upstream: warn when the agent returns a signature type that was

different to what was requested. This might happen when an old/non-OpenSSH
agent is asked to make a rsa-sha2-256/512 signature but only supports
ssh-rsa. bz#2799 feedback and ok markus@

OpenBSD-Commit-ID: 760c0f9438c5c58abc16b5f98008ff2d95cb13ce

1 files changed, 34 insertions, 7 deletions

diff --git a/sshconnect2.c b/sshconnect2.c
index 8138e46e0..2f0a57fac 100644
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect2.c,v 1.268 2018/02/07 22:52:45 dtucker Exp $ */
+/* $OpenBSD: sshconnect2.c,v 1.269 2018/03/03 03:01:50 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2008 Damien Miller.  All rights reserved.
@@ -1002,17 +1002,44 @@ key_sign_encode(const struct sshkey *key)
         return key_ssh_name(key);
 }
 
+/*
+ * Some agents will return ssh-rsa signatures when asked to make a
+ * rsa-sha2-* signature. Check what they actually gave back and warn the
+ * user if the agent has returned an unexpected type.
+ */
+static int
+check_sigtype(const struct sshkey *key, const u_char *sig, size_t len)
+{
+        int r;
+        char *sigtype = NULL;
+        const char *alg = key_sign_encode(key);
+
+        if ((r = sshkey_sigtype(sig, len, &sigtype)) != 0)
+                return r;
+        if (strcmp(sigtype, alg) != 0) {
+                logit("warning: agent returned different signature type %s "
+                    "(expected %s)", sigtype, alg);
+        }
+        free(sigtype);
+        /* Incorrect signature types aren't an error ... yet */
+        return 0;
+}
+
 static int
 identity_sign(struct identity *id, u_char **sigp, size_t *lenp,
     const u_char *data, size_t datalen, u_int compat)
 {
         struct sshkey *prv;
-        int ret;
+        int r;
 
         /* the agent supports this key */
-        if (id->key != NULL && id->agent_fd != -1)
-                return ssh_agent_sign(id->agent_fd, id->key, sigp, lenp,
-                    data, datalen, key_sign_encode(id->key), compat);
+        if (id->key != NULL && id->agent_fd != -1) {
+                if ((r = ssh_agent_sign(id->agent_fd, id->key, sigp, lenp,
+                    data, datalen, key_sign_encode(id->key), compat)) != 0 ||
+                    (r = check_sigtype(id->key, *sigp, *lenp)) != 0)
+                        return r;
+                return 0;
+        }
 
         /*
          * we have already loaded the private key or
@@ -1031,10 +1058,10 @@ identity_sign(struct identity *id, u_char **sigp, size_t *lenp,
                    __func__, id->filename);
                 return SSH_ERR_KEY_NOT_FOUND;
         }
-        ret = sshkey_sign(prv, sigp, lenp, data, datalen,
+        r = sshkey_sign(prv, sigp, lenp, data, datalen,
             key_sign_encode(prv), compat);
         sshkey_free(prv);
-        return (ret);
+        return r;
 }
 
 static int