sign/ed25519: Verify signatures are minimum length

The ed25519 signature verification code does not
check that the signature is a minimum/correct length.
As a result, if the signature is too short, libsodium will end up
reading a few bytes out of bounds.

Reported-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Co-authored-by: Demi Marie Obenour <demi@invisiblethingslab.com>

Closes: GHSA-gqf4-p3gv-g8vw

src/libostree/ostree-sign-ed25519.c

@@ -209,6 +209,9 @@ gboolean ostree_sign_ed25519_data_verify (OstreeSign *self,
       g_autoptr (GVariant) child = g_variant_get_child_value (signatures, i);
       g_autoptr (GBytes) signature = g_variant_get_data_as_bytes(child);
 
+      if (g_bytes_get_size (signature) != crypto_sign_BYTES)
+        return glnx_throw (error, "Invalid signature length of %" G_GSIZE_FORMAT " bytes, expected %" G_GSIZE_FORMAT, (gsize) g_bytes_get_size (signature), (gsize) crypto_sign_BYTES);
+
       g_autofree char * hex = g_malloc0 (crypto_sign_PUBLICKEYBYTES*2 + 1);
 
       g_debug("Read signature %d: %s", (gint)i, g_variant_print(child, TRUE));