pki_crypto: fix DSA signature extraction

Fix the DSA portion of 'pki_signature_to_blob': before this change, it
is possible to sometimes observe DSA signature validation failure when
testing with OpenSSH clients.  The problem ended up being the following
snippet which did not account for the case when 'ssh_string_len(x)' may
be less than 20:

  r = make_bignum_string(sig->dsa_sig->r);
  ...
  memcpy(buffer,
         ((char *) ssh_string_data(r)) + ssh_string_len(r) - 20,
         20);

Above consider the case that ssh_string_len(r) is 19; in that case the
memcpy unintentionally starts in the wrong place.  The same situation
can happen for value 's' in this code.

To fix, adjust the offsets used for the input and output pointers, taking
into account that the lengths of 'r' and 's' can be less than 20.  With
the fix I am no longer able to reproduce the original failure mode.

BUG: https://red.libssh.org/issues/144

Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/pki_crypto.c

@@ -1094,41 +1094,65 @@ static ssh_string _RSA_do_sign(const unsigned char *digest,
     return sig_blob;
 }
 
+static ssh_string pki_dsa_signature_to_blob(const ssh_signature sig)
+{
+    char buffer[40] = { 0 };
+    ssh_string sig_blob = NULL;
+
+    ssh_string r;
+    int r_len, r_offset_in, r_offset_out;
+
+    ssh_string s;
+    int s_len, s_offset_in, s_offset_out;
+
+    r = make_bignum_string(sig->dsa_sig->r);
+    if (r == NULL) {
+        return NULL;
+    }
+
+    s = make_bignum_string(sig->dsa_sig->s);
+    if (s == NULL) {
+        ssh_string_free(r);
+        return NULL;
+    }
+
+    r_len = ssh_string_len(r);
+    r_offset_in  = (r_len > 20) ? (r_len - 20) : 0;
+    r_offset_out = (r_len < 20) ? (20 - r_len) : 0;
+
+    s_len = ssh_string_len(s);
+    s_offset_in  = (s_len > 20) ? (s_len - 20) : 0;
+    s_offset_out = (s_len < 20) ? (20 - s_len) : 0;
+
+    memcpy(buffer + r_offset_out,
+           ((char *)ssh_string_data(r)) + r_offset_in,
+           r_len - r_offset_in);
+    memcpy(buffer + 20 + s_offset_out,
+           ((char *)ssh_string_data(s)) + s_offset_in,
+           s_len - s_offset_in);
+
+    ssh_string_free(r);
+    ssh_string_free(s);
+
+    sig_blob = ssh_string_new(40);
+    if (sig_blob == NULL) {
+        return NULL;
+    }
+
+    ssh_string_fill(sig_blob, buffer, 40);
+
+    return sig_blob;
+}
+
 ssh_string pki_signature_to_blob(const ssh_signature sig)
 {
-    char buffer[40] = {0};
-    ssh_string sig_blob = NULL;
     ssh_string r;
     ssh_string s;
+    ssh_string sig_blob = NULL;
 
     switch(sig->type) {
         case SSH_KEYTYPE_DSS:
-            r = make_bignum_string(sig->dsa_sig->r);
+            sig_blob = pki_dsa_signature_to_blob(sig);
-            if (r == NULL) {
-                return NULL;
-            }
-            s = make_bignum_string(sig->dsa_sig->s);
-            if (s == NULL) {
-                ssh_string_free(r);
-                return NULL;
-            }
-
-            memcpy(buffer,
-                   ((char *)ssh_string_data(r)) + ssh_string_len(r) - 20,
-                   20);
-            memcpy(buffer + 20,
-                   ((char *)ssh_string_data(s)) + ssh_string_len(s) - 20,
-                   20);
-
-            ssh_string_free(r);
-            ssh_string_free(s);
-
-            sig_blob = ssh_string_new(40);
-            if (sig_blob == NULL) {
-                return NULL;
-            }
-
-            ssh_string_fill(sig_blob, buffer, 40);
             break;
         case SSH_KEYTYPE_RSA:
         case SSH_KEYTYPE_RSA1: