Alternative fix for CVE-2022-4304

This is about a timing leak in the topmost limb
of the internal result of RSA_private_decrypt,
before the padding check.

There are in fact at least three bugs together that
caused the timing leak:

First and probably most important is the fact that
the blinding did not use the constant time code path
at all when the RSA object was used for a private
decrypt, due to the fact that the Montgomery context
rsa->_method_mod_n was not set up early enough in
rsa_ossl_private_decrypt, when BN_BLINDING_create_param
needed it, and that was persisted as blinding->m_ctx,
although the RSA object creates the Montgomery context
just a bit later.

Then the infamous bn_correct_top was used on the
secret value right after the blinding was removed.

And finally the function BN_bn2binpad did not use
the constant-time code path since the BN_FLG_CONSTTIME
was not set on the secret value.

In order to address the first problem, this patch
makes sure that the rsa->_method_mod_n is initialized
right before the blinding context.

And to fix the second problem, we add a new utility
function bn_correct_top_consttime, a const-time
variant of bn_correct_top.

Together with the fact, that BN_bn2binpad is already
constant time if the flag BN_FLG_CONSTTIME is set,
this should eliminate the timing oracle completely.

In addition the no-asm variant may also have
branches that depend on secret values, because the last
invocation of bn_sub_words in bn_from_montgomery_word
had branches when the function is compiled by certain
gcc compiler versions, due to the clumsy coding style.

So additionally this patch stream-lined the no-asm
C-code in order to avoid branches where possible and
improve the resulting code quality.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20284)

1 files changed, 10 insertions, 0 deletions

diff --git a/CHANGES b/CHANGES
index b19f1429bb..430e32e624 100644
--- a/CHANGES
+++ b/CHANGES
@@ -9,6 +9,16 @@
 
  Changes between 1.1.1t and 1.1.1u [xx XXX xxxx]
 
+  *) Reworked the Fix for the Timing Oracle in RSA Decryption (CVE-2022-4304).
+     The previous fix for this timing side channel turned out to cause
+     a severe 2-3x performance regression in the typical use case
+     compared to 1.1.1s. The new fix uses existing constant time
+     code paths, and restores the previous performance level while
+     fully eliminating all existing timing side channels.
+     The fix was developed by Bernd Edlinger with testing support
+     by Hubert Kario.
+     [Bernd Edlinger]
+
   *) Corrected documentation of X509_VERIFY_PARAM_add0_policy() to mention
      that it does not enable policy checking. Thanks to
      David Benjamin for discovering this issue. (CVE-2023-0466)