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  1. Feb 25, 2019
    • Matt Caswell's avatar
      Ensure bn_cmp_words can handle the case where n == 0 · 576129cd
      Matt Caswell authored
      
      
      Thanks to David Benjamin who reported this, performed the analysis and
      suggested the patch. I have incorporated some of his analysis in the
      comments below.
      
      This issue can cause an out-of-bounds read. It is believed that this was
      not reachable until the recent "fixed top" changes. Analysis has so far
      only identified one code path that can encounter this - although it is
      possible that others may be found. The one code path only impacts 1.0.2 in
      certain builds. The fuzzer found a path in RSA where iqmp is too large. If
      the input is all zeros, the RSA CRT logic will multiply a padded zero by
      iqmp. Two mitigating factors:
      
      - Private keys which trip this are invalid (iqmp is not reduced mod p).
      Only systems which take untrusted private keys care.
      - In OpenSSL 1.1.x, there is a check which rejects the oversize iqmp,
      so the bug is only reproducible in 1.0.2 so far.
      
      Fortunately, the bug appears to be relatively harmless. The consequences of
      bn_cmp_word's misbehavior are:
      
      - OpenSSL may crash if the buffers are page-aligned and the previous page is
      non-existent.
      - OpenSSL will incorrectly treat two BN_ULONG buffers as not equal when they
      are equal.
      - Side channel concerns.
      
      The first is indeed a concern and is a DoS bug. The second is fine in this
      context. bn_cmp_word and bn_cmp_part_words are used to compute abs(a0 - a1)
      in Karatsuba. If a0 = a1, it does not matter whether we use a0 - a1 or
      a1 - a0. The third would be worth thinking about, but it is overshadowed
      by the entire Karatsuba implementation not being constant time.
      
      Due to the difficulty of tripping this and the low impact no CVE is felt
      necessary for this issue.
      
      Reviewed-by: default avatarPaul Dale <paul.dale@oracle.com>
      Reviewed-by: default avatarViktor Dukhovni <viktor@openssl.org>
      (Merged from https://github.com/openssl/openssl/pull/8326)
      576129cd
    • David von Oheimb's avatar
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