[PATCH 3/3] block: print the server key type and fingerprint on failure

["Daniel P. Berrangé" <berrange@redhat.com>]

When validating the server key fingerprint fails, it is difficult for
the user to know what they got wrong. The fingerprint accepted by QEMU
is received in a different format than openssh displays. There can also
be keys for multiple different ciphers in known_hosts. It may not be
obvious which cipher QEMU will use and whether it will be the same
as openssh. Address this by printing the server key type and its
corresponding fingerprint in the format QEMU accepts.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 block/ssh.c | 37 ++++++++++++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 7 deletions(-)

diff --git a/block/ssh.c b/block/ssh.c
index fcc0ab765a..967a2b971e 100644
--- a/block/ssh.c
+++ b/block/ssh.c
@@ -386,14 +386,28 @@ static int compare_fingerprint(const unsigned char *fingerprint, size_t len,
     return *host_key_check - '\0';
 }
 
+static char *format_fingerprint(const unsigned char *fingerprint, size_t len)
+{
+    static const char *hex = "0123456789abcdef";
+    char *ret = g_new0(char, (len * 2) + 1);
+    for (size_t i = 0; i < len; i++) {
+        ret[i * 2] = hex[((fingerprint[i] >> 4) & 0xf)];
+        ret[(i * 2) + 1] = hex[(fingerprint[i] & 0xf)];
+    }
+    ret[len * 2] = '\0';
+    return ret;
+}
+
 static int
 check_host_key_hash(BDRVSSHState *s, const char *hash,
-                    enum ssh_publickey_hash_type type, Error **errp)
+                    enum ssh_publickey_hash_type type, const char *typestr,
+                    Error **errp)
 {
     int r;
     ssh_key pubkey;
     unsigned char *server_hash;
     size_t server_hash_len;
+    const char *keytype;
 
     r = ssh_get_server_publickey(s->session, &pubkey);
     if (r != SSH_OK) {
@@ -401,6 +415,8 @@ check_host_key_hash(BDRVSSHState *s, const char *hash,
         return -EINVAL;
     }
 
+    keytype = ssh_key_type_to_char(ssh_key_type(pubkey));
+
     r = ssh_get_publickey_hash(pubkey, type, &server_hash, &server_hash_len);
     ssh_key_free(pubkey);
     if (r != 0) {
@@ -410,12 +426,16 @@ check_host_key_hash(BDRVSSHState *s, const char *hash,
     }
 
     r = compare_fingerprint(server_hash, server_hash_len, hash);
-    ssh_clean_pubkey_hash(&server_hash);
     if (r != 0) {
-        error_setg(errp, "remote host key does not match host_key_check '%s'",
-                   hash);
+        g_autofree char *server_fp = format_fingerprint(server_hash,
+                                                        server_hash_len);
+        error_setg(errp, "remote host %s key fingerprint '%s:%s' "
+                   "does not match host_key_check '%s:%s'",
+                   keytype, typestr, server_fp, typestr, hash);
+        ssh_clean_pubkey_hash(&server_hash);
         return -EPERM;
     }
+    ssh_clean_pubkey_hash(&server_hash);
 
     return 0;
 }
@@ -436,13 +456,16 @@ static int check_host_key(BDRVSSHState *s, SshHostKeyCheck *hkc, Error **errp)
     case SSH_HOST_KEY_CHECK_MODE_HASH:
         if (hkc->u.hash.type == SSH_HOST_KEY_CHECK_HASH_TYPE_MD5) {
             return check_host_key_hash(s, hkc->u.hash.hash,
-                                       SSH_PUBLICKEY_HASH_MD5, errp);
+                                       SSH_PUBLICKEY_HASH_MD5, "md5",
+                                       errp);
         } else if (hkc->u.hash.type == SSH_HOST_KEY_CHECK_HASH_TYPE_SHA1) {
             return check_host_key_hash(s, hkc->u.hash.hash,
-                                       SSH_PUBLICKEY_HASH_SHA1, errp);
+                                       SSH_PUBLICKEY_HASH_SHA1, "sha1",
+                                       errp);
         } else if (hkc->u.hash.type == SSH_HOST_KEY_CHECK_HASH_TYPE_SHA256) {
             return check_host_key_hash(s, hkc->u.hash.hash,
-                                       SSH_PUBLICKEY_HASH_SHA256, errp);
+                                       SSH_PUBLICKEY_HASH_SHA256, "sha256",
+                                       errp);
         }
         g_assert_not_reached();
         break;
-- 
2.31.1