oss-security - bug in OpenSSL's CVE-2012-0884 fix

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Date: Fri, 11 May 2012 07:54:45 +0400
From: Solar Designer <solar@...nwall.com>
To: oss-security@...ts.openwall.com
Cc: openssl-team@...nssl.org, Ivan Nestlerode <inestlerode@...ibm.com>
Subject: bug in OpenSSL's CVE-2012-0884 fix

Hi,

This is just a heads up that OpenSSL 1.0.1c, 1.0.0j, and 0.9.8x fix an
additional issue that is believed to have only little/potential security
relevance in obscure cases and thus is being treated as a non-security
fix currently - yet is desirable to include in backports of distros that
make those instead of simply upgrading.

Here's the fix, by Stephen Henson of the OpenSSL core team:

"Make sure tkeylen is initialised properly when encrypting CMS messages."
http://cvs.openssl.org/chngview?cn=22537

or with an additional compiler warning fix:

http://cvs.openssl.org/filediff?f=openssl/crypto/cms/cms_enc.c&v1=1.11&v2=1.13

(Note: the "tkeylen = 0" portion is just to silence a possible compiler
warning; the actual fix is to the code.)

The bug had been introduced in:

"Fix for CMS/PKCS7 MMA. If RSA decryption fails use a random key and
continue with symmetric decryption process to avoid leaking timing
information to an attacker."
http://cvs.openssl.org/chngview?cn=22251

and more specifically in this portion:

http://cvs.openssl.org/filediff?f=openssl/crypto/cms/cms_enc.c&v1=1.10&v2=1.11

which introduced the tkey and tkeylen variables.  Of these, tkey is
initialized to NULL, tkeylen is left uninitialized (but is only meant to
be used when tkey is non-NULL?)  However, this line:

	if (ec->keylen != tkeylen)

may use the uninitialized tkeylen if (enc && ec->key) is true (this is
the opposite of the condition used to decide when to generate a random
key).  When ec->keylen != tkeylen happens to be true (which is likely),
the following block may then potentially use both tkey (which is NULL)
and tkeylen (uninitialized) if the call to
EVP_CIPHER_CTX_set_key_length() returns failure (shouldn't happen in a
bug-free app?)  Perhaps the program/thread would at least segfault in
this unlikely case.

Possibly more interesting is what will happen if the uninitialized
tkeylen value happens to match ec->keylen in the line quoted above
(unlikely, but possible).  I tried patching the line to:

	if (tkey && ec->keylen != tkeylen)

This simulates the tkeylen == ec->keylen case because tkey is left at
NULL precisely in the same cases when tkeylen is left uninitialized.

When this happens, the call to EVP_CIPHER_CTX_set_key_length() is
skipped.  This actually results in misbehavior seen on "make test":

[...]
data content test streaming PEM format: OK
encrypted content test streaming PEM format, 128 bit RC2 key: OK
encrypted content test streaming PEM format, 40 bit RC2 key: verify error
make[2]: *** [test_cms] Error 1

Thus, I conclude that the same kind of error might happen in actual
usage.  What impact this may have, I don't know.  Wrong encryption?
With what consequences?

Anyhow, at this point the bug is believed to be mostly non-security.

Alexander

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