path, r);
                 r = skerr_to_ssherr(r);
@@ -437,6 +517,7 @@ sshsk_enroll(int type, const char *provider_path, const char *application,
         key = NULL; /* transferred */
         r = 0;
  out:
+        sshsk_free_options(opts);
         sshsk_free(skp);
         sshkey_free(key);
         sshsk_free_enroll_response(resp);
@@ -528,6 +609,7 @@ sshsk_sign(const char *provider_path, struct sshkey *key,
         struct sk_sign_response *resp = NULL;
         struct sshbuf *inner_sig = NULL, *sig = NULL;
         uint8_t message[32];
+        struct sk_option **opts = NULL;
 
         debug("%s: provider \"%s\", key %s, flags 0x%02x%s", __func__,
             provider_path, sshkey_type(key), key->sk_flags,
@@ -571,7 +653,7 @@ sshsk_sign(const char *provider_path, struct sshkey *key,
         if ((r = skp->sk_sign(alg, message, sizeof(message),
             key->sk_application,
             sshbuf_ptr(key->sk_key_handle), sshbuf_len(key->sk_key_handle),
-            key->sk_flags, pin, &resp)) != 0) {
+            key->sk_flags, pin, opts, &resp)) != 0) {
                 debug("%s: sk_sign failed with code %d", __func__, r);
                 r = skerr_to_ssherr(r);
                 goto out;
@@ -617,6 +699,7 @@ sshsk_sign(const char *provider_path, struct sshkey *key,
         /* success */
         r = 0;
  out:
+        sshsk_free_options(opts);
         explicit_bzero(message, sizeof(message));
         sshsk_free(skp);
         sshsk_free_sign_response(resp);
@@ -645,8 +728,8 @@ sshsk_free_sk_resident_keys(struct sk_resident_key **rks, size_t nrks)
 }
 
 int
-sshsk_load_resident(const char *provider_path, const char *pin,
-    struct sshkey ***keysp, size_t *nkeysp)
+sshsk_load_resident(const char *provider_path, const char *device,
+    const char *pin, struct sshkey ***keysp, size_t *nkeysp)
 {
         struct sshsk_provider *skp = NULL;
         int r = SSH_ERR_INTERNAL_ERROR;
@@ -654,6 +737,7 @@ sshsk_load_resident(const char *provider_path, const char *pin,
         size_t i, nrks = 0, nkeys = 0;
         struct sshkey *key = NULL, **keys = NULL, **tmp;
         uint8_t flags;
+        struct sk_option **opts = NULL;
 
         debug("%s: provider \"%s\"%s", __func__, provider_path,
             (pin != NULL && *pin != '\0') ? ", have-pin": "");
@@ -663,11 +747,13 @@ sshsk_load_resident(const char *provider_path, const char *pin,
         *keysp = NULL;
         *nkeysp = 0;
 
+        if ((r = make_options(device, NULL, &opts)) != 0)
+                goto out;
         if ((skp = sshsk_open(provider_path)) == NULL) {
                 r = SSH_ERR_INVALID_FORMAT; /* XXX sshsk_open return code? */
                 goto out;
         }
-        if ((r = skp->sk_load_resident_keys(pin, &rks, &nrks)) != 0) {
+        if ((r = skp->sk_load_resident_keys(pin, opts, &rks, &nrks)) != 0) {
                 error("Security key provider \"%s\" returned failure %d",
                     provider_path, r);
                 r = skerr_to_ssherr(r);
@@ -710,6 +796,7 @@ sshsk_load_resident(const char *provider_path, const char *pin,
         nkeys = 0;
         r = 0;
  out:
+        sshsk_free_options(opts);
         sshsk_free(skp);
         sshsk_free_sk_resident_keys(rks, nrks);
         sshkey_free(key);